Symposium Organizers
Juergen Christen Otto-von-Guericke-Universität Magdeburg
Leonard J. Brillson Ohio State University
Hiroshi Fujioka University of Tokyo
H. Hoe Tan The Australian National University
H1: Devices
Session Chairs
Jürgen Christen
Naoki Ohashi
Monday PM, November 30, 2009
Ballroom C (Hynes)
9:30 AM - **H1.1
ZnO-based MESFET Devices.
Marius Grundmann 1
1 Institut fur Experimentelle Physik II, Universitaet Leipzig, Leipzig Germany
Show AbstractBased on our research on Schottky contacts on ZnO [1-4] we present metal-semiconductor field-effect transistors (MESFET) and integrated circuites (inverters) based on them. ZnO MESFETs on sapphire exhibit high channel mobility and large on-off ratio (mobility > 20 cm2/Vs, Ion/Ioff > 108) [5], similar to the performance of MISFET devices (with insulating gate electrode). The small operation voltages (+/- 1V) allow high transconductance and the smallest voltage swing (80 mV/dec.) reported for transparent semiconductor FETs [6]. Normally-on and normally-off devices are presented. For the first time Schottky gates on ZnO on glass allow for the fabrication of transistors with still rather good performance (mobility > 1 cm2/Vs, Ion/Ioff> 106). Inverters from two Mg0.003Zn0.097O channel MESFETs on sapphire exhibit unrivalled large peak gain maximum > 100 for supply voltage of only 3V. The oxide thin films have been prepared using pulsed laser deposition. Ohmic contacts are made using Au. The Schottky gates are made from various noble metal oxides, typically AgxO yielding the best results. In addition temperature dependent performance data, and results on the effect of annealing and bias stress on performance are presented.[1] H. von Wenckstern, E. M. Kaidashev, M. Lorenz, H. Hochmuth, G. Biehne, J. Lenzner, V. Gottschalch, R. Pickenhain, M. Grundmann, Lateral homogeneity of Schottky contacts on n-type ZnO, Appl. Phys. Lett. 84, 79 (2004)[2] H. v. Wenckstern, G. Biehne, R. Abdel Rahman, H. Hochmuth, M. Lorenz, M. Grundmann, Mean barrier height of Pd Schottky contacts on ZnO thin films, Appl. Phys. Lett. 88, 092102 (2006)[3] A. Lajn, H. von Wenckstern, Z. Zhang, C. Czekalla, G. Biehne, J. Lenzner, H. Hochmuth, M. Lorenz, M. Grundmann, S. Wickert, C. Vogt, R. Denecke, Properties of reactively sputtered Ag, Au, Pd, and Pt Schottky contacts on n-type ZnO, J. Vac. Sci. Technol. 27, 1769 (2009)[4] M.W. Allen, S.M. Durbin, X. Weng, J.M. Redwing, K. Sarpatwari, S.E. Mohney, H. von Wenckstern, M. Grundmann, Temperature Dependent Properties of Nearly Ideal ZnO Schottky Diodes, IEEE Transact. Electr. Dev. (2009), in press[5] H. Frenzel, A. Lajn, M. Brandt, H. von Wenckstern, G. Biehne, H. Hochmuth, M. Lorenz, M. Grundmann, ZnO metal-semiconductor field-effect transistors with Ag-Schottky-gates, Appl. Phys. Lett. 92, 192108 (2008)[6] M. Grundmann, H. Frenzel, A. Lajn, M. Lorenz, F. Schein, H. von Wenckstern, Transparent Semiconducting Oxides: Materials and Devices, phys. stat. sol. (b) (2009), in pressThis work has been performed in cooperation with H. Frenzel, A. Lajn, M. Lorenz, F. Schein, and H. von Wenckstern. This work is funded by Deutsche Forschungsgemeinschaft within the framework of Sonderforschungsbereich 762 ”Functionality of oxide interfaces” and the Leipzig Graduate School of Natural Sciences ”Building with Molecules and Nano-objects (BuildMoNa)” and the European Social Fund.
10:00 AM - H1.2
Enhanced Chemical Sensitivity of ZnO based Thin Film Resistors and Transistors via Illumination and Electric Field Effects.
George Whitfield 1 , Harry Tuller 1
1 Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States
Show AbstractZnO based thin film resistors (TFRs) and buried-gate thin film transistors (TFTs) have been fabricated and characterized as chemical sensors, to detect oxidizing and reducing gases. Conventional chemical sensors based on semiconducting metal-oxides often suffer from a number of drawbacks including the need to operate at elevated temperature (~200 to 400 degrees Celsius) with resultant degradation in selectivity. [1,2,3] In this study, the influence of illumination and field effect are investigated as means of accelerating surface reactions at reduced temperatures. Sensor selectivity and sensitivity are shown to be influenced through control of wavelength of illumination and magnitude of applied gate bias. Models are presented which explain the observed phenomena.[1] J. W. Gardne and P.N. Bartlett, Electronic Noses: principles and application, Oxford Science Publications: Oxford, 1999.[2] R. Gutierrez-Osuna, IEEE Sensors J. Vol. 2 , p. 189, 2002.[3] I.-D. Kim, A. Rothschild, T. Hyodo, and H. L. Tuller, Nano Lett., Vol. 6, pp. 93-198, 2006.
10:15 AM - H1.3
Identifying Individual n- and p-type ZnO Nanowires by the Output Voltage Sign of Piezoelectric Nanogenerator.
Shisheng Lin 1 2 , Jinhui Song 1 , Yangfan Lu 2 , Zhizhen Ye 2 , Zhonglin Wang 1
1 , School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 , State Key Laboratory of Silicon Materials, Hangzhou China
Show AbstractEffective identification of p-type ZnO is of critical importance for its applications in light-emitting diodes (LEDs). Recently, other than the thin film technology, interests have been focused on single crystalline p-type ZnO nanowires (NWs), which are the fundamental building blocks for a wide range of nano-optoelectronic and nano-electronic devices, such as nano-LEDs, nanogenerator and nanopiezotronic devices. For ZnO films, the conductivity type of the sample is determined in two ways: measurements of Hall effect and Seebeck effect, which are well established techniques. However, due to low dimensionality, it is hard to carry out such measurements for ZnO NWs. Up to now, the transport measurement of single NW based field effect transistor (FET) is being applied to identify the carrier type ZnO NWs, the process of which is rather complex and usually destroy the samples. We demonstrate a reliable and reproducible way how to measure the conductivity type of a single ZnO NW by piezoelectric effect without destroying the sample. This technique utilizes the unique different majority carrier and charge distribution between the n-type and p-type NWs being bent. A schottky barrier is needed to rectify the current direction. For a bent NW, the electrons, majority carrier in the n-type ZnO, tend to accumulate at the positive piezoelectric potentials, leaving the negative potentials at the compressed side unscreened. On the other hand, the holes, majority carrier in p-type ZnO, tend to accumulate at the negative piezoelectric potentials side, leaving the positive potentials at the stretched side unscreened. For the Pt-ZnO contact, a forward bias is produced only when the p-type ZnO side is applied positive voltage with regard to the Pt side and the reverse holds for the n-type ZnO. Under this scenario, in the real AFM tests, when the Pt coated AFM tip contact with the compressed side of p-type ZnO NWs, a positive output is produced. In contrast, negative voltage output is produced when the Pt coated AFM tip touches the compressed side of the n-type NWs. The unique and striking difference between the sign of the voltage output establishes the non-destructive method for discriminating the conductivity type of the ZnO NWs. [1]. Z. L. Wang, J. H. Song, Science, 312, 242 (2006). [2]. S. S. Lin, J. H. Song, Y. F. Lu, Z. Z. Ye, Z. L. Wang, submitted.[3]. M-P. Lu, J. H. Song, M-Y. Lu, M-T. Chen, Y. F. Gao, L-J. Chen and Z. L. Wang, nano letter 9, 1223 (2009).[4]. For more information: http://www.nanoscience.gatech.edu/zlwang/
10:30 AM - H1.4
ZnO:Polydiacetylene Films as Chromatic Sensors.
Anitha Patlolla 1 , Zafar Iqbal 1
1 Chemistry, New Jersey Institute of Technology, Newark, New Jersey, United States
Show AbstractFilms of the conjugated polydiacetylenes (PDAs) can function as chromatic temperature and stress sensors and can be prepared by spin-coating from monomer solutions, followed by polymerization using UV- or γ- irradiation. The polymer backbone of PDAs is comprised of alternating ene-yne groups which are responsible for intriguing stress, temperature and chemically-induced chromatic (blue to red) transitions. PDAs thus have the potential for advanced applications as sensors, but these applications are to some degree limited by the irreversibility of the blue to red transition in PDAs obtained from commercially available, cost-effective diacetylene monomers. We have discovered that ZnO uniquely forms a weak complex with acidic diacetylenes containing carboxylic groups: 10, 12-pentacosadiynoic acid (PCDA), 10, 12-docosadiynedioic acid (DCDA), 10, 12-tricosadiynoic acid (TCDA), resulting in reversal of the chromatic blue to red transition. Nanocomposite ZnO:PDA thin films have been prepared and their structural properties have been characterized by a variety of techniques including fourier transform infrared and Raman spectroscopy, differential thermal calorimetry (DSC) and extended x-ray absorption fine structure measurements. The infrared and DSC measurements are particularly indicative of chemical interaction between ZnO and the end –COOH groups on the side chains. We have also conducted testing and calibration of the thermal and stress sensing properties of the composite films. It was observed that the PDA based on DCDA which has a –COOH group at the end of each side chain remains in the red phase when it is encapsulated in a KBr matrix. The use of this property to simultaneously sense temperature/stress as a function of time will be discussed.
10:45 AM - H1.5
Conductance Modulation of ZnO Nanowires Through Surface Functionalization.
Tej Belagodu 1 , Hongbin Yu 1
1 Department of Electrical Engineering and Center for Solid State Electronic Research, Arizona State University, Tempe, Arizona, United States
Show AbstractAmong the II-VI semiconductors, ZnO has generated huge interest in the research community owing to certain favorable properties such as direct wide band-gap, high exciton binding energy, strong room-temperature luminescence, piezoelectricity and transparency in the visible range. However, the surface of ZnO nanowire (NW) can influence its optical and electrical properties, thus understanding and controlling the surface is vital. In this work, three terminal ZnO NW devices with a universal back-gate were fabricated by photolithography and metal deposition over dispersed ZnO NWs. First, raw I-V measurements in air and nitrogen ambient are recorded. Next, the NW surface is modified by functionalizing it with different molecular species to study the effects. I-V measurements after functionalization with carboxylic acid show an increase in the source-drain current whereas functionalization with a hydrocarbon-amine results in an effective decrease in conductance. Device fabrication, surface functionalization mechanisms, 2,3-terminal I-V measurements and applications will be discussed. These observed device behaviors are due largely to NW geometry. By their very morphology, NWs possess a very high surface-area to volume ratio which translates to the ability to modify the NWs’ properties by working on its surface. The surface of ZnO NWs generally carries many defects, surface imperfections and oxygen vacancies. Oxygen molecules from the ambient air capture free electrons from the bulk and get adsorbed on the surface [ O2 (gas) + e- -> O2- (ad) ]. Charge carriers .i.e., electrons and holes can be captured together by water molecules when they dissociate. These mechanisms reduce the number of free electrons in the material and form a depletion layer below the surface leading to a decrease in current when in air. This is substantiated by the fact that current increases considerably when the same IV measurements are carried out in a nitrogen ambient. Surface modification using Hydrocarbon chains can be broadly categorized into two – negative and positive functional groups. Carboxylic acids are of the type RCOOH which hydrolyze to give RCOO- and H+. The carboxyl ions get adsorbed at the surface and function as a negative surface gate impeding the motion of electrons and reducing the current. Alternately, using amines of the form RNH2 results in an increase in the current as amine groups tend to bond with a proton to form positively charged RNH3+, which acts as a positive surface gate to the n-type ZnO NWs. Factors such as functional group coverage of the surface and reversibility of functionalization are being investigated.
11:30 AM - H1.6
ZnO UV Photodetector with Tunnable Quality Factor.
Leonardo Campos 1 , Marcos Guimaraes 1 , Alem-Mar Goncalves 1 , Rodrigo Lacerda 1
1 Physics, UFMG, Belo Horizonte, Minas Gerais, Brazil
Show AbstractZinc oxide has proven to be an interesting material with properties like piezoelectricity and high optical and luminescent activity. It is well known that Zinc oxide has a great potential to be used as an ultra-violet photodetector with a band gap of 3.37eV. In this work we present a photodetector made by an isolated nanowire contacted by Cr/Au forming a field effect transistor. By changing the gate voltage we can modulate the effective photocurrent and control the quality factor of the photodetector. Our results show that we can increase the quality of the phodetector from low quality to high quality allowing thedetection of a 372nm wavelength with precision better than 2 nm (Q > 180). The ZnO-NWs were grown by a Zn foil thermal oxidation method. The field effect transistors were constructed using optical lithography techniques in a n-Si/SiO2 (300 nm thick) substrate with Cr (10 nm) and Au (100 nm) metals contacts. The transistors were built in back gate architecture and have shown a n-type behavior with an on/off gain of about 103. Low temperature transport measurements were performed to probe the activation energy for different gate voltages. Based on these results, we will present a model to explain the modulation that allows photocurrent control of the transistor. We believe that this new behavior can have an important impact on the applications for ZnO and other types of nanowire transistors as photodetectors.
11:45 AM - H1.7
Visible Blind UV Photocathode Based on MgZnO Thin Film Alloys and Nanostructures for Space Applications.
Ratnakar Vispute 1 , D. Pugel 2 , B. Woodgate 2 , G. Hilton 2 , T. Norton 2
1 , Blue Wave Semiconductors, Columbia, Maryland, United States, 2 , NASA, Greenbelt, Maryland, United States
Show AbstractVisible blind ultraviolet detector technologies especially for the spectral range between 90 and 300 nm are in demand for space application. This is due to the fact that this spectral range is accessible only from space. This UV region mainly contains the highest known density of spectral information for planets, stars, interstellar and intergalactic gas, and galaxies of any electromagnetic band. To study UV signatures coming from space, future UV missions will require improvements in visible bind detector sensitivity. Wide band gap semiconductor materials are ideal for this application. We are developing a novel and wide band gap Zn1-xMgxO (x=0 to 1) system that is capable of tuning a band gap from 3.3 eV to 7.9 eV for visible blind UV photocathode application. The primary reason for selecting MgZnO system is that the photocathode thin film and nanostructure based materials can be developed on variety of substrates. In this presentation, we will report a wider selection of MgZnO material compositions by stabilizing metastable phases in thin films and nanostructures to tune desired UV cut-off for detector application. We will also discuss selective area growth of ZnO and ZnMgO nanowires using patterned diamond-like-carbon (DLC) nucleation layers for fabrication of detector. Our results indicate that the selective growth and its control for the fabrication of nanowires could be useful for development of new and efficient photocathode materials. We will also highlight tailored ZnMgO thin film heterostructures and nanostructures (wires, tips, rods, fibers) with amorphous carbon (a-C), amorphous carbon nitride (a-CNx), and Cs to develop an efficient and visible blind UV photocathode material. Composition tuning and doping in nanostructures and their implications on visible blind UV photocathode advancement will be discussed.
12:00 PM - H1.8
GaN/ZnO and AlGaN/ZnO Heterostructure LEDs: Growth, Fabrication, Optical and Electrical Characterization.
Julian Benz 1 , Sebastian Eisermann 1 , Peter Klar 1 , Bruno Meyer 1 , Theeradetch Detchprohm 2 , Christian Wetzel 2
1 I. Physics Institute, Justus-Liebig University of Giessen, Giessen Germany, 2 Department of Physics, Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractThe wide band gap polar semiconductors GaN and ZnO and their related alloys offer fascinating properties in terms of band gap engineering, carrier confinement, internal polarisation fields and surface terminations. With a small lattice mismatch of -1.8 % between GaN and ZnO and the possibility to adjust the lattice constant of MgZnO to GaN, the system AlGaN/MgZnO offers a unique possibility to study quantum heterostructures with hetero- and isovalent interfaces with a great flexibility to tune band offsets, polarisation fields, surface termination, to vary strain and composition and to realize and explore a new class of opto-electronic devices. We report on the growth of undoped ZnO films on MOCVD grown p-type GaN and AlGaN on sapphire templates by chemical vapour deposition and on the fabrication of corresponding light-emitting diode (LED) structures. Electrical and optical properties of the n ZnO/p-GaN and n-ZnO/p-AlGaN LEDs will be compared and the observed differences will be discussed in terms of the band alignment at the hetero interface.
12:15 PM - H1.9
Blue to Cyan Electroluminescence from ZnO based Heterojunction Diodes with CdZnO Active Layers.
Lin Li 1 , Zheng Yang 1 , Jieying Kong 1 , Zheng Zuo 1 , Jianlin Liu 1
1 Electrical Engineering, University of California, Riverside, Riverside, California, United States
Show Abstractp-ZnO/CdZnO/n-ZnO and n-ZnO/CdZnO are grown on n-type and p-type Si substrates by plasma-assisted molecular-beam epitaxy (MBE) respectively. Heterojunction diodes are fabricated by standard photolithography techniques. Rectified I-V curves show typical diode characteristics in both samples. Blue/cyan electroluminescence (EL) emissions are observed at around 460nm and 480nm at room temperature from both structures respectively. Temperature dependent EL measurements from 9K to 300K show peak positions red shift due to heat effect. Room temperature photoluminescence (PL) measurements verify that the peak positions of blue EL emissions correspond to the PL peak positions from CdZnO active layers. Secondary ion mass spectroscopy (SIMS) measurements also confirm the incorporation of Cd in ZnO in active layer. Blue to cyan emitting LED devices with ZnO as p-type layer are demonstrated for the first time.
12:30 PM - H1.10
Light-Emitting Devices Based on ZnO-Nanowire Arrays Coated with P-Conductive Polymers.
Abdelhamid Elshaer 1 , Apurba Dev 1 , Jan-Peter Richters 1 , Julia Waltermann 3 , Siegfried Waldvogel 2 , Wolfgang Schade 3 , Tobias Voss 1
1 Institute of Solid State Physics, University of Bremen, Bremen Germany, 3 Institute for Energy Research and Physical Technologies, Clausthal University of Technology, Clausthal-Zellerfeld Germany, 2 Kekule-Institute for Organic Chemistry and Biochemistry, University of Bonn, Bonn Germany
Show AbstractDue to their large surface-to-volume ratio and high crystalline quality, ZnO nanowires are promising candidates for applications in nanoscale sensing technology and optoelectronics in the blue-UV spectral region. Especially low-temperature grown ZnO nanowires offer interesting properties for the large-scale and low-cost production of environmentally friendly solar cells and light-emitting diodes [1]. When a p-conductive polymer is used to form a hybrid pn-junction, solar cells with efficiencies of a couple of percent have been reported, and UV emission from such a heterojunction has also been observed.We have employed a wet-chemical synthesis method to fabricate ZnO nanowire arrays on conductive glass substrates with very good homogeneity for areas as large as 5 × 5 cm2. The typical dimensions of the nanowires are about 100 nm in diameter and 2 μm in length. TEM measurements prove the single-crystalline structure of the wires. Photoluminescence measurements (performed at temperatures between T = 4 and 300K) show clear excitonic emission lines which indicate an overall good optical quality, combined with weak visible deep-level luminescence. We fabricated ZnO nanowire/PEDOT:PSS compound structures that act as light emitting diodes. For these purposes, the PEDOT formulation was modified. This includes the addition of conductivity enhancers based on dimethylsulfoxide or ionic liquids. The polymer/additive mixture was applied as predominantly aqueous solution by an air brush technique.We studied the electroluminescence and the IV-characteristics of the devices for different polymers and processing parameters and found rectification factors of up to 105 and currents of up to 50 mA/mm2. Especially, the annealing time and layer thickness of the polymer layer were found to need precise adjustment to ensure optimized device characteristics. We found that an additional insulating polymer layer (polystyrene) coated directly onto the nanowires helps to improve the device characteristics. This is in agreement with our previous investigations of n-ZnO nanowire/p-silicon light emitting diodes where an insulating silicon dioxide layer between the n-ZnO and the p-silicon is needed to allow for tunnel injection of holes into the ZnO valence band [2]. We will discuss the relevant mechanisms of the electron transfer between the ZnO nanowire and the p-polymer that allows us to optimize the hybrid LEDs and solar cells.[1]R. Tena-Zaera, J. Elias, C. Levy-Clement, C. Bekeny, T. Voss, I. Mora-Sero, and J. Bisquert, J. Physical Chemistry C 112, 16318 (2008).[2]M. A. Zimmler, D. Stichtenoth, C. Ronning, W. Yi, V. Narayanamurti, T. Voss, and F. Capasso, Nano Letters 8, 1695 (2008).
12:45 PM - H1.11
p-n Homojunction Zinc Oxide Nanowires Based Light Emitting Device.
Philippe Gilet 1 , Anne -Laure Bavencove 1 , Pierre Ferret 1 , Jonathan Garcia 1 , Matthieu Lafossas 1 , Francois Levy 1 , Patrice Noel 1 , Pascal Marotel 1 , Nicolas Olivier 1 , Emilie Pougeoise 1 , Eddy Romain-Lattu 1 , Robin Thierry 1 , Guy Feuillet 1 , Daniel Le Si Dang 2
1 DOPT, CEA LETI Minatec, Grenoble France, 2 Institut Néel, CNRS, Grenoble France
Show AbstractIn this communication, we report on the electroluminescence from p-n homojunctions in vertically aligned ZnO nanowires (NWs). The samples were grown by catalyst free MOCVD on sapphire substrates. As or P were used as p-type dopants whereas no specific dopants are used for n-type doping . The nanowires have been embedded and contacted by n- and p-type electrodes (Ti/Au and Ni/Au respectively) through a technological integration process.Room temperature I-V characteristic of the devices exhibits diode like behaviour which unambiguously confirms the achievement of p-type doping in the nanowires. Light emission is clearly visible for low voltage bias (i.e. below 10V) and has been recorded with an UV CCD camera. The electroluminescence spectra clearly indicate well resolved room temperature band edge emission at 385nm. Voltage threshold around 3V and serial resistance of 31ohms have been measured for a 350µmx350µm device. Current densities as high as 120A/cm-2 can be injected without any degradation of the devices. Since first observations (within one month), no ageing effect has been noticed. One band is present at lower energy (around 2eV) but with lower intensity (three times) and is attributed to the defect band of the material. Local optical spectroscopy measurements (Photoluminescence and Cathodoluminescence) will be compared to macroscopic electroluminescence spectra of integrated NWs in order to discuss about the specific signature of p-type doping. Scanning Probe Microscopy (SPM) characterizations were performed on reference samples containing p type dopants in the Scanning Capacitance Microscopy (SCM) mode. These measurements reveal p-type space charge regions and add one more evidence of p-type doping achievement. Finally, we shall dwell upon the local transport measurements carried out on single nanowire FET devices in order to measure the p-type doping concentration in the nanowires and precisely localize the p-n junction.
H2: Doping
Session Chairs
Leonard Brillson
Yufeng Dong
Monday PM, November 30, 2009
Ballroom C (Hynes)
2:30 PM - **H2.1
Revealing Local Structures around Dopants in ZnO by X-ray Absorption Spectroscopy.
Sukit Limpijumnong 1 2
1 School of Physics, Suranaree University of Technology, Nakhon Ratchasima Thailand, 2 , Synchrotron Light Research Institute, Nakhon Ratchasima Thailand
Show AbstractThe role of a given dopant depends strongly on the site it assumed in a crystal. Therefore, various techniques have been employed to probe the local structures around dopants. Synchrotron x-ray absorption spectroscopy (XAS) is a very powerful technique to selectively study local structures, especially when used in conjunction with electronic state calculations. The main advantage of the technique is that it can selectively excite only the core electrons of the dopant. By using the x-ray energy that matches the core electron energy a specific element (the dopant), only the core electrons of that element are excited into the allowed empty states. The shape of the allowed density of states depends on the arrangement of the surrounding atoms. I will show our recent studies of aluminum-doped and arsenic-doped ZnO. For Al-doped ZnO, the comparison between the XAS measurements and calculations reveals that majority of Al substitutes for Zn in low Al-content samples. However, for high Al-content samples, the Al starts to form Al2O3-like structures. This is consistent with the observed reduction in conductivity for samples with high Al-contents. The detailed interpretation of the XAS will be discussed. For As-doped ZnO, several As configuration models are used for XAS simulations and compared with available measurements. The results indicate that it is unlikely that As will replace O in As-doped ZnO.
3:00 PM - H2.2
On the Lattice Position of Li in ZnO.
Filip Tuomisto 1 , Asier Zubiaga 1 , Katja Kuitunen 1 , Pekka Neuvonen 2 , Andrej Kuznetsov 2 , Bengt Svensson 2
1 , Helsinki University of Technology, TKK Espoo Finland, 2 Department of Physics, Centre for Material Science and Nanotechnology, Oslo University, Blindern Norway
Show AbstractBulk ZnO crystals grown by the hydrothermal (HT) method typically contain a fairly high concentration of Li (~1017 cm-3) that makes this substrate material highly resistive and unstable. It is commonly accepted that Li is amphoteric (resulting in efficient self-compensation), and has donor character in the interstitial position, while substitutional Li has acceptor character. Positron annihilation spectroscopy is a method that is particularly well suited for detecting vacancy defects in their neutral and negative charge states and also other point defects (e.g., impurities) in their negative charge states. Interestingly, as-grown HT ZnO always exhibits a slightly higher positron lifetime compared to high-purity vapor-phase or melt grown ZnO, indicating the presence of vacancy-type defects, and the temperature dependence of the positron parameters shows that these defects are neutral. In addition, the positron data indicate that these defects need to have significantly smaller open volume than Zn vacancies. No signs of negatively charged defects have been detected in as-grown HT ZnO [1].Recent theoretical calculations [2] show that substitutional Li in the negative charge state is located very close to the ideal lattice position, i.e., the changes in the bond lenghts to the neighbouring O atoms are of the order of 1 % compared to the defect-free lattice case. On the other hand, substitutional Li in the neutral state relaxes very strongly away from one of the O atoms, creating a noticeable open volume. Indeed, theoretical calculations of positron states show that positrons will localize in this open volume (that can be essentially thought of as a Zn vacancy – Li interstitial pair), increasing the positron lifetime by about 10 ps compared to the defect-free lattice. This difference is in excellent agreement with the experimental lifetime differences between HT and vapor-phase grown ZnO. In addition, the removal of Li from HT ZnO by a high-temperature annealing – polishing procedure reduces the positron lifetime down to that of vapor-phase grown ZnO. We conclude that a significant fraction of Li is in the neutral charge state in the substitutional site in HT ZnO, while no negatively charged (isolated) Li is observed. Hence, if Li is the cause of self-compensation, the negatively charged Li must be spatially correlated with the donors it compensates (producing neutral complexes) and as such invisible to positrons.[1] F. Tuomisto and D. C. Look, Proc. SPIE 6474, 647413 (2007).[2] S. Lany and A. Zunger, arXiv:0905.0018v1 [cond-mat.mtrl-sci]
3:15 PM - H2.3
Divacancy-Hydrogen Complexes in Zinc Oxide.
Jan Kuriplach 1 , Gerhard Brauer 2 , Oksana Melikhova 1 , Jakub Cizek 1 , Ivan Prochazka 1 , Wolfgang Anwand 3
1 Low Temperature Physics, Charles University, Prague Czechia, 2 Institut für Ionenstrahlphysik und Materialforschung, Forschungszentrum Dresden-Rossendorf, Dresden Germany, 3 Institut für Strahlenphysik, Forschungszentrum Dresden-Rossendorf, Dresden Germany
Show AbstractThe nature of point defects in zinc oxide is not yet fully understood, but this subject attracts wide attention as it is of vital importance for various technological applications. An important class of ZnO point defects is represented by hydrogen-related defects that are also often considered in relation to n-type character of ZnO materials [1]. Recently, it has been found that commercially available ZnO single crystals contain an appreciable amount of hydrogen at a level of at least 0.3 at.% [2]. The forms in which such hydrogen atoms are incorporated into the ZnO lattice are not precisely known at present, and there are several possibilities, in principle. In particular, hydrogen may occupy interstitial positions and/or may be present in the form of H2 molecules in 'channels' along the c-axis of the hexagonal ZnO lattice. Hydrogen may also form complexes with other impurities though their concentrations are usually much smaller than that of H, as found in [2]. Furthermore, hydrogen atoms can enter open volume defects, like oxygen and zinc vacancies and related defects. Positron annihilation techniques are sensitive to such kind of defects and, as shown in [2], positron trapping in ZnO occurs in Zn-vacancy–hydrogen related complexes. Namely, a suggestion has been given that positrons may get trapped in a Zn-vacancy occupied by one hydrogen atom. In the present work, we extend our study to Zn+O divacancies filled with varying amount of hydrogen atoms. Besides the structure and energy related properties of such defects, we also investigate their capability to trap positrons taking into account positron induced forces acting on the ions surrounding studied defects. We show that the Zn+O divacancy may trap positrons when up to two hydrogen atoms are located inside the divacancy, which provides an additional explanation for saturated positron trapping observed in ZnO single crystals, though further experimental evidence is needed to prove this hypothesis. [1] C. G. Van de Walle, Phys. Rev. Lett. 85, 1012 (2000). [2] G. Brauer et al., Phys. Rev. B 79, 115212 (2009).
3:30 PM - H2.4
Properties of Nitrogen Molecules in Zinc Oxide.
Norbert Nickel 1 , Marc Andy Gluba 1
1 , Helmholtz-Zentrum Berlin für Materialien und Energie, Berlin Germany
Show Abstract Nitrogen plays an important role in the field of semiconductor physics. During growth it can be readily incorporated to achieve alloying or doping. For ZnO, which is a very promising material for blue and UV light emitting diodes and lasers, the use of nitrogen has been suggested to achieve p-type doping. Unfortunately, the doping efficiency is rather low. To attain a sufficient hole concentration the nitrogen content has to exceed the hole concentration by 4 orders of magnitude and a vacuum anneal at high temperatures is required to activate the acceptors and to remove residual hydrogen from the sample [1]. This extremely low doping efficiency can have its origin in a number of effects. First, ZnO displays a native n-type behavior and thus, the responsible donors have to be compensated by nitrogen acceptors. Second, infrared and Raman measurements have shown that the presence of nitrogen in ZnO gives rise to the formation of N-H, N-O, and C≡N complexes [2], in addition to the formation of N2 molecules observed by electron spin resonance measurements [3]. The presence of these complexes will effectively lower the doping efficiency. Using density-functional theory, we show that nitrogen molecules are capable of causing the formation of localized states in the band gap of ZnO, thereby contributing to the low doping efficiency in an hitherto unexpected way. It is found that N2 causes localized states in the band gap either by forming an N2O molecule or by breaking a Zn-O bond. ZnO crystallizes in the wurtzite structure and thus N2 can be accommodated parallel to the c-axis or the basal plane with its unique axis. For the former case the equilibrium position of N2 is located in the center of a hexagon and one N atom resides in the same plane as the surrounding oxygen atoms When the N2 molecule is placed parallel to the basal plane two stable configurations are found. In the first configuration the nitrogen molecule forms nitrous oxide (N2O) by breaking a Zn-O bond. For the second, energetically more favorable site, the nitrogen molecule breaks a Zn-O bond thereby pushing the O and Zn atoms towards the interstitial sites. Although the N2 molecule does not form a chemical bond with the ZnO lattice a localized state appear in the band gap at about 170 meV above the valence band. This state is attributed to an oxygen p-orbital. Our results are discussed in the light of p-type doping of ZnO. [1] A. Tsukazaki, et al., Nature Materials 4, 42-46 (2005). [2] N. H. Nickel, F. Friedrich, J. F. Rommeluère, and P. Galtier, Appl. Phys. Lett. 87, 211905 (2005). [3] N. Y. Garces, L. Wang, N. C. Giles, L. E. Halliburton, G. Cantwell, and D. B. Eason, J. Appl. Phys. 94, 519-524 (2003).
3:45 PM - H2.5
Diffusion of Na and Li in Hydrothermally Grown ZnO.
Pekka Neuvonen 1 , Lasse Vines 1 , Anders Hallen 2 , Bengt Svensson 1 , Andrej Kuznetsov 1
1 Department of Physics, Centre of Material Science and Nanotechnology, University of Oslo, Oslo Norway, 2 School of ICT, Dept. of Microelectronics and Applied Physics, Royal Institute of Technology, Kista Sweden
Show AbstractZnO is a direct band gap semiconductor which has received increasing amount of attention because of its superior optical properties compared to GaN and other wide band gap materials. However, realizing p-type conductivity has proven to be difficult. This difficulty has been suggested to arise from the compensating effect of intrinsic defects, such as oxygen vacancies and zinc interstitials or impurity elements, such as lithium and hydrogen.To study the effects of group-I elements in ZnO, we have performed a series of sodium implantations into hydrothermally (HT) grown ZnO using either as-received samples with high Li concentration (~3×1017 cm-3) or pre-treated ones with low Li concentration (~3×1015 cm-3), followed by annealings at temperatures ranging from 450 °C to 950 °C. The reduced Li concentration has been accomplished by heat treatment at 1500 °C, which also decreases Zn-vacancy concentration, as shown by positron annihilation spectroscopy. After the implantation and annealing steps, the samples were measured with secondary ion mass spectrometry (SIMS). A strong interaction between sodium and lithium is observed in the samples containing high concentration of Li after annealing at 500 °C, where Li disappears from the Na rich region; Up to two orders of magnitude reduction in the Li concentration is detected. A shoulder on the Na concentration profile penetrating into the bulk starts to appear at temperatures around 550 °C, indicating trap limited diffusion. In addition, the sum of the Li and Na concentrations stays essentially constant in the bulk region and this anti-correlation between Na and Li persists up to annealing temperatures where the implantation peak of Na is depleted. This indicates strongly that Li and Na compete for the same trapping site, X, and where the X–Na complex has a lower formation energy (higher thermal stability) than that of X–Li. The concentration of the trap limited diffusion tail in the Na profile is 3×1017 – 5×1017 cm-3. In the samples having a low Li concentration, diffusion of Na is not observed at concentrations above ~1×1016 cm-3, except for a loss out of the samples. In the low Li samples, Li is observed to redistribute itself more according to implantation damage profile. In addition, scanning spreading resistance microscopy (SSRM) shows high resistivity in both, the Na and Li-rich regions.From these experiments, the diffusivity of Na is being extracted. Moreover, the studies may support Li-VZn -complex to be a compensating defect in n-type HT ZnO, and the concentration of this complex can be lowered with pre-treatment or by introducing Na, which is then combining with the Zn-vacancy, forcing Li to an interstitial site and to diffuse out of the Na-rich region.
4:30 PM - **H2.6
Novel Mobility Analysis of Highly Conducting ZnO Thin Films.
David Look 1 2 , Kevin Leedy 3 , David Tomich 2 , Burhan Bayraktaroglu 3
1 , Wright State University, Dayton, Ohio, United States, 2 Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Dayton, Ohio, United States, 3 Sensors Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Dayton, Ohio, United States
Show AbstractHighly conducting ZnO thin films are useful in many applications, including window defrosters and transparent contacts for displays and solar cells. A key figure of merit for such films is the resistivity ρ, which fundamentally depends on both mobility µ and carrier concentration n. Even more fundamentally, µ and n depend on donor (ND) and acceptor (NA) concentrations, which must be determined for further materials development. To illustrate this point, if n = 1 x 1021 cm-3 and NA/ND = 0.5, both obtainable in ZnO, then ρ = 2.2 x 10-4 Ω-cm, a competitive value; however, if NA can be reduced such that NA/ND ≈ 0, then ρ = 7.5 x 10-5 Ω-cm, a superb value. The determination of ND and NA requires an analysis of mobility µ, which often depends on film thickness d, especially if the substrate is not lattice-matched to the film. Here we present a new mobility analysis that makes possible the accurate determination of both ND and NA, for arbitrary d: [µmeas(d)]-1 = [µB-H(NA,nmeas)]-1 + hbar(3π2nmeas)1/3/(eCd), where C is a constant, nmeas = ND - NA, and µB-H is the degenerate form of the well-known Brooks-Herring formula for ionized-impurity scattering. From our experience with samples grown by pulsed laser deposition (PLD), the measured value of n is not a strong function of d, and the same is true of ND and NA; thus, µB-H(NA,nmeas)is a constant and can be determined, along with C, by fitting a plot of µmeas(d) vs d. Ga-doped ZnO samples spanning a thickness range of 3 to 130 nm were grown by PLD using a 99.99%-pure ZnO target containing 3 wt% Ga2O3. The substrate was Si, coated with a 1-µm-thick layer of SiO2, for electrical isolation, and the substrate temperature during growth was 400 °C. Hall-effect measurements yielded an average carrier concentration nmeas of about 3 x 1020 cm-3 with mobilities ranging from about 1 to 35 cm2/V-s, depending on thickness. A good fit to the data set µmeas(d) vs d was obtained for µB-H(NA,nmeas) = 43 cm2/V-s and C = 0.24. Then, from the standard formula for µB-H (not shown) we get NA = 2.1 x 1020 cm-3, and thence, from the value of nmeas, ND = 5.1 x 1020 cm-3. These values of ND and NA immediately yield two important results: (1) the percentage of Ga atoms in the target entering the ZnO lattice as donors is about 45%; and (2) the compensation ratio NA/ND is about 0.4. Both of these quantities would be very difficult to obtain by any other method. Further, if NA can be reduced, such that NA << ND, then we can use our formula to determine the potential improvements in µ and ρ: µ = 93 cm2/V-s and ρ = 1.3 x 10-4 Ω-cm, both excellent values. It is almost certain that the acceptors in our samples are defect-related, because there are no acceptor-type impurities with concentrations as high as 2 x 1020 cm-3. These defects are probably Zn vacancies, which have a low formation energy in n-type ZnO. Finally, the value of C may be related to grain size, and efforts are underway to examine grain size as a function of thickness.
5:00 PM - H2.7
Optimisation of the Post-Implantation Annealing Conditions for Doping ZnO p-Type.
Guy Feuillet 1 , Pascal Marotel 1 , Patrice Gergaud 1 , Guillaume Perillat-Merceroz 1 , Isabelle Bisotto 1 , Carole Granier 1 , Fabienne Ponthenier 1 , Philippe Gilet 1 , Francois Levy 1
1 LETI, CEA, Grenoble France
Show AbstractIon implantation is a possible way to dope ZnO p type. For efficient doping, ion implantation requires that the implantation induced structural defects be removed and that the dopants be activated. This is usually done by thermal annealing treatments. The temperatures needed to reach each of these two goals may be different, depending on the implanted species. We have undertaken combined structural, optical and electrical studies to try and find the optimum annealing temperatures. In a first step, argon ions were implanted in ZnO substrates in order to induce a high density of implantation defects. Subsequently the substrates were annealed in-situ in an X-ray diffractometer to follow the evolution of the lattice parameter distribution as a function of the temperature and to gain a better understanding of the structural recovery kinetics. Transmission electron microscopy was also used to assess the structure of the extended defects created upon ion implantation – mainly basal dislocation loops- and their evolution with annealing temperature. In a second step, ZnO substrates were implanted with different species with a view to obtain p type doped layers. As in the case of Ar, annealing was carried out in situ in the X- ray diffractometer and the structure was found to recover its cristallinity but at lower temperatures. Power and temperature dependant photoluminescence measurements indicated emissions from Donor-Acceptor Pair transitions (DAP) and from the related band-level (e-A0) transition, for intermediate annealing temperatures, in clear correlation with the X-ray data. These results constitute an indication of implantation induced p type doping in ZnO, and they will be presented together with recent Hall measurements and I-V characteristics.
5:15 PM - H2.8
P-type Nitrogen Doped ZnO Films Grown By Thermal Evaporation.
Wei Mu 1 , Lei Kerr 1 , David Look 2 , Tim Cooper 2
1 Dept. of Paper and Chemical Engineering, Miami University, Oxford, Ohio, United States, 2 Semiconductor Research Center, Wright State University, Dayton, Ohio, United States
Show AbstractWe report the effect of nitrogen dopant precursors on the electrical and optical properties of nitrogen doped ZnO (ZnO:N) grown by thermal evaporation deposition. Two different N dopant precursors, pure NO and 5% NO/N2 mixture are used to investigate the controversial issue of single atom N and diatom N2 dopant effect on film conductivity. This study showed that it is not simple to say that single atom N dopant gas is a more effective p-type dopant. The dopant gas effectiveness depends on the process. Formation of Zn2N3 plays critical role in obtaining p-type conductivity. The photoluminescence (PL) data has demonstrated the correlation between the p-type conductivity and the presence of large red band.
5:30 PM - H2.9
Nitrogen Doping of Homoepitaxial c- and a-plane ZnO.
Sebastian Eisermann 1 , Stefan Lautenschlaeger 1 , Bruno Meyer 1 , Markus Wagner 2 , Axel Hoffmann 2
1 1. Physics Institute, Justus-Liebig-University Giessen, Giessen Germany, 2 Institute of Solid State Physics, Technical University Berlin, Berlin Germany
Show AbstractThe impurity incorporation in polar and non-polar ZnO films is of prime importance for the understanding of controlled doping. We used chemical vapour deposition to grow ZnO films on ZnO substrates on polar (Zn- and O-termination) and non-polar surfaces. As nitrogen is expected to generate shallow acceptor levels in ZnO, doping experiments have been carried out using NH3 as a precursor for nitrogen doping. The impurity incorporation into the thin films has been analyzed by secondary ion mass spectrometry and Raman measurements. Optical features of doped and the undoped layers have been investigated using low temperature and temperature dependent photoluminescence measurements. Luminescence excitation experiments provide insight into the radiative recombination of donors and acceptors.
5:45 PM - H2.10
Properties of MgxZn1-xO:P Thin Films Grown on (000-1) ZnO.
Holger von Wenckstern 1 , Matthias Brandt 1 , Michael Bonholzer 1 , Martin Lange 1 , Robert Heinhold 1 , Kertin Brachwitz 1 , Florian Schmidt 1 , Michael Lorenz 1 , Marius Grundmann 1
1 Experimentelle Physik II, Universität Leipzig, Leipzig Germany
Show AbstractZnO and its ternary compounds are promising candidates for UV optoelectronics. However, the production of high quality p-type material is a necessary issue. Recently, it has been shown that p-type material can be achieved by phosphorous doping, however, the crystalline quality of the p-type sample is low compared to undoped material [1]. On the other hand, it has been shown, that homoepitaxial growth of ZnO:P thin films leads to material of very high quality, however a conversion towards p-type was not observed [2].It has been found that Mg0.1Zn0.9O:P can produce p-type conduction, however, free hole concentrations are low [3]. Within the present study, homoepitaxial MgxZn1-xO:P thin films with variable magnesium content (x < 0.15) have been grown by pulsed-laser deposition. All samples were investigated by high-resolution X-ray diffraction and low temperature photoluminescence. Hall effect measurements show electron mobilities of the as-grown films as high as 820 cm2/Vs at 55 K exceeding values observed for ZnO:P homoepitaxial thin films [4]. From the free electron concentration a critical donor concentration of Nc =(8.3±1)×1018 cm-3 has been derived and a donor activation energy of ED0 =56±3 meV in the dilute limit has been found for the dominant donor, consistent with an effective mass type donor.We used reactively sputtered Pt Schottky contacts to determine the depth dependence of the net doping concentration from capacitance-voltage measurements. The data suggests that an accumulation of impurities stemming from the ZnO substrates occurs at the thin film/substrate interface. For selected samples, thermal admittance spectroscopy was used to determine the thermal activation energy and the capture crosssection of donor-like defect states. A shallow defect with thermal activation energy of about 35 meV was detected in all samples and is tentatively attributed to interstitial zinc.Upon annealing the free carrier concentration was reduced by several orders of magnitude, depending on the oxygen partial pressure during growth in MgZnO:P films, while it remained nearly unchanged in ZnO:P samples. The role of the Mg concentration in the films in acceptor defect formation is discussed in detail.[1] A. Allenic et al., Adv. Mater. 19, 3333 (2007).[2] M. Brandt et al., J. Appl. Phys. 104, 013708 (2008).[3] Y. W. Heo et al., Appl. Phys. Lett. 84, 3474 (2004).[4] M. Brandt et al., J. Vac. Science. Technol. B 27, 1604 (2009).
Symposium Organizers
Juergen Christen Otto-von-Guericke-Universität Magdeburg
Leonard J. Brillson Ohio State University
Hiroshi Fujioka University of Tokyo
H. Hoe Tan The Australian National University
H3: Electronic and Optical Properties
Session Chairs
Zhanghai Chen
Marius Grundmann
Tuesday AM, December 01, 2009
Ballroom C (Hynes)
9:30 AM - **H3.1
Hard-X-ray Photoelectron Spectroscopy on Zinc Oxide and its Related Alloys.
Naoki Ohashi 1 , Yutakai Adachi 1 , Isao Sakaguchi 1 , Kenji Matsumoto 1 , Shigenori Ueda 1 , Hideki Yoshikawa 1 , Yoshiki Wada 1 , Yoshiyuki Yamashita 1 , Keiske Kobayashi 1 , Hajime Haneda 1
1 , National Institute for Materials Science, Tsukuba Japan
Show Abstract The electronic structure of ZnO and its related alloys were investigated by using hard-x-ray photoelectron spectroscopy (HX-PES). The most characteristic feature of HX-PES is that very large escaping depth of photoelectrons, which enables us to investigate more bulk-sensitive analysis than conventional PES using soft-x-ray. With HX-PES technique, we investigated the electronic structure of doped ZnO and (Zn,Mg)O alloys in terms of charge compensation and crystalline polarity. As reported in our previous papers,1-4) it was clear that Fermi level (EF) in ZnO is pined to be close to the bottom of conduction band, even if the sample shows semi-insulating behavior with less electron concentration, e.g., 1014 cm-3 at room temperature. Moreover, it was evidently seen that EF in (Zn,Mg)O alloy is close to the bottom of conduction band. Note that the location of EF was unchanged with the crystalline orientation of the observed surface, e.g., (0001) or (000-1) face. These results were strange for us, if we assume that band bending due to surface state or permanent polarization occurred. Thus, we investigated angle resolved HX-PES for systematic analyses of the surface band bending behavior. As a result, we realized that the band bending did not occur at "ordinary" ZnO surface. Here, "ordinary" means that ZnO close to the thermal equilibrium state. On the other hand, band bending behavior was obviously seen in the heavily (>> 1019 cm-3) electron doped ZnO and it was indicated that an electron accumulation layer exists at the surface of heavily doped ZnO. Note that solubility limit of cationic donors, Al, Ga, and In, is about 1-2 × 1019 cm-3 under thermal equilibrium conditions5,6). Thus, we can conclude that the electron accumulation layer exists when the samples were under non-equilibrium state having too much dopant in comparison to the solubility limit. [References] 1 T. Ohsawa et al., APL 92 (2008) 232108.2 T. Ohsawa et al., Chem. Mater. 21 (2009) 144.3 T. Ohsawa et al., APL 94 (2009) 042104.4 N. Ohashi et al., APL 94 (2009) 042104.5 T. Nakagawa et al., JJAP 47 (2008) 7848.6 H. Ryoken et al., JMR 20 (2005) 2866.
10:00 AM - H3.2
Band Gap Engineering of ZnO and Related Materials through Uniaxial Strain.
Satyesh Yadav 1 , Rampi Ramprasad 1
1 Chemical, Materials, and Biomolecular Engineering, Institute of Materials Science, University of Connecticut, Storrs, Connecticut, United States
Show AbstractAlthough ZnO and related II-VI semiconductors such as ZnS, ZnSe and ZnTe lend themselves to facile synthesis in thin film or nanostructured form, their large band gap (> 3.5 eV) prevents their application in optical devices, such as solar cells and LEDs. In this ab initio work, we show that ZnX (X = O, S, Se, Te) in the wurtzite crystal structure exhibits the following universal behavior: under uniaxial compressive strain along the wurtzite c-axis, the band gap of the system initially increases, goes through a maximum, and then decreases abruptly, allowing for a wide range of possible uniaxial strain-induced band gaps. Our ab initio calculations were based on density functional theory (DFT). Uniform uniaxial strain in bulk wurtzite ZnX was achieved by systematically varying the c-axis lattice parameter, and optimizing the a-axis lattice parameter and the atomic coordinates. Since ZnX is a covalent sp3 -hybridized systems displaying tetrahedral bonding, the band gap increases with small amounts of compressive strain, owing to increased splitting of the “bonding” valence band energies from the “anti-bonding” conduction band energies. We however find that for larger uniaxial compressive strains (> 4%), these systems display the onset of a phase transformation to a graphite-like phase: each Zn atom abruptly becomes coplanar with the 3 X atoms below in one layer; likewise, in the adjacent layer, each X atom becomes coplanar with the underlying Zn atoms. This sp3 to sp2 phase transformation is accompanied by a drastic drop in the band gap values. Although DFT has the well-known deficiency of underestimating band gaps, trends in changes in the band gaps, and structural distortions due to strain are predicted accurately. We thus believe that this new discovery may lead to important applications.
10:15 AM - H3.3
In Situ Surface Photovoltage Spectroscopy of ZnO Processed by Remote Plasmas.
Raul Peters 1 , Stephen Glancy 2 , Antonio Paramo 1 , Yuri Strzhemechny 1
1 Department of Physics and Astronomy, Texas Christian University, Fort Worth, Texas, United States, 2 Department of Engineering Science, Trinity University, San Antonio, Texas, United States
Show AbstractIn many instances the quality of the ZnO surface and the near-surface region is a key performance-defining parameter. By the same token, the surface itself could be a very significant source of lattice defects as well as contaminating impurities, and this influence may extend into the sub-surface vicinity. In our work, key element of the surface analysis is the surface photovoltage (SPV) spectroscopy known for its notable advantages, which include: identification of conduction vs. valence band nature of the defect-related transitions and the defect level positions within the band gap; ability to measure relatively low densities of surface defects as well as their cross sections. Additional information can be deduced from the SPV transient measurements. In our system SPV characterization is run in high vacuum, complemented by in situ remote plasma treatment. This combination of surface-sensitive and surface-specific techniques is uniquely suited for studying surface properties with a very high degree of reliability since in between processing and characterization cycles there is no exposure to common air contaminants. Thus, availability of remote plasma yields a double benefit. On the one hand, it is an efficient tool for surface cleaning, and on the other hand, it allows one to tailor surface properties. We employed different remote plasma treatments of ZnO surfaces (single-crystalline and nano-crystalline) as a tool for elucidating and controlling surface properties. In situ SPV spectra and transient measurements of the as-received and processed samples revealed significant plasma-driven changes not only in the specific defect properties but in the overall electronic and optical surface characteristics. We correlated the effects revealed by SPV with the results from other complementary experimental probes.
10:30 AM - H3.4
Fermi Level Stabilization Energy in Cadmium Oxide.
Derrick Speaks 1 2 , Kin Yu 2 , Samuel Mao 3 4 , Eugene Haller 1 2 , Wladek Walukiewicz 2
1 Materials Science & Engineering, UC Berkeley, Berkeley, California, United States, 2 Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 4 Mechanical Engineering, UC Berkeley, Berkeley, California, United States
Show AbstractRecent experimental evidence suggests that the position of the conduction band edge (CBE) of Cadmium Oxide is located close to the InN CBE with reference to the Fermi Stabilization energy (EFS). Our previous work [1] has demonstrated that due to the low lying CBE, native defects in InN are donor-like, hence energetic particle irradiation can be used to create uniform concentrations of donors in InN. High quality Cadmium Oxide thin films were synthesized via Pulsed Laser Deposition on c-plane sapphire. Films are n-type with a typical electron concentration of ~9.0x1019 - 5x1020 /cm3 and a mobility ranging from 110 to 80 cm2/Vs, respectively. The optical absorption edge of these films varied from 2.26 eV to 2.80 eV. The shift is well described by the Burstein Moss effect. Improvement of mobility to ~ 120 to 130 cm2/Vs was observed after 2 hours of annealing in O2 at atmospheric pressure. X-ray diffraction and ion channeling show that the films grow epitaxially with high density of dislocations for strain release. Irradiation with high energy particles (He+, Ne+) was used to introduce point defects into CdO. Increasing irradiation damage increases the electron concentration until a saturation level of 5x1020 cm-3 is reached. This saturation of the carrier concentration is due to Fermi level pinning at EFS. EFS was found to lie at about 1 eV above the conduction band of CdO, which is very unusual for semiconductors which normally have EFS located inside the band gap. This result explains the propensity of CdO for n-type, and suggests that CdO shares many similarities with InN. We also found that due to the high ionicity of CdO, strong dynamic annealing occurs during irradiation and therefore the saturation level is dependent on the damage dose rate. Thermal annealing of irradiated CdO leads to films with electron mobilities much higher than observed in as-grown materials (180 cm2/Vs). This suggests that the annealing reduces ionized impurity scattering by ordering of the point defects in irradiated CdO, a behavior similar to that found in irradiated InN films.Reference[1] S. X. Li, K. M. Yu, J. Wu, R. E. Jones, W. Walukiewicz, J. W. Ager III, W. Shan, E. E. Haller, H. Lu and W. J. Schaff, Phys. Rev. B 71 161201(R) (2005).
10:45 AM - H3.5
Ultrafast Exciton Dynamics in ZnO: Excitonic Versus Electron-Hole-Plasma Lasing.
Tobias Voss 1 , Tina Shih 2 , Jan-Peter Richters 1 , Juergen Gutowski 1 , Eric Mazur 2
1 Institute of Solid State Physics, University of Bremen, Bremen Germany, 2 School of Engineering and Applied Sciences and Department of Physics, Harvard University, Cambridge, Massachusetts, United States
Show AbstractLarge bandgap semiconductors like ZnO are currently of interest as light emitters, detectors, and lasing media in the blue-UV spectral range. The increased use of ZnO nanostructures has lead to a renewal of the interest in the high-density processes of ZnO, particularly the mechanism responsible for lasing in ZnO after excitation with intense femtosecond laser pulse.With a pump-probe reflectometry technique, we investigated the ultrafast exciton-polariton dynamics of bulk c- and m-plane single-crystalline ZnO wafers after femtosecond laser excitation with excitation densities close to lasing conditions. Using 266-nm femtosecond pump and white-light femtosecond probe pulses, we find that the exciton-polariton resonances are completely bleached directly after excitation and do not recover for several picoseconds. This strong damping of the resonances indicates a significant screening of the Coulomb interaction by the excited free carriers. Even below the typical lasing threshold of ZnO nanostructures, we observe damping of the exciton-polariton resonances for several picoseconds, establishing that the primary mechanism for lasing in ZnO induced by femtosecond laser pumping is emission of an electron-hole plasma. We describe the results with a phenomenological exciton-polariton model and quantitatively extract the time-resolved damping constants for the exciton-polariton resonances at different excitation fluences.
11:30 AM - **H3.6
Optical and Vibrational Properties of High Quality ZnO.
Axel Hoffmann 1 , G. Callsen 1 , M. Wagner 1 , R. Kirste 1 , S. Lautenschlaeger 2 , J. Sann 2 , B. Meyer 2
1 Solid State Physics, Technische Universitaet Berlin, Berlin Germany, 2 Physics Institute, Justus Liebig University Giessen, Giessen Germany
Show AbstractWe report on the impact of substrate surface polarity on the optical properties of high quality homoepitaxial ZnO. Furthermore, the influence of impurities and strain on the optical and structural properties of high quality ZnO is performed by Raman, luminescence and absorption measurements. The pressure and polarization dependent measurements allow the determination of lattice and structural parameters as well as of the symmetries and ordering of the valence bands. The application of uniaxial pressure leads to a shift of the free and bound excitonic recombinations. The size of the shift is compared to the Raman and XRD measurements in order to determine lattice and structural parameters. The variations in the bound exciton spectra are discussed regarding the presence of different chemical impurities. The symmetry ordering of the crystal field and spin-orbit split valence bands are determined by polarization and pressure dependent luminescence and transmission spectra. Finally, we report on the incorporation of deep defects such as Cu and Fe by electro-paramagnetic resonance spectroscopy and Fourier transform infrared spectroscopy (FTIR). Based on this comprehensive investigation of ZnO substrates we give suggestions for the choice of ZnO substrates in order to achieve high quality ZnO homoepitaxial layers and devices.
12:00 PM - H3.7
The Near-surface Electronic Properties of Clean, Well-ordered ZnO Surfaces of Both Polarities.
Louis Piper 1 , Andrew Preston 1 , Sang-Wan Cho 1 , Alex DeMasi 1 , Martin Allen 2 , Steven Durbin 2 , Kevin Smith 1
1 Department of Physics, Boston University, Boston , Massachusetts, United States, 2 Department of Electrical Engineering, University of Canterbury, Christchurch New Zealand
Show AbstractThe physical properties of ZnO surfaces/interfaces are of interest both fundamentally and technologically. Here we present our recent results from clean (1x1) reconstructed ZnO surfaces of both Zn- and O-polarity single crystals using synchrotron-based x-ray photoemission spectroscopy (XPS). By employing Ar ion bombardment and a combination of vacuum and oxygen annealing cycles, we were able to prepare clean (1x1) reconstructed surfaces of both polarities. Comparison of our O 1s core-level XPS provides supporting evidence of hydrogen stabilizing the O-polar (1x1) surface, as claimed by Kunat et al.[1] The valence band spectra of clean ZnO (of both polarities) are found to agree well with recently reported bulk quasi-particle calculations (within the GW approximation) used to successfully to interpret our earlier bulk x-ray emission and absorption spectra of ZnO.[2] In both cases the Fermi level is pinned close to the conduction band. Further inspection reveals a difference of 300 meV in the Fermi level pinning of the O-polar and Zn-polar surfaces from extrapolation of their valence band regions and work-functions. We go further by estimating the charge-neutrality level of ZnO by saturating the surface with monolayers of K atoms. Earlier studies of K-deposited III-V compounds revealed that after saturating the surface the Fermi level is eventually pinned close to or at the charge-neutrality level.[3] In a similar manner, we find the Fermi level is finally pinned close to the 3.40 eV predicted for the charge neutrality level of ZnO by Schleife et al.[4] This work supported in part by the US DOE under DE-FG02-98ER45680, and by the ACS Petroleum Research Fund.[1] M. Kunat, St. Gil Girol, Th. Becker, U. Burghaus, and Ch. Woll, Phys. Rev. B 66, 081402R (2002)[2] A.R.H. Preston, B.J. Ruck, L.F.J. Piper, A. DeMasi, K.E. Smith, A. Schleife, F. Fuchs, F. Bechstedt, J. Chai, and S.M. Durbin, Phys. Rev. B. 78, 155114 (2008)[3] M. G. Betti, V. Corradini, G. Bertoni, P. Casarini, C. Mariani, and A. Abrama Phys. Rev. B 63, 155315 (2001)[4] A. Schleife, F. Fuchs, C. Rödl, J. Furthmüller, and F. Bechstedt, Appl. Phys. Lett. 94, 012104 (2009)
12:15 PM - H3.8
Effect of Donor-bound and Free Excitons on Luminescence of MgxZn1-xO Thin Films.
Christof Dietrich 1 , Alexander Mueller 1 , Marko Stoelzel 1 , Gabriele Benndorf 1 , Joerg Lenzner 1 , Michael Lorenz 1 , Marius Grundmann 1
1 Institut für Experimentelle Physik II, Universität Leipzig, Leipzig Germany
Show AbstractWe report on the properties of the near-band edge photoluminescence (PL) of MgxZn1-xO thin films with Mg-contents in the range 0.005 ≤ x ≤ 0.09. The MgxZn1-xO thin films used in this study were grown in an oxygen atmosphere on a-plane sapphire by pulsed-laser deposition.
For x ≤ 0.031 donor-bound and the free excitonic transition could be resolved individually. This enabled the unambiguous determination of the standard deviation σ of the potential fluctuations caused by the alloying.
The transitions of donor-bound (I6a-line) and free excitons (XA-line) are separately resolved for x ≤ 0.031 by low temperature PL. The temperature-dependent PL measurements of these samples showed the dominance of the recombination of donor-bound excitons up to 100 K; for T > 100 K the recombination of free excitons dominates. A temperature-dependent S-shape behavior of the spectral position of the PL maximum is observed and due to localization of excitons in alloy potential fluctuations [1].
For samples with x > 0.031 a more pronounced S-shape was observed. It is attributed to the influence of the potential fluctuations mentioned above and also to the change of the dominant recombination process from I6a to XA at temperatures around 100 K. We show that without considering these effects the strength of the potential fluctuation is considerably overestimated [2,3].
These findings were confirmed by time-resolved PL measurements. The time-delayed PL spectra of a MgxZn1-xO sample with x = 0.08 showed a blue shift of the maximum over time, which proves the observation of a superposition of two different decay processes, in particular the recombination of fast donor-bound excitons and slow free excitons.
[1] A. Müller et al., Solid State Comm. 148, 11-12, 570-572 (2008)
[2] S. Heitsch et al., J. Appl. Phys. 101, 083521 (2007)
[3] H. Murotani et al., J. Appl. Phys. 104, 053514 (2008).
12:30 PM - H3.9
The Lasing Characteristic of Epitaxial ZnO Films on Si (111) using Y2O3 Buffer Layer.
Chin-Chia Kuo 1 , Wei-Rein Liu 1 2 , Wen-Feng Hsieh 1 3 , Chia-Hung Hsu 1 2 , Wei-Chin Lee 4 , Minghwei Hong 4 , Ray-Nien Kwo 5
1 Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu Taiwan, 2 , Research Division, National Synchrotron Radiation Research Center, Hsinchu Taiwan, 3 Institute of Electro-Optical Science and Engineering, National Cheng Kung University, Tainan Taiwan, 4 Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu Taiwan, 5 Department of Physics, National Tsing Hua University, Hsinchu Taiwan
Show AbstractWe have grown high-quality c-plane ZnO epitaxial films on Si(111) with a thin Y2O3 buffer layer by pulsed laser deposition. The optical characteristics of two different thicknesses of ZnO films (~ 1.2 μm and ~ 550 nm) were studied at room temperature under optical excitation by a frequency tripled Nd:YAG laser with wavelength of 355 nm at 1 kHz. We found the lasing characteristic peaks which correlated to the film thickness. Accompanied by increasing the ZnO layer thickness, the lasing thresholds raise that result from the different optical resonance structures. However, the lasing thresholds are still very low due to high crystal quality of the ZnO epitaxial films acts as microresonators. The thickness over 1μm, there is cracking observed on the ZnO surface. The lasing properties on the cracking ZnO film result in the random lasing mode; whereas, the lasing mode of the thinner film is liked the Fabry-Perot cavity mode that can be characterized by the transverse Gaussian modes.
12:45 PM - H3.10
Two- and Three-photon Absorption Spectra of ZnO.
Marcelo Vivas 1 , Tina Shih 2 , Tobias Voss 2 3 , Eric Mazur 2 , Cleber Mendonca 1 2
1 , University of Sao Paulo, Sao Carlos Brazil, 2 , Harvard University, Cambridge, Massachusetts, United States, 3 , University of Bremen, Bremen Germany
Show AbstractZinc oxide (ZnO) is a wide-band gap semiconductor that is being extensively studied for the development of optoelectronic devices operating in the blue-to-near-UV spectral region. Furthermore, ZnO possesses a large exciton binding energy and can easily be handled to yield uniform and high-quality ZnO nanostructures. To better understand ZnO’s optical processes under fs-laser excitation, it is necessary to characterize its nonlinear absorption behavior. Even though a few studies have been carried out on the nonlinear optical properties of ZnO, they were performed in a narrow wavelength range and using MHz repetition-rate lasers. Therefore, there has yet to be a complete multiphoton-absorption study of ZnO that describes the spectral regions of each nonlinear absorption mechanisms as well as the regions where there are mixture of nonlinear processes. In this work we present a broadband analysis of the nonlinear absorption in bulk ZnO employing the open aperture Z-scan technique. Using an optical parametric amplifier we generated tunable, from 460 nm to 960 nm, 1-kHz repetition rate laser pulses to investigate the nonlinear absorption spectrum of ZnO. We observed two- and three-photon absorption between 550 and 980 nm. Each multiphoton absorption process was interpreted using a theoretical model developed for specifically for semiconductors. For excitation wavelengths below 500 nm, we observed reverse saturable absorption due to one-photon excitation of ZnO. Such process was described using rate-equations. We also determined the spectral regions that exhibit a mixture of nonlinear absorption mechanisms, which were confirmed by photoluminescence measurements.
H4: Electric and Magnetic Properties
Session Chairs
Tuesday PM, December 01, 2009
Ballroom C (Hynes)
2:30 PM - **H4.1
Schottky Contacts to ZnO: What We Know, and Don't Know.
Steven Durbin 1 2 , Martin Allen 1 2
1 Department of Electrical and Computer Engineering, University of Canterbury, Christchurch New Zealand, 2 , The MacDiarmid Institute for Advanced Materials and Nanotechnology, Christchurch New Zealand
Show AbstractIn the last several years, significant progress has been made with regard to performance of Schottky contacts to n-ZnO, with barrier heights in excess of 1 eV and ideality factors at the image force limit routinely achievable. There is some evidence that oxygen vacancies play a major role in determining barrier height, but this is only part of the picture. Consistent differences between diodes formed on various low index faces are not so easily explained, nor is the apparent upper limit of 1.2 eV to the barrier height. More intriguing is the possible relationship of these issues to the difficulties often reported in achieving p-type material.
3:00 PM - H4.2
Schottky Barrier Contacts on MgxZn1-xO Thin Films.
Holger von Wenckstern 1 , Kerstin Brachwitz 1 , Oliver Kramer 1 , Matthias Schmidt 1 , Florian Schmidt 1 , Stefan Mueller 1 , Christoph Dietrich 1 , Martin Ellguth 1 , Michael Lorenz 1 , Marius Grunmann 1
1 Experimentelle Physik II, Universität Leipzig, Leipzig Germany
Show AbstractWe investigated the formation and the energy position of defect levels with respect to the conduction band minimum Ec of MgxZn1-xO thin films in dependence on x. Alloying of ZnO by Mg leads to an increase of the band gap, therefore MgxZn1-xO layers are commonly used as barrier material for ZnO quantum wells. The electronic properties of MgxZn1-xO have been studied in detail for a wide range of compositions mainly using low temperature luminescence measurements [1]. In contrast electrical properties of MgxZn1-xO thin films, like the dependence of the thermal activation energy Et of effective mass donors and deep levels or of the barrier height of Schottky contacts on the magnesium content were not investigated systematically up to now. We present a detailed and comprehensive investigation of properties of Schottky barrier contacts on MgxZn1-xO thin films for 0 < x < 0.2. All samples were grown heteroepitaxially on a-plane sapphire substrates by pulsed-laser deposition. Selected samples were annealed ex-situ for two hours in 700 mbar oxygen at 500°C, 700°C, and 900°C, respectively. The samples were characterized by X-ray diffraction, atomic force microscopy, cathodoluminescence and photoluminescence (PL). The magnesium content in the thin films was determined by using PL and Rutherford backscattering spectroscopy, respectively. Schottky contacts were realized by reactive dc-sputtering of Pd [2]. As ohmic contact we used a degenerately doped ZnO:Al layer deposited prior to the MgxZn1-xO layer allowing a front-to-back contact geometry [3]. This ensures a low series resistance of the resulting Schottky diodes allowing space charge spectroscopic methods with test frequencies of 1 MHz and above. The Pd/MgxZn1-xO Schottky diodes were characterized by current-voltage measurements for temperatures between 20 K and 320 K. The best contacts have ideality factors as low as 1.1, an effective barrier height of about 1 eV and a rectification ratio I(-1 V/ I(1 V) of 109 at room temperature.Further, we applied thermal admittance spectroscopy, deep level transient spectroscopy (DLTS) and for selected samples Laplace DLTS. Typical DLTS and admittance results show that the thermal activation energy of the E3 defect increases from 300 meV for x = 0 to about 405 meV for x = 0.09. We further observed a decrease of the defect emission rates en with increasing annealing temperature.[1] A. Müller, G. Benndorf, S. Heitsch, C. Sturm, M. Grundmann, Solid State Comm. 148, 570 (2008).[2] A. Lajn, H. von Wenckstern, Z. Zhang, C. Czekalla, G. Biehne, J. Lenzner, H. Hochmuth, M. Lorenz, M. Grundmann, S. Künzel, C. Vogt, R. Deneke, J. Vac. Sci. Technol. B 27, 1769 (2009).[3] Holger von Wenckstern, Gisela Biehne, R. Abdel Rahman, Holger Hochmuth, Michael Lorenz, and Marius Grundmann, Appl. Phys. Lett. 88, 092102 (2006).
3:15 PM - H4.3
Deep or Shallow? The Acceptor Puzzle in ZnO.
Stephan Lany 1 , Alex Zunger 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractIn the quest for
p-type ZnO, much research activity has been dedicated towards the prototype acceptors N and Li. Notwithstanding reports of high hole concentrations, the issue of
p-type doping of ZnO remains debated. Considering that shallow acceptors seem to be related to stacking faults [1], even the conventional point defect doping mechanism is in question. Indeed, the shallow acceptor levels observed in
p-doped ZnO roughly in the range of 100 - 200 meV above the valence band maximum (VBM) are difficult to reconcile with the spectroscopically established properties of substitutional Li
Zn [2] and N
O [3] acceptors, indicating strongly localized, Jahn-Teller distorted hole wavefunctions. Moreover, the acceptor state of Li
Zn has been established as a deep level 0.8 eV above the VBM [4], and also the photo-response of N
O [5] in the visible could indicate a deep level.
While support of p-doping by substitutional acceptors has been drawn from numerous theoretical calculations, the bulk of these studies was based on the local density or generalized gradient approximations (LDA or GGA), which notoriously over-delocalize acceptor-states and yield too shallow binding energies for localized acceptor states. In order to predict quantitatively the acceptor level in wide-gap oxides, we recently developed a method that ensures that the unoccupied hole-state is correctly placed in energy relative to the spectrum of occupied states [6], which is not the case in standard LDA or GGA calculations. When we use this method to study in detail the prototypical NO and LiZn acceptors in ZnO, we find that these acceptor-type point defects have deep levels and cannot produce p-type conductivity. At the same time, our calculated optical transition energies consistently explain experimentally observed absorption and photoluminescence energies due to N [5] and Li [4,7], including the recent observation of a shallow state of Li with a binding energy of 300 meV [4]. We interpret this shallow Li acceptor as a transient state that occurs due to a small energy barrier between the symmetric configuration of the initially ionized LiZn- acceptor and the Jahn-Teller symmetry broken deep LiZn0 ground state. We further compare the behavior of acceptors in ZnO with the Mg and Zn acceptors in GaN, where our present method predicts a considerable change in the localization of the hole-state, compared to LDA or GGA, but where the acceptor energies remain close to the VBM, allowing for p-type doping despite rather localized acceptor states.
[1] M. Schirra et al., Phys. Rev. B 77, 125215 (2008)
[2] O.F. Schirmer, J. Phys. Chem. Solids 29, 1407 (1968)
[3] W.E. Carlos at al., Physica B 308–310, 976 (2001)
[4] B.K. Meyer et al., Appl. Phys. A 88, 119 (2007)
[5] N.Y. Garces et al., Apply. Phys. Lett. 80, 1334 (2002)
[6] S. Lany and A. Zunger, arXiv:0905.0018 (2009)
[7] O.F. Schirmer, J. Phys. Cond. Mat. 18, R667 (2006)
3:30 PM - H4.4
Magneto-Transport Characterization of Epitaxial ZnO Films Grown on (111) Si Substrates.
Kui Zhang 1 , Wei Guo 1 , Tassilo Heeg 2 , Darrell Schlom 2 , Xiaoqing Pan 1
1 Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Department of Materials Science and Engineering, Cornell University, Ithaca, New York, United States
Show AbstractZnO films were grown by pulsed laser deposition on (111) Si substrates with Sc2O3 buffer layers at temperatures from 240 to 600 °C. Both x-ray diffraction and transmission electron microscopy (TEM) reveal the ZnO films are highly crystalline with good epitaxy. Rectifying effects were observed in I-V curves, exhibiting turn-on voltages as low as 0.4 V. Carrier transport properties have been investigated by low temperature Hall measurements under magnetic fields up to 10 T, where abnormally positive magneto-conductivity were observed below 150 K. It is found that the experimental data can be well fitted by the diffusive Fermi surface (DFS) model, taking into account the weak localization and electron dephasing theories, which indicate a possibly weak in-plane disorder within the film. The micro-structural parameters obtained during the fitting process are compared with the TEM images. Optical properties were also studied by temperature-dependent photoluminescence measurements.
3:45 PM - H4.5
Influences of Polarization Effects in the Electrical Properties of Polycrystalline MgZnO/ZnO Heterostructure.
Huai-An Chin 1 , Chih-I Huang 1 , Yuh-Renn Wu 1 , I-Chun Cheng 1 , Jian Chen 2 , Kuo-Chuang Chiu 3 , Tzer-Shen Lin 3
1 Graduate Institute of Photonics and Optoelectronics and Department of Electrical Engineering, National Taiwan University , Taipei Taiwan, 2 Institute of Applied Mechanics, National Taiwan University , Taipei Taiwan, 3 Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu Taiwan
Show AbstractThe study of transparent conducting oxide (TCO) has become an important area due to the applications of lighting and display technology. Therefore, high mobility and conductivity TCO materials would be a key issue to the industry. In this paper, we have applied the Monte Carlo method to analyze the mobility and polarization effect of polycrystalline MgxZn1-xO/ZnO heterostrcuture. The effects of grain boundary scattering, ionized impurity scattering as well as phonon scattering have been included in our program. The grain boundary potential size and carrier screening effect has been analyzed with our developed 2D Poisson and drift-diffusion solver. With a careful design of modulation doping and including the effect of spontaneous and piezoelectric polarization, the grain boundary potential can be suppressed and thus the mobility of the ZnO layer can be improved.Based on the theoretical prediction, we conduct a series of experiments to discuss the polarization effect within the polycrystalline MgxZn1-xO/ZnO heterosturcture. A 200nm-thick ZnO thin film with grain size of ~15nm is deposited as the first layer followed by the deposition of MgxZn1-xO with various thicknesses as the second layer. Our preliminary results show that the present of MgxZn1-xO improves the electrical conductance of polycrystalline ZnO thin films by 1 to 2 orders of magnitude. In general, the amount of enhancement first increases with the thickness of MgxZn1-xO and then saturates. The onset of saturation occurs at ~20nm for the Mg0.3Zn0.7O case and ~10nm for the Mg0.4Zn0.6O case. These phenomena are consistent with the simulation prediction. More systematic experiments are under going at present, and the results will be reported at the symposium.
4:30 PM - **H4.6
ZnO Polarity, Interface Defects and Schottky Barriers.
Yufeng Dong 1 , Zhaoqiang Fang 2 , David C. Look 2 , Daniel R. Doutt 1 , Tyler A. Merz 1 , Leonard J. Brillson 1
1 , The Ohio State University, Columbus, Ohio, United States, 2 , Wright State University, Dayton, Ohio, United States
Show AbstractDespite nearly sixty years of research, many fundamental issues surrounding ZnO remain unresolved. Among the most notorious of these have been the difficulty of p-type doping and the role of compensating native defects. While the importance of native defects as native donors and acceptors is recognized, it is not yet clear how various visible emissions correlate with Zn/O vacancies, interstitials, and antisite defects. Furthermore, there are few studies on the polarity dependence of such native defects and their correlation with ZnO physical properties. In addition, fabricating high quality ZnO contacts remains a challenge and there is little known about the comparison between two ZnO surface polarities (O or Zn-surface) on (i) surface morphologies, (ii) surface defect concentrations and energy levels, (iii) surface reactivities with various metal contacts, and (iv) the Schottky barrier heights and their metal correlations.We used depth-resolved cathodoluminescence spectroscopy (DRCLS), current-voltage and capacitance-voltage measurements, atomic force microscopy (AFM), surface photovoltage spectroscopy (SPS), positron annihilation spectroscopy (PAS) and deep level transient spectroscopy (DLTS) to probe the physical properties of surface and near surface defects at (0001) Zn- and (000-1) O- polar surfaces and metal interfaces of remote oxygen plasma (ROP) cleaned ZnO, and to determine the energy levels of Zn vacancies and vacancy complexes in bulk ZnO crystals. ROP provides a means to remove surface contamination and native point defects. The transport properties of the Schottky barrier diodes (SBDs) are not only dependent on metal but also very sensitive to the surface polarity. They correlate to surface morphologies, DRCLS defect emissions, CV carrier profiles and surface and bulk traps revealed by DLTS. Indeed, effective carrier concentrations in the surface space charge region exhibit a strong dependence on specific defects and their densities as well as surface polarity. Our findings demonstrate the importance of polar effects and oxygen plasma treatment on forming surface and near-surface defects that control the physical properties of ZnO. In particular, the role of Zn vacancies may provide insight into methods of compensating n-type donors in order to achieve p-type doping. Furthermore, with the consistency in CL and PAS depth profile for the Zn vacancy, we provide “fingerprints” of specific native defects such as Zn and O vacancies and determination of their energy levels, which can help assess various theoretical methods of calculating gap states in ZnO and other semiconductors.
5:00 PM - H4.7
Zn Treated Co Doped ZnO for Spintronics.
Lubna Shah 1 , Hao Zhu 1 , Weigang Wang 1 , Bakhtyar Ali 2 , Tao Zhu 3 , Fan Xin 1 , S. Shah 1 2 , John Xiao 1
1 Physics and Astronomy, University of Delaware, Newark, Delaware, United States, 2 Material Science and Engineering, University of Delaware, Newark, Delaware, United States, 3 State Key Laboratory of Magnetism, Institute of Physics, Chinease Academy of Sciences, Beijing China
Show AbstractField of spintronics provides new class of devices (e.g. integrated memory chips and polarized light emitters) by making use of carrier spin in addition to its charge. Ferromagnetic semiconductors, especially wide band gap e.g. ZnO and TiO2, appear to be promising materials to be used in spintronics devices. The ongoing quest to fabricate ferromagnetic (FM) semiconductors with Tc well above room temperature has promoted the intensive study on the transition metal (Co, Ni, Mn) doped ZnO. We have successfully fabricated bulk samples of Co doped ZnO having room temperature ferromagnetism by performing Zn vapor treatment. Our structural characterization (XRD and XPS) on both as-prepared and Zn treated samples reveal inclusion of Co into the oxide lattice without any evidence of metallic Co clusters or secondary phase existence. Through systematic magnetic and transport study we have demonstrated that the bound magnetic polaron (BMP) model is responsible for ferromagnetism in Co-ZnO semiconductors, where the carriers are provided by the interstitial Zinc (Zni). Our experiment is unique since we are able to, by changing temperature, cross the carrier concentration threshold, above which a long-range ferromagnetic order is established. Consequently, ferromagnetic order is observed at room temperature but it exhibits decreasing trend at temperatures below 100 K. To support our conclusion we have performed a systematic investigation on structure, magnetic properties, and transport properties which give consistent results in the context of our proposed two-region model, i.e. (a) Zni region/layer where carriers are sufficient to couple Co ions ferromagnetically, and (b) the region with little carriers that remain in paramagnetic state. The ferromagnetic exchange mechanism responsible for ferromagnetism in our semiconducting samples is further verified by “reverse” experiment as well as electronic structure of magnetic impurity (Co) and carrier associated with the defect (Zni).* Corresponding authors. Email address:
[email protected],
[email protected]5:15 PM - H4.8
Structural and Magnetic Studies on Ho Doped ZnO Nanoparticles.
M. S. Ramachandra Rao 1 , J. N. Divya Deepthi 1 , M. S. Ramachandra Rao 1
1 , IIT Madras, Chennai India
Show AbstractRecent results on Gd doped GaN, indicating high magnetic moments [Dhar et al., Phys. Rev. Lett. 96 (2005) 037205.] have motivated us to investigate ZnO thin films doped with rare earth (RE) metal ions. For the 3d transition metals, the 3d electrons are exterior and delocalized, leading to strong direct exchange interactions and high Curie temperatures, but often the orbital momentum is zero, leading to small total magnetic moments per atom. In the case of RE doping, the 4f electrons are localized, exchange interactions are indirect as they occur via the 5d or 6s conduction electrons. However the high orbital momentum leads to high total magnetic moment per atom. RE doped ZnO also has potential applications as visible light emitting phosphors in high power lasers and other optoelectronic devices [John et al.Appl. Phys. Lett. 77 1635 (2000), Deng et al. Phys. Chem. C 111 13013(2007)]. Literature survey reveals that there is no such study on Ho doped ZnO compounds. Hence, in this work we investigated the structural, morphological, optical and magnetic properties of Ho doped ZnO nanoparticles. These compounds were synthesized by a chemical route with different contents of the RE metal, Ho3+. The diameter of 1 mol% Ho doped nanoparticles obtained using the chemical route lies within 15-20 nm. This decrease in particle size led to an increase in band gap by 18 meV as compared to the bulk commercial undoped compound. Dielectric measurements on the bulk samples show a higher dielectric constant for Ho doped samples as compared to undoped samples. Impedance and loss measurements were also carried out on these samples. To investigate the magnetic properties of the Zn1-xHoxO, isothermal magnetization hysteresis measurements were performed at different temperatures. Interestingly, M-H measurements of 1 mol% Ho doped sample revealed a hysteresis loop at 5 K while the undoped sample was diamagnetic in nature. It can be seen from M-T curve that there is no discrepancy in magnetization between the ZFC and FC curves in the whole measuring temperature region. The possible reason for this behaviour has been explained in this work. Thin films of the above material were grown on SiO2 substrates and further measurements are being carried out. The results will be presented in detail.
5:30 PM - H4.9
Ferromagnetism in Transition Metal doped ZnO.
Michael Snure 1 , Ashutosh Tiwari 1 , Dhananjay Kumar 2
1 Materials Science and Engineering , University of Utah, Salt Lake City, Utah, United States, 2 Mechanical Engineering, North Carolina A&T, Greensborro, North Carolina, United States
Show AbstractSpintronics has the potential to revolutionize semiconductor device technology; however, in order to realize the full potential of such devices ferromagnetic semiconductors with Curie temperatures above room temperature are required. For nearly a decade transition metal (TM) doped ZnO has been considered a prime candidate for realizing these ferromagnetic semiconductors. Over this time almost every TM doped ZnO system has been tried and ferromagnetism has been reported in many of these systems. However, these reports have generated a considerable amount of controversy too. The research community is still split over whether the observed ferromagnetism in these materials originates from ferromagnetic TM clusters, or if the ferromagnetism is an intrinsic property of the material. Here we report our work that shows strong examples of both intrinsic and extrinsic ferromagnetism in TM doped ZnO films. In this work conventional ferromagnetic elements like Ni as well as non-ferromagnetic elements like Cu were selected as potential magnetic dopants in ZnO. Thin films of TM doped ZnO were deposited by pulsed laser deposition (PLD) using high purity single-phase targets. We observed ferromagnetic hysteresis above room temperature in single phase Cu and Co doped ZnO thin films. The observed ferromagnetism in Cu doped ZnO was quite unexpected considering the fact that all elements present in the film (as well as their oxide phases) are non-ferromagnetic, and hence the observed magnetic behavior cannot be simply attributed to the existence of ferromagnetic metal clusters. In Ni doped ZnO ferromagnetic hysteresis was not found to be an intrinsic property of the material. By varying the deposition environment we controlled the solubility of Ni in ZnO. This allowed us to produce films with different quantities of Ni metal clusters. From these films a direct correlation between measured ferromagnetic hysteresis and the presence of Ni metal clusters was made. In samples containing large quantities of Ni metal ferromagnetic hysteresis was observed, while the films with Ni completely dissolved into the ZnO matrix showed paramagnetism.
H5: Poster Session I
Session Chairs
Leonard Brillson
Jürgen Christen
Hiroshi Fujioka
Hoe Tan
Wednesday AM, December 02, 2009
Exhibit Hall D (Hynes)
9:00 PM - H5.1
Ab-initio Band Structures and Optical Properties of MgO, ZnO, and CdO.
Andre Schleife 1 , Claudia Roedl 1 , Frank Fuchs 1 , Friedhelm Bechstedt 1 , Tim Veal 2 , Philip D. King 2 , Chris McConville 2 , Munise Rakel 3 , Christoph Cobet 3
1 , Institut fur Festkorpertheorie und -optik, Friedrich-Schiller-Universitat Jena and European Theoretical Spectroscopy Facility, Jena Germany, 2 , Department of Physics, University of Warwick, Coventry United Kingdom, 3 , Institute for Analytical Sciences, Berlin Germany
Show AbstractThe accuracy of modern parameter-free calculations has not only been driven to excellent agreement with experimental results, but even pushed towards being predictive, e.g., for non-equilibrium crystal structures. Nowadays, even spatially nonlocal hybrid functionals in combination with sophisticated GW calculations of quasiparticle energies are available and computationally affordable. They can be refined by the inclusion of the spin-orbit interaction.We introduce the theoretical concepts of one of the currently most reliable ab-initio approaches to the computation of electronic structures. The nonlocal hybrid HSE03 functional, as an approximation to exchange and correlation, is used to compute the starting point for the solution of the quasiparticle equation using the GW approximation. Spin-orbit coupling is also included. We then combine these electronic structures with the treatment of the screened attractive electron-hole Coulomb interaction as well as local-field effects in the form of the unscreened electron-hole exchange in the kernel of a Bethe-Salpeter equation.The highly accurate electronic band structures resulting for wz-MgO, wz-ZnO, and wz-CdO are used to derive unknown quantities: fundamental gaps, effective electron/hole masses, crystal-field and spin-orbit splittings, optical matrix elements, and exciton binding energies. We apply Tersoff's approach to the computed band structures to derive branch point energies for the three oxides as a universal energy level of reference. We study the resulting band alignments for rs-MgO, wz-ZnO, and rs-CdO as well as In2O3 and group-III nitrides and derive natural band discontinuities. The results are discussed in the light of measured values.Furthermore, we compute dielectric functions that compare excellently to experimental results in a wide photon energy range. They allow us to derive reflectivity curves and electron-energy loss functions. We are able to clarify in detail the origin of peaks in the reflectivity and energy-loss functions in terms of optical interband transitions and plasmon effects. The impact of many-body effects is pointed out in detail and chemical trends for the three oxides are discussed.
9:00 PM - H5.10
A Study of Dual Acceptor Behaviour in p-ZnO: Ag-N and Sb-N.
Eliana Kaminska 1 , Iwona Pasternak 1 , Anna Piotrowska 1 , Oksana Volnianska 2 , Piotr Boguslawski 2 3 , Elzbieta Dynowska 2 , Adam Barcz 1 2 , Ewa Przezdziecka 2 , Tomasz Wojciechowski 2
1 , Institute of Electron Technology, Warsaw Poland, 2 , Institute of Physics PAS, Warsaw Poland, 3 , University of Bydgoszcz, Bydgoszcz Poland
Show AbstractOne of the major obstacles to overcome for the development of ZnO-based electronics is the lack of a reliable p-type doping procedure. The most often studied acceptors in ZnO are group-V N, As, and Sb, and belonging to group I – Li and Ag atoms. Of these only nitrogen substitutes for O, while As and Sb are suggested to occupy Zn sites and form complexes with O vacancies [1]. Li and Ag substitute for Zn. Recently, we reported on the formation of p-type ZnO in a two-step process in which Zn-based material containing N [2],Sb [3] or Ag [4], was first sputter-deposited on sapphire substrate and then thermal oxidized to form an uniformly doped ZnO film. Here, motivated by theoretical and experimental suggestions according to which the use of dopant complexes or codoping may create shallower acceptors, we have studied the efficiency of dual acceptor doping. We compare the properties of p-type ZnO achieved by Ag-N and Sb-N dual acceptor doping. 0.5-1 micron thick layers of Zn-Ag-N and Zn-Sb-N compounds were grown by magnetron reactive sputtering and oxidized by furnace annealing in oxygen flow. The chemical composition and microstructure of the layers were assessed with SIMS, XRD, SEM and AFM. Transport properties were determined from Hall measurements. Optical properties of ZnO films were studied by LT photoluminescence and optical transmission.The net result of these experiments is an improvement of p-type conductivity manifested by increased hole concentration and mobility obtained by using nitrogen-assisted Ag- and Sb-doping. The lowest resistivity was found to be 0.19 ohmcm for ZnO:Ag:N with hole concentration 1.2*1019cm-3 and mobility 3.9 cm2/Vs. The resistivity of p-ZnO:Sb:N was 4.3 ohmcm with hole concentration of 2.0*1017cm-3.and mobility 7.4 cm2/Vs.The experimental results were supported by theoretical calculations of the doping efficiency of Ag, as well as the efficiency of the dual acceptor doping with Ag-N or Sb-N. These were conducted by using density functional calculations.The research was partially supported by the European Union within European Regional Development Fund, through grant Innovative Economy (POIG.01.03.01-00-159/08, "InTechFun")[1]. S. Limpijumnong et al., Phys. Rev. Lett. 92, 155504 (2004).[2] E. Kaminska et al., Solid State Commun., 135, 11(2005)[3] E. Przezdziecka et al., Phys. Rev. B 76, 193303 (2007) [4] E. Kaminska et al., AIP Conf. Proc. 893 337 (2007)
9:00 PM - H5.11
Electrical and Thermal Stress Analysis of InGaZnO Thin-Film Transistor.
Mami Fujii 1 , Takashi Fuyuki 1 , Ji Sim Jung 2 , Jang Yeon Kwon 2 , Yukiharu Uraoka 1 3
1 Materials Science, Nara Institute of Science and Technology, 8916-5, Takayama, Ikoma, Nara, Japan, 2 , Samsung Advnced Institute of Technology, Mt. 14-1,Nongseo-Dong, Giheung-Gu, Yongin-Si, Gyeonggi-Do, Korea (the Republic of), 3 , CREST, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama, Japan
Show AbstractIn recent years, thin film transistors (TFT) using oxide semiconductors have attracted much attention as promising materials for the driving device of next generation displays. However, several papers have reported their poor against electric stress. Therefore, reliability is one of the serious problems. In this study, we evaluated the reliability of oxide semiconductor TFTs with amorphous IGZO by applying gate and drain voltage stress, and thermal stresses. At first, we investigated the difference in degradation mode caused by only gate voltage stress and both gate and drain voltage stress. And next, a thermal imaging system was used to analyze a thermal distribution during the stress. Furthermore, we measured the change of TFT characteristics under thermal stress.We examined the drain voltage dependence in addition to the gate voltage dependence. Transfer curve shifted parallel to the positive direction under the only gate voltage stress and both gate and drain voltage stress. However, the amount of threshold voltage shift under gate and drain voltage stress was smaller than case of only gate voltage stress and shifted with sub-threshold swing (S value) change. We analyzed thermal distributions in TFT during gate and drain voltage stress. The results of thermal distributions was measured under the various drain voltage stress (0, 10, 20 V) at the fixed of gate voltage stress (20 V). Joule heating caused by drain current was observed. Under the thermal stress in a range from room temperature to 150 degrees, transfer curve shifted to negative direction with S value change.From the results of transfer curve change, we supposed that there is a possibility of the transfer curve shifting parallel to positive direction by gate voltage and to negative direction with S value change by drain voltage at same time under gate and drain voltage stress. And, from the results of thermal distributions, maximum temperature increased with increasing drain voltage stress. We thought that this thermal heating causes the threshold voltage shift when drain voltage stress was applied. However, the amount of threshold voltage shifts decreased with increasing drain voltage stress. Furthermore, under the thermal stress conditions, transfer curve changed in the same manner with the drain voltage stress condition. Therefore, we supposed that two kinds of degradation modes occurred by applying gate and drain voltage stresses.
9:00 PM - H5.12
Characteristics of In2O3 Films Grown on (0001)-plane Al2O3 Substrates by Atomic Layer Deposition using Trimethylindium and Nitrous Oxide.
Wei-Hsu Chi 1 , Kuo-Yi Yen 1 , Shao-Cian Li 1 , Jyh-Rong Gong 1
1 Department of Physics, National Chung Hsing University, Taichung Taiwan
Show AbstractIndium oxide (In2O3) films were deposited on (0001) α-Al2O3 substrates by the atomic layer deposition (ALD) using trimethylindium (TMIn) and nitrous oxide (N2O). The characteristics of In2O3 films were analyzed by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission spectroscopy, and four point probe method. Under certain TMIn flow rates, In2O3 films grown by ALD at 400°C exhibited resistivities in the range of 10^-1~10^-2 Ω-cm. Optical transmittances more than 80% were found in the wavelength range of 400-800 nm for the ALD-grown In2O3 films with an optical band gap value of ~3.66 eV.
9:00 PM - H5.13
Al-doped and Ga-doped ZnO Nanorods : Doping During Growth vs Thermal Diffusion.
Alan Man Ching Ng 1 , Fang Fang 1 , Xin Yi Chen 1 , Aleksandra B. Djurisic 1 , Wai Kin Chan 2
1 Physics, The University of Hong Kong, Hong Kong, Hong Kong SAR, China, 2 Chemistry, The University of Hong Kong, Hong Kong, Hong Kong SAR, China
Show AbstractDue to its wide band gap (~3.3 eV) and large exciton binding energy (60 meV), ZnO has been of great interest for its potential use in optoelectronic devices. In recent years, attention has been focused on ZnO 1-D nanostructures. Various nanostructures were fabricated and their optical and electrical properties were studied as they are the building blocks of many nanoscale electronic and photonic devices. Doping of ZnO nanostructures has also been investigated in order to enhance the optical quality and lower the resistivity. Group III elements, such as Ga and Al, have been reported as the n-type dopants for ZnO nanostructures. N-type doping is of interest for fabrication of p-i-n LED structures, improving the contact resistance and thus lowering the turn-on voltage of ZnO-based devices. However, the effect of doping method on the nanostructure properties has not been comprehensively studied.In this work, we have investigated different doping methods for electrodeposited ZnO nanorods. Al-doped and Ga-doped ZnO nanorods were prepared by doping during growth and thermal diffusion. For solution growth, the nanorods were fabricated by electrodepositon with solution containing 1%, 2% and 5% of Ga or Al precursors (compared to the concentration of Zn precursor). For thermal diffusion, un-doped ZnO nanorods were first prepared by electrodeposition. A thin layer of Ga or Al was then deposited by thermal evaporator followed by thermal treatment. The samples were characterized by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and photoluminescence (PL). The current-voltage (I-V) characteristic of the nanorods was also investigated.
9:00 PM - H5.14
Fabrication, Processing and Characterization of Thin Film ZnO for Integrated Optical Gas Sensors.
Eliana Kaminska 1 , Anna Piotrowska 1 , Iwona Pasternak 1 , Michal Borysiewicz 1 , Marek Ekielski 1 , Krystyna Golaszewska 1 , Zbigniew Adamus 1 , Tomasz Wojciechowski 2 , Elzbieta Dynowska 2 , Przemyslaw Struk 3 , Tadeusz Pustelny 3
1 , Institute of Electron Technology, Warsaw Poland, 2 , Institute of Physics, Polish Academy of Sciences, Warsaw Poland, 3 , Silesian University of Technology, Gliwice Poland
Show AbstractRecently, optical gas sensors built of optical waveguide with sensing layer on top of it, integrated with input and output couplers attract considerable interest. In this respect, ZnO with its unique combination of optical properties and sensing capabilities offers an opportunity to use it as waveguide and sensing element. To realize practical devices, however, there are still important technological steps to be mastered. A special challenge presents the growth of non-doped material. It is commonly recognized that intrinsic defects such as vacancies, interstitials and extended structural defects as well as residual impurities contribute to background n-type conductivity and lattice constraints in nominally undoped ZnO. In this communication we report on the fabrication and pattering of ZnO thin films and demonstrate integrated optical sensor with planar Bragg grating couplers. The numerical computation using OptiFDTD software has been performed for the design of the optical sensor’s elements. ZnO films were deposited by reactive magnetron sputtering. The dependence of the growth parameters (total pressure, gas flow ratio Ar/O2 and power) on the properties of ZnO films was investigated. For optimized deposition conditions, an additional heat-treatment allowed to reduce the residual stress in ZnO and to obtain highly resistive, high-quality material. Planar Bragg grating structures were obtained by DUV photolithography and ICP etching in BCl3/Ar plasma. High-resolution XRD, AFM, SEM and SIMS were used to study the microstructure, composition and deviations from stoichiometry. The residual and thermal stress in ZnO films was determined by wafer curvature technique. Transport measurements were performed to evaluate the conductivity and free carrier concentration in ZnO films. Spectroscopic ellipsometry and transmission measurements were applied to study optical characteristics of the films. The characteristics of the ZnO waveguides with Bragg grating couplers were determined at 677 nm using a setup equipped with a fiber detector, goniometer and homodyne nanovoltmeter. The propagation losses were recorded by digital camera and interpreted using procedure in LabView environment.For the optimized fabrication conditions of ZnO waveguide the attenuation coefficient was below 10dB/cm.
9:00 PM - H5.15
Fabrication of ZnO Ceramics using ZnO-Al2O3 Nanocomposite Particles Prepared by Mechanical Treatment.
Satoko Tasaki 1 , Junichi Tatami 1 , Toru Wakihara 1 , Katsutoshi Komeya 1 , Takeshi Meguro 1 , Hiromi Nakano 2
1 , Yokohama National University, Yokohama Japan, 2 , Toyohashi University of Technology, Toyohashi-shi, Aichi Japan
Show AbstractZnO ceramics have excellent electrical characteristics to be used as a varistor, sensor, and piezoelectric device, transparent electrode and so on. It is well known that ZnO ceramics are able to control electrical properties adding trace impurities. Typical dopants to ZnO are Cr2O3, MoO3, Bi2O3 and Pr6O11, which exhibits nonlinear of electrical property. Al2O3 is also one of the most important impurities because Al dissolved into ZnO to generate electron. In order to improve the property of ZnO ceramics, such impurity must be dispersed uniformly. In this study, nano-composite particles of ZnO/nano Al2O3 prepared by mechanical treatment were used to fabricate ZnO ceramics. ZnO and γ-Al2O3 powders were used in this study. The amount of added γ-Al2O3 was 0.1, 1.0 and 3.7 mol %. These powders were mixed thorough mechanical powder composite process or ball milling process. The powder mixtures were observed by TEM and evaluated by BET specific surface area. They were molded into φ15x3mm cylindrical pellets by uniaxial pressing at 50 MPa followed by cold isostatic pressing at 200MPa. After binder was burn out in air at 500OC for 3h, the green bodies were fired at 1400OC in air for 2h. The microstructure was observed by TEM and SEM. Electrical conductivity was measured by four-terminal method.As a result of TEM observation of the mechanically treated ZnO powder, it was confirmed that γ-Al2O3 nano-particles were directly bonded on the surface of a ZnO particle. Specific surface area of powder mixture decreased by mechanical treatment, which also means bonding between γ-Al2O3 and ZnO occurred by mechanical treatment. The relative density of ZnO sintered body fabricated from the composite particles prepared by mechanical treatment was slightly lower than that prepared ball milling process even at the same firing temperature. This phenomenon resulted from inhibition of mass transfer by grain boundary diffusion in the sintering process because the γ-Al2O3 nano-particles uniformly dispersed on the surface of ZnO by mechanical treatment. The ZnO ceramics prepared by mechanical treatment had higher electrical conductivity than conventional ball milling. Such improvement of electrical property should result from uniform dispersion of nanosized Al2O3 in the powder mixture.
9:00 PM - H5.16
Electrical and Optical Properties of CVT-grown ZnO Crystals.
Koji Abe 1 , Masanori Oiwa 1
1 Department of Electrical and Electronic Engineering, Nagoya Institute of Technology, Nagoya Japan
Show AbstractZinc oxide (ZnO) is a wide band gap semiconductor which is considered as a promising material for optoelectronic devices in the near UV spectral region. To achieve high-performance ZnO devices, high-quality ZnO crystals will be required. ZnO crystals have been grown by vapor phase, hydrothermal, and other methods. Chemical vapor transport (CVT) methods can produce ZnO crystals with low concentrations of residual impurities because stable oxide such as Li2O and CaO are not transported toward seed crystals. We have studied electrical and optical properties of CVT-grown ZnO crystals.The seed crystals used in the present study were hydrothermally grown ZnO (0001) substrates. ZnO crystal growth was performed in a graphite crucible placed inside a vertical quartz tube. The crucible was filled with ZnO feed powder, and the seed crystals were fixed on the lid of the crucible. Prior to the crystal growth, the quartz tube was evacuated below 2×10−5 Torr. Subsequently, the quartz tube was filled with a gas mixture of Ar, H2O, and CO2. The temperatures of the seed crystal and feed powder were set to 860 and 1020°C, respectively. Photoluminescence (PL) measurements were performed at room temperature by using a He-Cd laser (325 nm) as excitation source. To measure electrical properties of CVT-grown crystals, the seed crystal was removed by mechanical polishing. Electrical properties were measured at temperatures from 79 to 297 K. Strong near-band-edge emission at 380 nm and broad green emission at around 500 nm were observed in PL spectra. This green emission could be due to the oxygen vacancies in the crystals. The intensity of the green emission decreased with increasing CO2 pressure in the quartz tube. This result suggests that CO2 reacts with carbon of the graphite crucible and produces CO used as oxygen source in this CVT system. In addition to optical properties, electrical properties were improved by using CO2. Carrier concentration at room temperature was decreased by one order of magnitude. The carrier concentration and Hall mobility measured at 295 K were 3.2×1016 cm−3 and 205 cm2/Vs, respectively. Hall mobility at 79 K was 1690 cm2/Vs. The donor ionization energy estimated from the dependence of the carrier concentration on the temperature was 30–40 meV. It was found that CO2 in the quartz tube has no effect on the donor ionization energy. Taking account of the decrease in the carrier concentration caused by CO2, the donor ionization energy will be related to structural defects.
9:00 PM - H5.17
Growth and Characterization of ZnO Nanodots on Si Substrate by PLD.
Premkumar Thirugnanam 1 , Suresh Sundaram 1 , Ganesh Vottikondala 1 , Manoravi Periyasami 2 , Panigrahi Bk 3 , Baskar Krishnan 1
1 Crystal Growth Centre, Anna University, Chennai, Tamilnadu, India, 2 chemistry Group, IGCAR, kalpakkam, Tamilnadu, India, 3 Materials Science Division, IGCAR, kalpakkam, Tamilnadu, India
Show AbstractZinc oxide (ZnO) nanodots were grown on the Silicon (100) substrates by pulsed laser deposition (PLD). The ZnO films were simultaneously deposited on both Gallium Nitride (GaN) and Si substrates to study the effect of lattice mismatch on the morphology of the ZnO film. The experiments were carried out at a substrate temperature 600 °C and the argon pressure of 1×10-4 mbar. The atomic force microscopy image clearly shows the ZnO nanodots on Si (100) substrate. The mean height and width of the ZnO nanodots is 8 nm and 90 nm, respectively. Due to large lattice mismatch between ZnO and Si (~ 40%) the growth mode has been attributed to the island growth mode (Volmer – Weber). The ZnO film deposited on GaN substrate shows homogeneous columnar grains. The mean height and grain size is 39 nm and 190 nm, respectively. The FWHM of (0002) peak of ZnO nanodots is higher compared to ZnO film grown on GaN. The broadening of (0002) peak may be due to size effect of ZnO nanodots. The ZnO film has compressive stress of -1.57 GPa on Si substrate. The film grown on GaN has tensile stress of 2.268 GPa. The intensity ratio of near band edge emission to visible emission was found to be higher for ZnO nanodots. This was attributed to the low oxygen vacancy and Zinc interstitials in ZnO nanodots even though the growth was carried out in argon ambient. The blue shift has been observed in the near band emission of ZnO nanodots than the ZnO film grown on GaN substrate
9:00 PM - H5.18
Effect of Metal Oxide Nanoparticles on the Mechanical Properties and Tacticity of Poly(methyl methacrylate).
Wantinee Viratyaporn 1 , Richard Lehman 1
1 Materials Science and Engineering, Rutgers University, Piscataway, New Jersey, United States
Show AbstractIn particulate/polymer composite systems, the polymer-particle surface interaction is a critical factor that influences the final properties of the polymer composites system. The quality of the interaction determines to a significant extent the load-transfer efficiency and hence the mechanical properties. Naturally, this factor increases in significance as the size of the particles decreases, and the effect becomes dominant when the particles are at the nanometer scale. Nanoparticles have special effects on polymers since the particle size is of the same order as the polymer chain gyration. Consequently, the nanoparticles can have a strong affect on the configuration and conformation of the surrounding polymer which leads to alterations in the bulk properties of the composite. In this work, we sought to investigate this bound interfacial layer in nanoparticle/polymer composites using Raman spectroscopy and mechanical characterization. Our experimental system was comprised of aluminum oxide and zinc oxide nanoparticles in a PMMA matrix. Various particle sizes and volume fractions were chosen as experimental variables. The polymer nanocomposites were prepared by in situ bulk (radical) polymerization. To minimize the need for in-depth dispersion studies, nanoparticles pre-dispersed in propylene glycol methyl ether acetate were selected. The aluminum oxide particles were spherical shape whereas the zinc oxide particles were acicular shape. With regard to mechanical behavior, electron microscopy images show that the nanoparticles in the PMMA lead to the debonding followed by shear yielding mechanism around the nanoparticles. Moreover, only at a particular volume fraction, unusually high (but reproducible) impact strength was observed, apparently resulting from debonding of the matrix and crack deflection arising from the fast loading rate of the 3.5 m/s impact test. This phenomenon was observed only for the zinc oxide particle composites and seems to be due to the acicular shape of these particles. Raman spectroscopy showed changes in the matrix polymer tacticity when nanoparticles were incorporated, an effect that almost certainly arises from the interaction between the polymer chain and nanoparticle surface. The molecular processes by which these conformational changes in the polymer are generated were addressed from a chemical perspective. Metal oxides are known to have hydroxyl layer on their surface. These polar groups have the potential to interact with polar groups on the polymer chains via secondary bonding and reduced the localized Coulombic energy. Such a mechanism, which is essentially one of dipole-dipole interaction, appears to cause the PMMA chains to orient such that the polar acrylic pendant group is closest to the oxide particle surface. Results of these considerations will be presented using qualitative chemical structures of the bonded interfacial region and the association of the polymer/oxide dipole moieties.
9:00 PM - H5.2
Ga Doped ZnO Nanowire Arrays on Conducting Substrates for Nanostructured Solar Cell Applications.
Minjie Zhou 1 , Haojun Zhu 1 , Quan Li 1
1 Department of Physics, the Chinese University of Hong Kong, Shatin, New Territories, Hong Kong China
Show AbstractThe ordered ZnO nanowire arrays on conducting substrates may serve as effective electrode for nanostructured solar cells (e.g dye or quantum dots sensitized solar cells). Nevertheless, the undoped ZnO could be highly resistive as its native defects (such as oxygen vacancies) are not efficient donors. In fact, the oxygen related defect centers are detrimental to the device performance by acting as non-radiative recombination centers for the photo generated electrons and holes. Substitutional doping of ZnO with group III elements can improve its conductivity and the Al doped ZnO thin film has been widely used in the Cu(InGa)Se2 thin film based solar cells. In the present study, we demonstrate the growth of Ga doped ZnO nanowire arrays on conducting substrates including high-doped Si and ITO. We have found that the Ga incorporation plays an important role in the resulted nanowire morphology, size, and optical properties. In particular, Ga doping at low concentration effectively suppresses the native oxygen defects in the ZnO nanowires, while higher concentration results in additional defect centers, which lead to defect luminescence at ~650 nm. The electrical behaviors of the doped nanowires are compared with the undoped samples based on the transport properties measured from a number of individual nanowires, from which the Ga doping level is semi-quantitatively analyzed.
9:00 PM - H5.20
Effect of Growth Conditions on Structural and Electrical Properties of Ga-doped ZnO Films Grown by Plasma-assisted MBE.
Vitaliy Avrutin 1 , Huiyong Liu 1 , Natalia Izyumskaya 1 , Hadis Morkoc 1
1 Electrical and Computer Engineering, Virginia Commonwealth University, Richmond, Virginia, United States
Show AbstractZnO heavily doped with group-III elements has recently attracted a considerable interest as a potential candidate for replacing conventional transparent oxides. Among the group-III elements, Ga is the most attractive dopant, since the Ga–O covalent bond length (1.91 Å) is much closer to that of Zn–O (1.97 Å) than In–O (2.1 Å) and Al–O (2.7 Å) [1]. Owing to its advanced control over the growth conditions and in situ monitoring capabilities, molecular-beam epitaxy (MBE) allows the insight into the effects of growth conditions on dopant incorporation and electrical properties of the resulting material, albeit it is not the most suitable technique for mass production. In this contribution, we report on the MBE growth of heavily Ga-doped ZnO films on a-plane sapphire substrates for transparent electrode applications. We studied the effects of Ga flux, substrate temperature, and oxygen pressure on the electrical and structural properties of the films as well as on their growth mode. The higher incorporation of Ga was observed for the films grown at higher oxygen pressure but the same temperature of Ga effusion cell. The pronounced effect of Ga flux on the growth mode and surface morphology has been observed. The increase in Ga flux led to the change from two-dimensional to three-dimensional growth mode and eventually to the polycrystalline material. The highest electron concentrations (10^20-cm^-3 range) and lowest resistivities (10^-4 –Ω cm range) were achieved in the films grown in the three-dimensional mode. Also almost two-fold increase in the growth rate was observed for the ZnO:Ga films grown under Ga fluxes corresponding to electron concentration above ~mid-10^19 cm^-3. This change in the growth rate can be tentatively attributed to the change from oxygen surface polarity typical for ZnO grown on sapphire to Zn polarity. It should be noted that the growth rates for Zn-polar ZnO is about 1.5 times than that for the O-polar material for the same Zn flux [2]. Another possible explanation is the increase in Zn sticking coefficient on the surface covered with a layer of Ga adatoms.1. W. B. Pearson, Crystal Chemistry and Physics of Metals and Alloys,Wiley, New York, 1972, p. 76.2. D.C. Oh, T. Suzuki, J.J. Kim, H. Makino, T. Hanada, M.W. Cho, T. Yao, J.S. Song, and H.J. Ko, J. Vac. Sci. Technol. B, 23, 1281 (2005).
9:00 PM - H5.21
Effect of Annealing on Structural and Optical Properties of Heavily Carbon-Doped ZnO.
He Huang 1 , Weiyang Jiang 1 , Elena Barbir 1 , Zhiwei Deng 1 , Michael Chen 1 , Dichen Li 1 , Simon Watkins 1
1 Department of Physics, Simon Fraser University, Burnaby, British Columbia, Canada
Show AbstractMetalorganic vapor phase epitaxy (MOVPE) is of high interest for growth of ZnO. Nevertheless, in contrast to III-V MOVPE, there have been remarkably few studies on the incorporation of carbon in this material. Carbon is expected to act as a double acceptor when residing on oxygen sites, and there have been reports of graphitic clusters in MOVPE grown ZnO, as well of some reports of ferromagnetism in intentionally carbon doped films. In this study we discuss the growth of ZnO using a flow modulation epitaxy technique in which the group II and VI precursors are alternately exposed to the growth surface. We show here that at low growth temperatures, in conjunction with certain precursors (diethlyzinc and N2O) remarkably high concentrations of carbon can be achieved without extrinsic dopant sources by this technique. In this work we report on the effect of thermal annealing on the structural and optical properties of heavily carbon-doped ZnO films. (101) pole figure scans show 6 well resolved peaks showing single crystal alignment for both as-grown and annealed films. Remarkable narrowing of the (002) x-ray rocking curves is observed upon annealing under air at temperatures between 800 and 1100oC, indicating significant structural rearrangement of the epilayers. Raman spectra show strong graphitic bands at 1341 and 1575 cm-1 in the as-grown samples. Annealing at 1000oC results in complete removal of the graphitic bands, and a strong increase in the intensity of the ZnO lattice phonon E1 mode at 435 cm-1. Annealing also greatly improves the optical transparency of the films. Films grown at low temperatures are generally brownish in appearance, correlating with the presence of large amounts of unintentional carbon. Annealing under air at 1000oC results in clear, transparent films, indicating the removal of the graphitic carbon observed in Raman measurements.
9:00 PM - H5.22
Controlled Annealing Induced Structure and Photoluminescence Property Evolution in Solution-processed Mg–alloyed ZnO Nanowires.
Paresh Shimpi 1 , Pu-Xian Gao 1
1 Material Science and Engineering, CMBE, IMS, University of Connecticut, Storrs, Connecticut, United States
Show AbstractSolution-processed Mg-alloyed ZnO nanowire arrays have been achieved recently without using high temperature annealing process. These nanowire arrays exhibited a blue-shifted ultraviolet (UV) near-band-edge (NBE) emission compared to ZnO nanowire arrays at room temperature. By introducing thermal annealing processes in oxygen-rich ambient condition, the UV NBE emission was mitigated until disappeared with the annealing temperature increasing from 400 °C to 800 °C. As the annealing temperature increased, the intensity of UV peak decreased while the intensity of visible peak (490-520 nm) increased. The structure evolution upon thermal annealing was revealed to be responsible for these abnormal photoluminescence properties variations. Furthermore, the nanoscale alloying localization in the nanowires was successfully kept intact through suppressing the slow surface diffusion using a rapid thermal annealing process. The successful nanoscale localized alloying in semiconductor nanowires could bring up new opportunities in optoelectronics, spintronics, and sensors.
9:00 PM - H5.23
A Study of the Effect of ZnO Content on the Transport Properties of Transparent Amorphous In2O3 - ZnO Thin Film Transistors.
Sunghwan Lee 1 , Jordan Chesin 1 , Hongsik Park 1 , Kathryn Schwink 1 , Julia Edel 1 , David Paine 1
1 Division of Engineering, Brown University, Providence, Rhode Island, United States
Show AbstractHigh performance (μ≈10-50 cm2/Vsec) amorphous IZO (In2O3-ZnO) thin film transistors (TFT’s) fabricated using room temperature processing have been demonstrated [(1),(2)]. Room temperature processing of the channel material allows the potential use of polymer-based substrates and the amorphous phase provides for planar interfaces (channel IZO rms roughness = ~3.6nm) and isotropic wet etch characteristics. The key to these devices is the stabilization of the amorphous phase by the presence of tetrahedrally coordinated Zn cations in the InO6–based amorphous network. In this study, we have explored the effect of Zn content on the performance of a-IZO TFT’s fabricated using a Si backgate metallization/SiO2 dielectric/a-IZO channel device. A series of amorphous IZO-based transparent TFT’s were fabricated using DC magnetron sputtering of custom fabricated sintered In2O3-ZnO targets containing 0, 2, 5, 7 and 10 wt% ZnO. This study examines the effect of varying Zn content in the deposited TFT channel layers on device characteristics such as threshold voltage, field effect mobility and on-off ratio. We report, for example, that increasing Zn content from 0 to 10 wt.% in the channel layers of identically prepared devices, shifts the threshold voltage from highly positive (39.1V) to negative (-3.2V) while increasing the field effect mobility from 1.5 to 20.4 cm2 / Vs, and on-off ratio of 1E4 to 1E8.References:(1) B. Yaglioglu, H. Y. Yeom, R. Beresford, and D. C. Paine, Appl. Phys. Lett. 89, 062103 (2006)(2) P. Barquinha, A. Pimentel, A. Marques, L. Pereira, R. Martins, E.J. Fortunato, J. Non-Cryst. Solids 352, 1749 (2006)
9:00 PM - H5.24
Effect of Aluminum Nitrate Concentrations in Zinc Acetate Precursor on ZnO:Al Thin Films Prepared by Spray Pyrolysis.
Samerkhae Jongthammanurak 1 , Sirirak Phakkeeree 2 , Yot Boontongkong 1 , Chanchana Thanachayanont 1
1 , National Metal and Materials Technology Center, Pathumthani Thailand, 2 Materials Engineering, Kasetsart University, Bangkok Thailand
Show AbstractZinc oxide films were deposited by spray pyrolysis using the solution of 0.05M zinc acetate in methanol. Different concentrations of aluminum nitrate were added into the zinc acetate solution, in making of the precursor for ZnO:Al film deposition. The undoped ZnO and ZnO:Al films were deposited at 430 degree Celsius for 150 minutes. X-Ray Diffraction (XRD) results show that the undoped ZnO and ZnO:Al films are polycrystalline with the hexagonal wurtzite structure. The (002) orientation is preferred in the undoped ZnO films, and the (101) orientation is preferred in ZnO:Al films. Images from Scanning Electron Microscope (SEM) are in agreement with the XRD data, showing that ZnO morphology changes from hexagonal-plate like grains densely packed in the undoped films to the more porous structure of the ZnO:Al films. Average crystallite size slightly increases with the addition of aluminium nitrate in the precursor solution. Thickness measurements by cross-section SEM show that the Al:ZnO films using the 2% aluminum nitrate containing precursor is half as thick as the undoped ZnO films. We postulate that the addition of the aluminum nitrate in the precursor solution has an effect of reducing the deposition rate of the growing films, which explains the observed trends of decreasing film thickness and increasing average crystallite size. We will also discuss electrical resistivity and thermoelectric properties of these films.
9:00 PM - H5.25
A Homoepitaxially Grown High-quality p-type N and Te Co-doped ZnO Film.
Seunghwan Park 1 2 , Tsutomu Minegishi 1 , Jisub Park 1 , Hyunjae Lee 1 , Toshinori Taish 3 , Ichiro Yonenaga 3 , Dongcheol Oh 4 1 , Mina Jung 5 , Jiho Chang 5 , Soonku Hong 6 , Takafumi Yao 1
1 , CIR, Tohoku University, Sendai Japan, 2 , National Institute for Materials Science, Tuskuba Japan, 3 , IMR, Tohoku University, Sendai Japan, 4 , Hoseo University, Ansan Korea (the Republic of), 5 , Korea Maritime University, Pusan Korea (the Republic of), 6 , Chungnam National University, Daejeon Korea (the Republic of)
Show AbstractZnO, with a wide-bandgap energy of 3.37 eV and a large exciton binding energy of 60 meV, has attracted a considerable attention as a promising material in the the optoelectronic applications of the UV region. However, low solubility of acceptor and self-compensation hampered its p-type conductivity. For the successful p-type conductivity control, high-quality ZnO film with high acceptor concentration is inevitably required. However, the realization of high crystal quality p-type ZnO is very difficult because nitrogen incorporation in ZnO film is usually accompanied by degradation of crystalline quality due to the formation of numerous structural defects. Therefore, new growth technique for high-quality p-ZnO layers with high acceptor concentration is necessary to achieve the mission. Tellurium atoms act as pseudo-donor, which can reduce the Madelung energy by co-doping with nitrogen atom. Therefore, it is expected to grow high-quality ZnO doped with high N concentration beyond the solubility limit. In this presentation, nitrogen and tellurium codoping is proposed as a new solution to achieve p-type ZnO films. Nitrogen doped ZnO film (ZnO:N) and nitrogen+tellurium codoped ZnO film (ZnO:[N+Te]) were prepared on (0001) ZnO substrate by plasma-assisted molecular beam epitaxy. These are compared with un-doped ZnO (u-ZnO) in terms of optical, structural and electrical properitesZnO:N showed the n-type conductivity with an electron concentration of 2.5×1017 cm-3, which was greater than that of u-ZnO (4×1016 cm-3). On the other hand, ZnO:[N+Te] showed reliable p-type conductivity with a hole concentration in the range of 1~4×1016 cm-3, even though high acceptor concentration of ~1021cm-3 was obtained. Photoluminescence spectra measured at 10 K revealed that the n-type ZnO:N film exhibited only broad and weak donor-acceptor pair emission, while the p-type ZnO:[N+Te] films have predominantly strong acceptor-bound excitons (AoX). Especially, narrow acceptor bound exciton (AoX ~ 1.2 meV) and weak deep level emission prove the considerable improvement of optical properties, in comparison with those of previously reported ZnO:N. Furthermore, in compare to ZnO:N, ZnO:[Te+N] revealed superior properties such as narrow x-ray rocking curve line width (30 arcsec), smooth surface morphology (RMS~1.0 nm). Such behaviors are attributed to the decrease of Madelung energy of ZnO by incorporation of Te, which increases the solubility and decreases the compensation ratio of acceptors.In summary, a high crystal quality p-type ZnO with high acceptor concentration has been achieved by co-doping of nitrogen and tellurium.
9:00 PM - H5.26
Comparative Study of Wet Chemical Etching of ZnO, IZO and IGZO.
Jun Young Kim 1 , Jae-Kwan Kim 1 , Seung-Cheol Han 1 , Joon Seop Kwak 1 , Han-Ki Kim 2 , Ji-Myon Lee 1
1 Materials Science and Metallurgical Engineering, Sunchon National University, Sunchon, Chonnam, Korea (the Republic of), 2 Display Material Engineering, Kyung Hee University, Yongin-si, Gyeonggi-do, Korea (the Republic of)
Show AbstractTransparent conductive oxides (TCOs) were applied in many areas, such as the transparent electrodes used in liquid crystal displays, solar cells, and light emitting diodes because of their high electrical conductivity and high optical transparency. Recently, oxide TFTs has attracted much attention for backplanes of new transparent displays. There have been a number of oxide TFT studies using ZnO, In-Zn-O (IZO), and In-Ga-Zn-O (IGZO) as active channel material. It is very important to develop well-defined etching process of various transparent oxides for applications of such devices, because thickness of channel layer is less than a few tens of nanometers. In this study, etch characteristics such as etch rate and etched surface morphology of ZnO, IZO, and IGZO were investigated by using various acidic solutions.ZnO, IZO, and IGZO with thickness of 300, 350, and 600 nm, were grown on sapphire, glass, and Si(001) substrate by radio-frequency magnetron sputtering method, respectively. Wet-etching experiments were performed by various acidic solutions such as H2SO4, formic acid, and oxalic acid. Etch rates were determined by using surface profilometer by averaging each 10 points in the samples. Surface morphologies and compositions of etched sample were characterized by using FE-SEM, AFM and EDX.In this study, we have found that the ZnO and IGZO could be easily etched by using H2SO4 aqueous solutions. However, it was also observed that the IZO could not be easily etched by H2SO4. Considering the fact that Zn-based oxide can be etched too easily by using conventional acidic solutions such as HCl and H2SO4, it is very interesting that the IZO is resistive to the etching by acid solution, indicating that every Zn-based oxide cannot be etched by acidic solutions. The etch rates for ZnO and IGZO were measured to be about 290 nm/min and 20 nm/min, respectively, under the condition of 0.1 mole H2SO4 at room temperature. While a high density of etch-residue was observed even after the over-etching time 5 min for IGZO, residue was not observed after etching of ZnO. Furthermore, although both ZnO and IGZO were easily etched by using 0.1 mole of oxalic acid, IGZO was not etched by less than 0.1 mole of oxalic acid, indicating that the etch-mechanism of ZnO and IGZO might be different from each other. In addition, it was also found that sidewall morphology of etched sample was different. Furthermore, we will also present etch mechanism and surface morphology of ZnO, IZO, and IGZO.
9:00 PM - H5.27
Catalyst Free MOCVD Growth of ZnO Nanorods: Proposal for a Growth Mechanism.
Robin Thierry 1 , Guillaume Perillat-Merceroz 1 , Milan Rosina 1 , Matthieu Lafossas 1 , Pierre Ferret 1 , Pierre-Henri Jouneau 2 , Francois Levy 1 , Guy Feuillet 1
1 LETI Minatec, CEA, Grenoble France, 2 INAC, CEA, Grenoble France
Show AbstractZnO is a promising material for the fabrication of light emitting devices. One approach to achieve this goal is to use ZnO nanorods because of their expected high crystalline and optical quality. Catalyst free growth of nanorods by Metalorganic Chemical Vapour Deposition (MOCVD) was carried out on (0001) sapphire substrates. Arrays of well-aligned, vertical nanorods were obtained with uniform lengths and diameters.Scanning electron microscopy (SEM) has been used to observe the morphology of the ZnO nanorods and X-ray diffraction and transmission electron microscopy (TEM) to investigate the crystalline structure of the nanorods. TEM observation as well as photoluminescence measurements confirm the very good crystalline quality of the nanorods. Samples with various deposition times have been studied to understand the growth mechanism. A thin wetting layer in epitaxy with the sapphire substrate is formed together with nanorods and pyramids located at their basis. Nanorods are of Zn polarity, their surrounding pyramids of O polarity, and the lateral facets of the hexagonal nanorods are {1-100}, suggesting that surface energies play a role in the nanorod growth. The in-plane mismatch between the sapphire substrate and ZnO is relaxed by dislocations. Most of them lie in the wetting layer, with only a few in the pyramids and none in the nanorods. A growth mechanism is proposed to explain the morphology of the growth and the arrangement of the dislocations.
9:00 PM - H5.29
Growing Zn0.90Co0.10O Diluted Magnetic Semiconductors by r. f. Sputtering System.
Musa Can 1 , Tezer Firat 1 , Sadan Ozcan 1
1 Physics Engineering, Hacettepe University, Ankara Turkey
Show AbstractIn this work, Zn0.90Co0.10O diluted magnetic semiconductors (DMS) synthesized by mechanical milling and thermal treatment. The formation of solid state reaction was monitored by differential thermal and thermo gravimetric methods (DT-TGA). Substitution of Co+2 ions with Zn+2 host atoms in ZnO lattice had been analyzed by using X-Ray Diffractometer (XRD), Fourier Transform Infrared (FT-IR) and X-Ray Photo Spectroscopy (XPS). Additionally, the impurity phases such as CoO, Co3O4 and ZnCo2O4 or Co clusters have not been detected in the samples. However, in the samples, the only detected impurity is tungsten (W) impurity from the mortar of the mill was observed which have been as ZnWO4 in sample.The prepared Zn0.90Co0.10O semiconductors pressed under force of 25ton to form 2inç pellet. Then, by using this target, thin films were grown on (0001) ordered sapphire (α-Al2O3) with 30W, 60W and 120W r. f. powers. The structural analyses were done by XRD, Energy Dispersive X-Ray Spectrometry (EDS) and XPS. After analysis, XRD results have been proved that the grown Zn0.90Co0.10O thin films are in form of ZnO single crystal structure with (0002) ordered, which is coherent to (0001) ordered α-Al2O3. Additionally, XRD patterns have proved that Zn0.90Co0.10O thin films were grown without impurity phases, such as Co cluster, CoO and Co3O4. Substituted Co atoms in host ZnO matrix have been understood due to binding energy peak of 781.3±0.4eV for Co2p3/2 and energy difference of ~15.61±0.03eV between Co2p1/2 and Co2p3/2. These results have also shown that there are no Co clusters or Co3O4 phases in the lattice. And, the homogeneity of Co atoms in the lattice has been shown by EDS spectrums. It is observed that the increase r. f. power to 120W causes homogeneity of Co and Zn atoms in thin film as %8.1±0.1 and %91.7±0.7 for Zn0.90Co0.10O target, respectively.
9:00 PM - H5.3
Correlation Between Residual Stresses and Defects in ZnO Thin Films Deposited by Magnetron Sputtering on Si.
Florine Conchon 1 , Pierre-Olivier Renault 1 , Philippe Goudeau 1 , Eric Le Bourhis 1 , Elin Sondergard 2 , Etienne Barthel 2 , Sergey Grachev 2 , Eric Gouardes 3 , Veronique Rondeau 3 , Rene Gy 3 , Remy Lazzari 4 , Jacques Jupille 4 , Nathalie Brun 5
1 Laboratoire PHYMAT UMR 6630 CNRS, Universite de Poitiers, Futuroscope-chasseneuil Cedex France, 2 SVI - UMR 125 , Saint-Gobain / CNRS, Aubervilliers France, 3 SGR, Saint-Gobain, Aubervilliers France, 4 , INSP - UMR 7588 , Paris France, 5 , LPS - UMR 8502 , Orsay France
Show AbstractZnO is a material of technological importance for, among other things, its practical and potential applications for short wavelength optoelectronic devices and transparent conductive oxide films, such as in UV-lasers, blue to UV light-emitting diodes and solar cells electrodes. This oxide is also used in glass stacking as a UV spectrum filter and can provide other promising technological applications thanks to its adjusting photoluminescence properties. Unfortunately, large compressive intrinsic stresses arising during the deposition process can induce changes of the physical properties and promote delamination or failures of the ZnO films. The control of their mechanical reliability can then be achieved by an in depth comprehension of the residual stresses build up or relaxation mechanisms occurring in the films in relation with the material structure.In this communication, we will show that the residual stresses are strongly correlated to the structure of the ZnO films. In particular, we will demonstrate with the aid of X-ray diffraction (sin2Ψ analysis) and in-situ curvature measurements that residual compressive stresses can be relieved by tuning the point defects concentration of the films. This can be achieved by adjusting the sputter-deposition parameters such as the oxygen partial pressure and the annealing temperature/atmosphere. The role played by protective Si3N4 coatings encapsulating the ZnO films will also be assessed. This work is done in the framework of an ANR project named Merethif.
9:00 PM - H5.30
Positions of Dopant Atoms in ZnO Determined by X-ray Standing Waves and X-ray High Energy Photoemission Methods.
B. Walz 1 , M. Tolkiehn 1 , S. Thiess 1 , J. Dumont 2 , M. Cubaka 2 , D. Novikov 1
1 HASYLAB, Deutsches Elektronensynchrotron DESY, Hamburg Germany, 2 Research Centre in Physics of Matter and Radiation, University of Namur, Namur Belgium
Show AbstractX-ray standing waves (XSW) is a method for direct determination of element-specific structural information in single crystals [1]. We are using its recent extention, kinematical XSW, for investigation of dopant metal atoms positions in zinc oxide single crystals.The position of impurity atoms was studied both for as-grown samples and for samples after thermal diffusion of Mn and Co from the surface. It is shown, that for a wide variety of metals the impurity atoms occupy the Zn position in the unit cell. The results are also supported by a direct comparison with high-energy X-ray photoelectron spectroscopy data [2]. The obtained results can be used for better understanding of ferromagnetic properties of ZnO-based materials. They can also give some insight into the critical technological issues, including control of oxygen stoichiometry and hetero-epitaxial structure growth [3,4].[1] X-Ray Standing Wave Technique - Principles and pplications. Ed. A.Kazimirov,J.Zegenhagen, World Scientific Publ. (2008)[2] J.Dumont et.al, HASYLAB Annual Report, 623 (2007)[3] Ü. Özgür et.al, Appl.Phys.Rev. 98, 041301 (2005)[4] R.Ramesh, D.G.Schlom Mater.Res.Bull. 33, 1006 (2008)
9:00 PM - H5.32
Effects of Growth Parameters on Surface-morphological, Structural, Electrical and Optical Properties of AZO Films by RF Magnetron Sputtering.
Shou-Yi Kuo 1 , Ming-Jer Jeng 1 , Liann-Be Chang 1 , Wei-Ting Lin 1 , Shng-Cheng Hu 2 , Yung-Tien Lu 2 , Ching-Wen Wu 2
1 Electronic Engineering , Chang Gung University, Tao-Yuan Taiwan, 2 Chemical Systems Research Division, Chung-Shan institute of Science & Technology, Tao-Yuan Taiwan
Show Abstract500 nm-thick aluminum-doped zinc oxide (ZnO:Al) thin film is usually used as front transparent conductive oxide (TCO) contact in CIGS2 thin film solar cells, and for this application is often deposited by reactive radio-frequency (RF) magnetron sputtering system from a ceramic target. This work reports on the prepared and characterization of AZO thin films on Corning 1737 glass substrate grown by reactive rf-magnetron sputtering from ZnO ceramic target with 2 wt% Al content. It was found that the growth parameters, such as chamber pressure, working power, and substrate temperature, have significant influences on properties of AZO films. The structural property, surface morphology and element of AZO thin films were analyzed by x-ray diffraction (XRD), atomic force microscope (AFM), field emission scanning electron microscopy (FESEM), and energy dispersive spectrometer (EDS). The optical properties were analyzed by ultraviolet-visible spectrometer. The electrical properties were measured by Hall Measurement system. According to the experimental results : (1) Films were polycrystalline showing a strong preferred c-axis orientation (002). (2) As the argon flow rate was decreased, the resistivity of AZO film was decrease, and the transmittances were decreased from 92% to 87.5% in the visible wavelength range. (3) The working power were increased, the resistivity values were decreased while showing higher mobility and the carrier concentration. (4) Lower substrate temperature leads to a decrease on resistivity, and the transmittance and lowest resistivity values are 81% and 2.5×10-4Ω-cm. Our experiment results could lead to better understanding for improving further CIGS-based photovoltaic devices and would be used on low temperature process devices.
9:00 PM - H5.33
Ab intio Investigations of Lattice Parameters in MgZnO Alloys.
Markus Heinemann 1 , Christian Heiliger 1
1 I. Physikalisches Institut, Justus Liebig University, Giessen Germany
Show AbstractWe present first principle calculations of MgxZn1-xO alloys. In particular, we calculate the equilibrium lattice parameters in wurtzite crystal structure as a function of Mg concentration. A pseudopotential method based on density functional theory is used to relax the structure to equilibrium. Different concentrations of Mg are described within a supercell approach averaging over a large number of different configurations. For the equilibrium structures we analyze the size of the band gap as a function of the Mg concentration. We discuss the influence of different exchange correlation potentials on the equilibrium lattice parameters as well as on the size of the band gap. Finally, we compare our results to measurements.
9:00 PM - H5.34
Electrodeposition and Characterization of ZnO Films on TCO Films.
Aparna Prabhakar 1 , Hariklia Deligianni 1 , Lubomyr Romankiw 1 , Supratik Guha 1
1 , IBM, Yorktown Heights, New York, United States
Show AbstractZinc oxide is a commonly used transparent conducting electrode (TCE) for thin film solar cells. It is however, mostly deposited using vacuum based processes. A significant advantage in production cost would arise if solution processing techniques, such as electrodeposition, could be developed for ZnO TCEs. In this paper we have investigated the effect of varying electrodeposition conditions on the quality of electroplated ZnO films on ~400 nm indium-tin-oxide (ITO) coated glass substrates. This includes variations in electrolyte compositions, pH and temperature. The films were investigated by X-ray diffraction, scanning electron microscopy, and optical transmittivity measurements. We will describe results from three chemical bath compositions: (a) Zn(NO3)2 6H2O with KCl (0.1M) as supporting electrolyte; (b), Zn(NO3)2 6H2O with KNO3 (0.1 M) as supporting electrolyte; (c), ZnCl2 with KNO3 (0.1M) as supporting electrolyte. It was observed that the current density in KNO3 based solutions declined over time due to instability of the nitrate at high temperatures greater than 50 C, leading to non-uniform films. Uniform films were achieved with the first solution (Solution a) with the concentration of Zn(NO3)2 6H2O varied from 0.5mM to 0.4M. At each concentration, CV curves were used to establish the desired deposition conditions. Desirable films, with no hydrochloride compounds, were obtained at Zn2+ concentrations < 0.1M . The highest transmissivity (~ 50%) was obtained at 0.01 M Zn(NO3)2 6H2O deposited at -1.3 V, 75 C, in comparison to transmissivity of 80% for the starting substrates. X-ray diffraction indicated the presence of both (wurtzite) ZnO and potassium zinc oxide, (K7Zn6O7, ) The ongoing work includes approaches for eliminating the potassium zinc oxide phase by exploring other electrolyte compositions, and the introduction of dopants into the plated ZnO.
9:00 PM - H5.35
Electrochemical Deposition of Pure and Doped Nanostructured ZnO Thin Films.
Elena Matei 1 , Nicoleta Preda 1 , Lucian Ion 2 , Stefan Antohe 2 , Ionut Enculescu 1
1 , National Institute of Materials Physics, Magurele, Ilfov, Romania, 2 , University of Bucharest, Bucharest Romania
Show AbstractElectrochemical deposition represents a low costs, highly scalable method to prepare ZnO films on conductive substrates. The methods allow the fabrication of both pure and doped films.We present in this paper our results regarding the fabrication of pure and transition metal doped thin films grown on platinum and on glass/ITO substrates from a zinc nitrate bath. Our focus was on how preparation conditions influence the properties of the ZnO deposits. During our experiments we varied the composition of the electrochemical bath components, the temperature and the deposition potential. We measured optical properties (transmission, reflection and luminescence spectroscopy), morphological and compositional properties, structural and transport properties. We found the conditions to fabricate either compact smooth films or nanostructured layers.A wide range of applications can be pursued for such films, the main advantage of the preparation method being its scalabbility.
9:00 PM - H5.36
Deep Levels in Ni Doped ZnO Thin Films.
Matthias Schmidt 1 , Kerstin Brachwitz 1 , Martin Ellguth 1 , Matthias Brandt 1 , Michael Lorenz 1 , Holger von Wenckstern 1 , Rainer Pickenhain 1 , Marius Grundmann 1
1 Semiconductor Physics Group, Universtät Leipzig, Leipzig, Sachsen, Germany
Show AbstractThe transparent semiconductor zinc oxide bears enormous potential regarding the production of transparent electronics and photonic devices operating in the near UV - range. Although a variety of defects investigated by different methods has been reported, the microscopic origin is uncertain or unknown in most of the cases.In our present study, we investigated deep levels present in a nickel doped ZnO thin film and a nominally undoped reference sample. Various capacitance spectroscopic methods, including capacitance voltage spectroscopy (C - V), thermal admittance spectroscopy (TAS), and deep level transient spectroscopy (DLTS) were used to characterise the samples.We employed the pulsed laser deposition technique for the growth of the ZnO thin films on a - plane sapphire substrates. For the Ni doped sample the target contained 20 ppm NiO. The Schottky contacts, essential for the capacitance spectroscopic measurements, were realised by reactively sputtered palladium, the ohmic back contacts by a aluminum doped ZnO layer showing metallic conduction. From C - V measurements the net - doping concentration is approximated to 4×1015 cm-3 in the Ni doped sample, while in the reference it was approx. 1.5×1016 cm-3. The DLTS measurements revealed the existence of a deep level, in the following labelled T4, in the Ni doped as well as in the reference sample. Whereas the concentration of T4 in the reference was close to the detection limit, its concentration in the Ni doped sample was almost equal to the net - doping concentration of the sample. We employed the double DLTS technique in order to determine the electric field dependence of the activation energy and the apparent electron capture cross - section T4. The mean activation energy of T4 amounts approx. 515 meV. We additionally performed TAS measurements also revealing T4 in the Ni doped sample. From standard Arrhenius analysis the activation energy is estimated to be 500 meV.
9:00 PM - H5.37
Conductive ZnO Nanostructures for Photovoltaic Applications.
Ralf Wehrspohn 1 , Martin Otto 1 , Johannes Uepping 1
1 Institute of Physics Institute of Physics, µMD, Martin-Luther-University Halle-Wittenberg, Halle Germany
Show AbstractConductive ZnO nanostucutures for photovoltaic applications toward first prototypes have been investigated. Aluminum doped zinc oxide (AZO) deposited via ALD was used for 3d opal inversion and as a conformal cover layer for plasma etched black silicon. Both structures can be incorporated into photovoltaic solar cells either as intermediate reflector layer in tandem cells or as transparent conductive oxide, respectively. The material conductivity is one critical property for both applications. Therefore, ZnO with different Al doping levels is studied using conductivity and electrical impedance spectroscopy measurements. Additionally, the effect of distinct illumination conditions is investigated. We show a significant enhancement of conductivity within the AZO in comparison to stoichiometric ZnO.
9:00 PM - H5.38
Dopant-induced Phase Separations in Mn-doped ZnO.
L. Saraf 1 , M. Engelhard 1 , P. Nachimuthu 1 , D. Baer 1
1 Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington, United States
Show AbstractThe overall importance of doped and updoped ZnO is quite clear due to its multi-functionality and several areas of wide band gap semiconductor related applications. Mn-ZnO system is also of interest because the ionic radius of Mn and Zn in the two plus valance state are comparable. It has been observed that existence of Mn2+ and related ordering of magnetic moments facilitates ferromagnetism in ZnMnO. It is crucial to synthesize phase pure ZnMnO because existence of secondary phases create some controversy in the way the data is interpreted. The stabilization and analysis of pure ZnMnO3 spinel phase is underrepresented in the literature. Such detailed analysis is needed due to immediate occurrence of this phase after the Mn-dopant reaches its saturation in the wurtzite structure form. Here we discuss in details the stabilization of ZnMnO3 phase and compare spectroscopic and structural data with widely observed and stable ZnO as well as ZnMn2O4 phases.
9:00 PM - H5.39
Electrically Air-stable ZnO Thin Film Produced by Reactive RF Magnetron Sputtering for Thin Film Transistors Applications.
Divine Ngwashi 1 , Richard B. Cross 1 , Shashi Paul 1
1 EMTERC, De Montfort University, Leicester United Kingdom
Show AbstractWide-bandgap semiconducting II-VI oxides are an important class of materials in optoelectronics devices. Zinc oxide (ZnO), with a bandgap of ~3.4 eV, is one such material, which when produced at room temperature, is compatible with cheap flexible and plastic substrates. However, its physical properties are strongly dependent on the deposition parameters and post-deposition treatments such as annealing, O2 gas and plasma treatment [1-3]. For example, its electrical resistivity is mainly determined by thickness, impurity distribution, grain size and grain boundaries; all of which are affected by growth/post-growth conditions. ZnO produced by a variety of methods has shown great potential for thin film transistors (TFTs), gas and chemical sensors, and for conducting electrodes in photovoltaics. Moreover, as it occurs mainly in polycrystalline form, higher carrier mobilities than hydrogenated amorphous silicon (a-Si:H) can be attained [4]. In all these applications, however, reliability and stability remain challenging issues. Electrical conduction of ZnO thin films is often found to degrade drastically in air or an oxygen environment. For TFT applications, attempts have been made to solve this instability problem by using suitable passivation layers [5], which could in turn affect the transparency of the whole structure.In this paper, we report that native defect control in ZnO growth by reactive RF magnetron sputtering can produce electrically air-stable films. A ZnO target (99.999 %) is used at a substrate sputtering distance of 6 cm. The film stoichiometry is controlled via the RF power and the O2/Ar gas ratio. The films were then post-treated in 0.2 mTorr O2 atmosphere for 1 hour at room temperature. The electrical (current-voltage (I-V)) and optical properties (ultra-violet/Visible spectroscopy (UV-Vis)) of the films as-grown and after 18 months exposure to air were investigated to determine the effects of deposition parameters and/or O2 post-treatment on film stability. The feasibility of incorporating these films into TFTs will also be presented References:[1] L. Mingjiao and K. Hong Koo, Applied Physics Letters 84, 173-5 (2004).[2] P. Won Il, K. Jin Suk, Y. Gyu-Chul, M. H. Bae, and H. J. Lee, Applied Physics Letters 85, 5052-4 (2004).[3] H. Woong-Ki, J. Gunho, K. Soon-Shin, S. Sunghoon, and L. Takhee, IEEE Transactions on Electron Devices 55, 3020-9 (2008).[4] R. Martins, Barquinha, P., Ferreira, I., Pereira, L., Gonalves, G., Fortunato, E. , Journal of Applied Physics 101, 044505 (2007).[5] W.-K. Hong, Kim, B.-J., Kim, T.-W., Jo, G., Song, S., Kwon, S.-S., Yoon, A., Lee, T. , Colloids and Surfaces A: Physicochemical and Engineering Aspects 313-314 (2008).
9:00 PM - H5.40
Growth of ZnS/ZnO Heterojunction Nanorods using ZnOS Buffer Layer.
Tomomasa Satoh 1 , Hiroaki Koishikawa 1 , Takaki Kaneshiro 1 , Makoto Yugi 1 , Takashi Hirate 1
1 Fuculty of Engineering, Kanagawa University, Yokohama Japan
Show Abstract One-dimensional(1-D) nanostructures such as nanowires, nonorods and nanobelts are very exciting and attractive materials for many electrical and optical applications. In particular, 1-D nanostructures of II-VI semiconductors such as ZnO and ZnS have large interest because of direct band gaps and comparatively easy controlling of growth morphology of their 1-D nanostructures. In addition, ZnS/ZnO heterojunction nanostructures with epitaxial growth are required to expand application range. Both ZnO and ZnS have the wurtzite crystal structure, although ZnS has also the zinc blend structure. The epitaxial growth of ZnS/ZnO is generally difficult due to a large lattice mismatch. In this study, using a ZnOS alloy layer by a unique growth method, we have successfully fabricated ZnS on ZnO nanorods mainly in the c-axis direction of ZnO nanorods at a low ZnS growth temperature to avoid sulfidation of ZnO. Firstly, ZnO nanorods were synthesized on an n-type Si wafer by a low pressure thermal CVD method combined with a laser ablation of Mn catalyst. The CVD precursors to synthesize ZnO nanorods were metal Zn vapor and O2. The growth temperature and growth time were 500 °C and 15 min, respectively. Mn target was laser-ablated with a pulsed Nd-YAG laser (λ=1.064 nm) during the initial stage (3 min) of the CVD growth of ZnO nanorods. Then, a ZnOS layer was synthesized on the ZnO nanorods by a CVD method using metal Zn vapor and O2 combined with a laser ablation of a single crystal target of ZnS. The growth temperature and growth time were 450 °C and 15 min, respectively. The laser ablation of ZnS was carried out all over the CVD growth time. Finally, a ZnS layer was synthesized on the ZnOS/ZnO nanorods by only a CVD method using metal Zn vapor and H2S. The growth temperature and growth time were 400 °C and 20 min, respectively. The ZnO nanorods had the shape of hexagon prism and flat tip, and were vertically well-aligned on the substrate. The diameter and length were ~50 nm and ~1.0 μm, respectively. The ZnO (1-x)S(x) layer with a composition of x=0.08 was obtained, and the thickness was 30 nm. A XRD analysis showed that after the growth of ZnOS and ZnS, the diffraction peaks of ZnO remain unchanged without sulfidation, and that the growth of ZnS has been obtained at such a low growth temperature. The ZnS growth thickness in the c-axis direction was ~60 nm. On the other hand, the lateral ZnS growth thickness was less than 10nm. In addition, the top and side surfaces of the ZnS/ZnO (0.92)S(0.08)/ZnO nanorods were very smooth, although those of the ZnS/ZnO nanorods without the ZnOS buffer layer were quite rough. These results suggest that in spite of such a low S concentration in the ZnOS, the ZnO (0.92)S(0.08) buffer layer have considerable effects on ZnS growth on ZnO nanorods, and that the ZnS growth on the ZnO nanorods using the ZnOS buffer layer could be epitaxial.
9:00 PM - H5.41
Current Transport Mechanisms for MSM-Photodetectors on ZnO:N Thin Films.
Tingfang Yen 1 , Alan Haungs 1 , Sung Jin Kim 1 , Alexander Cartwright 1 , Wayne Anderson 1
1 Electrical Engineering, University at Buffalo, The State University of New York, Buffalo, New York, United States
Show AbstractUltraviolet photodetectors (UV-PDs) operating in the short wavelength are important devices in various commercial and military applications. Most UV-PDs have been silicon-based. However, due to the indirect bandgap of 1.2 eV for Si, it requires bulky band filters to block solar radiation background and it is intolerant of high temperature and caustic environments1. ZnO is one of the potential materials to eliminate those disadvantages, and has been of interest in optical electronics applications for a few years due to its large exciton binding energy, wide bandgap, and some advantages over GaN, such as low cost of bulk single crystals, lower growth temperature, tolerance to high energy radiation, and ease of etching in most acids and alkalis.In this paper, we report on metal-semiconductor-metal photodetectors (MSM-PDs) on ZnO thin films with a focus on analysis of I-V-T current transport mechanisms. ZnO thin films were deposited by rf sputtering and doped by nitrogen ion implantation with post annealing by rapid thermal annealing (RTA). MSM-PDs were fabricated on the ZnO/Si structure. A 10 times reduction in dark current was observed compared to the devices on as-deposited ZnO. The performance of MSM-PDs gave a 2030 photo to dark current ratio and responsivity (R) = 2.7 A/W. The pulse response was 12.3 ns rise time and 15.1 ns fall time.Temperature-dependent I-V characteristics of the MSM-PDs were observed to determine the current transport mechanisms. Most current shows a non-exponential dependence of I on V, thus space charge limited current (SCLC) theory was applied. In the SCLC region, J~Vmgave m to determine the current transport mechanism. Thus, in the region of SCLC, we evaluated the temperature-dependence of the slope of the logI-logV curve. The dark current region can be separated into three regions where current transport changes with temperature. Two slopes were observed at 400K, both of which gave m <1, which might indicate SCLC dominated by recombinative injection2. Three slopes were consistently observed at temperatures from 200K to 350K: in the lower bias region, m was around 2.5, which indicates SCLC in the mobility regime with exponential distribution of trapping levels; m <1 in the middle voltage region; and m close to 1.5 at 300K and 350K in the higher bias region, which indicates SCLC in the ballistic regime. Photo current showed two current transport mechanisms: m > 2 in the lower bias region and m close to 0.5 in the higher bias region. Current transport is governed by the ZnO structure rather than the electrodes.
9:00 PM - H5.42
Appropriate Choice of Channel Ratio in Thin-film Transistors for the Exact Determination of Field-effect Mobility.
Koshi Okamura 1 , Donna Nikolova 1 , Norman Mechau 1 , Horst Hahn 1
1 Institute of Nanotechnology, Forschungszentrum Karlsruhe, Karlsruhe Germany
Show AbstractThin-film transistors (TFTs) fabricated in low-temperature and high throughput processes are in great demand for low-cost and large-area productions. For the evaluation of any kind of semiconducting materials for TFTs, the most important figure of merit is field-effect mobility. However, the field-effect mobility is sometimes extracted from the TFTs with the active semiconductor area undefined and in the geometry of the small channel ratio; the effect of the fringing electric field at the ends of source/drain electrodes are not taken into account. Therefore, the effect and the magnitude of the fringing electric field on the field-effect mobility are systematically investigated.ZnO nanoparticles were dispersed in 2-methoxyethanol with a stabilizer by a homemade dispersing equipment. TFTs in the bottom gate configuration were fabricated from the suspension, consisting of a Si substrate, a 200-nm-thick SiO2 layer, a spin-coated nanoparticulate ZnO layer and Al source/drain electrodes. The Al electrodes were deposited through a shadow mask to be in different channel ratios, such as 2.5, 5.5, 12, 32 and 70. The field-effect mobilities were extracted from the transfer characteristics in the saturation regime, and were evaluated as a parameter of the channel ratio. The field-effect mobility extracted from TFTs, with the active ZnO area undefined, at the small channel ratio of 2.5 showed the value by 418 % overestimated. In contrast, the field-effect mobility extracted from TFTs, with the active area defined, at the large channel ratio of 70 was nearly equivalent to the real value within the margin of error of 4 %. These results reveal that the active semiconductor area of TFTs should be defined for the exact determination of the field-effect mobility; otherwise, the channel ratio should be large enough to neglect the effect of the fringing electric field.
9:00 PM - H5.44
Effect of Oxygen Flow on the Synthesis of Zinc Oxide Nanomaterials.
Subhash Singh 1 , Raj Swarnkar 1 , Ram Gopal 1
1 Physics, Allahabad University, Allahabad, U.P., India
Show AbstractBinary semiconductor oxides such as ZnO, CdO, SnO2 and In2O3 have innumerable applications and are now widely used as transparent conductive oxides (TCOs). Zinc oxide nanoparticles are synthesized by pulsed laser ablation of Zinc metal in double distilled water with and without simultaneous flow of oxygen in the closed vicinity of produced plasma. Colloidal nanoparticles synthesized are found stable for several weeks. Nanoparticles synthesized by laser ablation with simultaneous flow of oxygen has smaller ratio of zinc hydroxide/zinc oxide as compared to that synthesize in the absence of oxygen flow. Instead of this particles synthesized in presence of oxygen flow are smaller in size and narrower distribution. The sample is characterized by using UV-visible absorption, scanning electron microscopy (SEM), thermo gravimetric analysis (TGA), Differential thermal analysis (DTA), X-ray diffraction (XRD) and Fourier transform infrared (FTIR) techniques
9:00 PM - H5.46
Comparison of APCVD to LPCVD Processes in the Manufacture of ZnO TCO for Solar Applications.
Wei Zhang 1 , T. Salagaj 1 , C. Jensen 1 , K. Strobl 1
1 CVD Applications Laboratory, CVD Equipment Corporation, Ronkonkoma, New York, United States
Show AbstractA 3” horizontal tube furnace, FirstNano EasyTube 3000 system, was used to investigate the optimization of both APCVD (Atmospheric Pressure Chemical Vapor Deposition) and LPCVD (Low Pressure Chemical Vapor Deposition) processes to grow both boron and fluorine doped ZnO films with a resistivity, sheet resistance and haze suitable for their potential utilizations as TCO (Transparent Conductive Oxide) layers for photovoltaic applications. Growth rates as high as 100 nm/min have been obtained in some parameter regions for both processes. In both cases the resulting material property parameters were the same or better than reported in the literature. Although the horizontal hot wall CVD R&D reactor is not optimum for uniform TCO thin film deposition it allowed us to investigate the interrelationship of the most critical parameters with the resulting material properties.The driving force for this work is the ultimate goal of demonstrating a process parameter solution suggesting that ZnO films (usable for either display system manufacturing and/or photovoltaic applications) can be deposited with optimized material properties that are comparable to LPCVD or sputtering processes, but that the APCVD solution could be more economical for large scale thin film ZnO coating implementation. Ultimately our desire is to transfer such a ZnO deposition process to our proprietary, APCVD CVDgCoat™ platform which has the ability to coat up to 4 m wide glass sheets and metal foils.
9:00 PM - H5.47
Electrical Properties and Growth of ZnO Nanoparticles in Humid Atmosphere.
Sonja Hartner 1 , Moazzam Ali 2 , Markus Winterer 2 3 , Hartmut Wiggers 1 3
1 Institute of Combustion and Gasdynamics, University of Duisburg-Essen, Duisburg, NRW, Germany, 2 Nanoparticle Process Technology, University of duisburg-Essen, Duisburg, NRW, Germany, 3 CeNIDE, Center for Nanointegration Duisburg-Essen, Duisburg, NRW, Germany
Show AbstractZinc oxide (ZnO) is a II-VI wide and direct band gap semiconductor and has shown promising results as an inexpensive alternative for transparent conductive materials which are used for flat panel displays, solar cells, varistors and sensors. Especially printing technologies based on nanoparticles containing inks and pastes are a promising way to cheap and large area functional structures. In our experiments we investigated the electrical properties of ZnO nanoparticles in humid atmosphere by impedance spectroscopy at temperatures ranging from 323 to 673 K. Gas compositions with different concentration of water were realized by conveying argon trough a water bubbler. The amount of water vapor was measured by means of a heated quadrupol mass spectrometer. ZnO was produced by gas phase synthesis in a hot wall reactor and the as-prepared powder was pressed into pellets with a diameter of 5 mm, and a green body density of about 60%.The impedance measurements of the ZnO nanoparticles show typical semiconductor behavior and changing electrical properties depending on the level of humidity. By increasing the concentration of water in the gas phase, the conductivity of the pressed ZnO pellets irreversibly increases by a factor of two. A detailed microscopic investigation of the measured samples revealed that the particles with an initial grain diameter of 18 nm grew in size achieving a saturation level at about 28-30 nm. The particle growth can be explained by Kelvin’s effect resulting in particles with a reduced amount of curvature, and the amphoteric character of ZnO, while the reduced amount of grain boundaries enhances the conductivity. Temperature and humidity level influence the speed of growth until the particles achieve the final size. The post-processing of printed ZnO films with humid gases is potentially a valuable step in increasing the performance of transparent conducting films.
9:00 PM - H5.48
Effects of Preferential Alignment of ZnO Nanorod Arrays on Broadband and Omnidirectional Antireflective Characteristics.
Yen-Chun Chao 1 , Chin-An Lin 1 , Jr-Hau He* 1
1 , Graduate Institute of Photonics and Optoelectronics, & Department of Electrical Engineering, National Taiwan University, Taipei Taiwan
Show Abstract Recently, there has been a popular interest in reducing the reflection of surfaces by fabricating antireflection (AR) coatings [1, 2]. Moreover, ZnO is a potentially important material due to its electrical and optoelectronic characteristics. Therefore, very recently ZnO nanostructures bring exciting possibilities for next-generation AR coatings to suppress reflection effectively. With paying more attention to optical characteristics of ZnO nanostructures, it is demanded to understand the relation between the different morphologies of ZnO nanostructures and AR properties. In this work, alignment-controlled ZnO nanorod arrays (NRAs) on the Si(100) substrates were synthesized using a hydrothermal method. ZnO NRAs exhibit broadband and omnidirectional antireflective characteristics up to an incidence angle of 60° for the wavelength ranges from 385 nm to 850 nm for unpolarized, TE-polarized, and TM-polarized lights, strongly associated with preferential alignment of NRAs. Due to growth on any surface of devices/substrates with ease, broadband and omnidirectional AR characteristics, ZnO NRAs are promising antireflection structures to apply to a wide variety of optoelectronic devices.References[1] T. Lohmuller, M. Helgert, M. Sundermann, R. Brunner, J. P. Spatz, Nano Lett. 2008, 8, 1429.[2] S. J. An, J. H. Chae, G.-C. Yi, G. H. Park, Appl. Phys. Lett. 2008, 92, 121108. *Corresponding author. Fax: +886-2-2367-7467; Tel: +886-2-33669646; E-mail:
[email protected]9:00 PM - H5.49
High Quality Epitaxial Zinc Oxide Thin Films Grown on Noble Metal Buffer Layers Using Pulsed Laser Deposition.
Chunming Jin 2 , Wei Wei 1 , Ravi Aggarwal 1 , Jagdish Narayan 1 , Roger Narayan 2
2 Joint Department of Biomedical Engineering, North Carolina State University and UNC Chapel Hill, Raleigh, North Carolina, United States, 1 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States
Show AbstractZinc oxide thin films have great potential for applications in a variety of opto-electronic devices such as UV-lasers and UV-diodes. To inject electrical current into opto-electronic devices, a high quality ohmic contact is required. Previous researches have shown that noble metals-platinum and gold can be used as high-quality ohmic contact materials for zinc oxide films. Hence, for practical device applications, it is desirable to grow high quality epitaxial zinc oxide films on noble metal electrodes. In this work, we report growth and characterization of epitaxial zinc oxide thin films on gold and platinum buffer layers using pulsed laser deposition. C- and R-plane sapphire wafers were used as substrates. Detailed X-ray diffraction (XRD) and transmission electron microscope (TEM) studies were performed to investigate structural and epitaxial quality of the ZnO/Au or Pt/ Sapphire heterostructures. XRD f-scans confirmed that the metal and ZnO layers were epitaxial. Gold buffer layer grew epitaxially on C-sapphire substrates with [111] direction of gold parallel to the [0001] direction of substrate and [11-2] direction of gold parallel to [2-1-10] direction of substrate. On the gold buffer layer, epitaxial zinc oxide film was obtained with [0001] direction of zinc oxide parallel to [111] direction of gold and [2-1-10] direction of ZnO parallel to [11-2] direction of gold. We have grown platinum buffer layer on R-sapphire substrate. Interestingly, platinum has grown epitaxially with [111] direction of platinum perpendicular to the R-sapphire surface and [11-2] direction of platinum parallel to [01-1-1] direction of sapphire. Epitaxial zinc oxide film was obtained on platinum buffer layer with [0001] direction of zinc oxide parallel to [111] direction of platinum and [2-1-10] direction of ZnO parallel to [11-2] direction of platinum. Photoluminescence studies showed that zinc oxide thin films grown on noble metal buffer layers exhibited stronger photoluminescence intensity compared to the films grown directly on sapphire substrates. This indicates that noble metal buffer layers can be used not only for metallic contacts, but also to improve the photoluminescence efficiency of zinc oxide films.
9:00 PM - H5.5
Ferromagnetic Behavior of High Purity ZnO Nanoparticles.
Ana Leticia Fernandez-Osorio 1 , Roberto Escudero 2
1 , Facultad de Estudios Superiores Cuautitlan., Cuautitlan Mexico, 2 Materia Condensada , Universidad Nacional Autonoma de Mexico, Mexico, D. F. , Mexico
Show AbstractZnO nanoparticles were prepared by chemical methods using low temperature in aqueous solutions. Nanoparticles produced are in the range from 9 to 20 nm. Very small nanoparticles below 12 nm, present a light grey color, whereas nanoparticles above 20 nm are white. High resolution TEM shows oxygen deficiencies. Ferromagnetism may be related to the oxygen deficiencies in the nanoparticles. The chemical synthesize and processing is the main factor for the magnetic behavior. Ferromagnetic characteristics were observed from 2 K to room temperature and above. Magnetic properties were determined by performing careful magnetization vs Temperature, M(T), and isothermal measurements M(H) at different magnetic field intensities. The coercive field clearly shows ferromagnetism above room temperature. All experimental previsions were taken into account in order to avoid 3d element contamination.
9:00 PM - H5.50
Electrical Properties and Stress Analysis of Laser-Annealed Flexible ZnO Thin Films.
Yeon Hwa Jo 1 , Bhaskar Chandra Mohanty 1 , Hong Rak Choi 1 , Yong Soo Cho 1
1 Materials Science & Engineering, Yonsei University , Seoul Korea (the Republic of)
Show AbstractOwing to suitable combination of optical and electrical properties, ZnO-based thin films are suitable as transparent electrodes in various optoelectronic and photovoltaic devices. Especially, low temperature deposited ZnO thin films on flexible polymer substrates are of significant interest for flexible solar cells and displays. In this work, Al-doped zinc oxide (ZnO:Al) thin films were grown on polyethersulphone (PES) substrates at low temperature (300 K) by RF magnetron sputtering of a compound target containing Al2O3 by 2 wt%. Subsequently, the as-deposited ZnO:Al thin films were laser annealed using a pulsed excimer laser (KrF; wavelength of 248 nm; pulse width 20 nm) in air. Structural, electrical and optical properties of the films laser annealed at different laser energies (from 75 to 225 mJ/cm2) were investigated. Analysis of X-ray diffraction patterns yielded that the films were polycrystalline with a tendency of (002) preferable orientation. With increase in laser power per unit area, the FWHM of the (002) peak decreased, indicating enhanced crystallinity of the films. Extensive study of surface microstructure using scanning electron microscopy showed that films laser treated at energy higher than 175 mJ/cm2 had cracks which had width of a few nm and length as high as 5 μm. A possible explanation of the presence of the cracks is offered, based on relief of stress in the films that was generated due to instantaneous laser annealing.
9:00 PM - H5.51
A Inverse Electrolysis Method to Synthesize Surface Nano-roughered ZnO Contact Electrode.
Yu-Hao Liao 1 , Yi-Hao Pai 1 , Gong-Ru Lin 1
1 Graduate Institute of Photonics and Optoelectronics, National Taiwan University , Taipei Taiwan
Show AbstractThe fabrication of surface nano-roughered zinc oxide (ZnO) is demonstrated by using a zinc/electrolyte fuel cell based chemical reactor, in which either the KOH with concentration 7M or the saturated NaCl aqueous solutions is employed as the electrolytes to react with the Zn plate based anode. The chemical reaction of oxygen and water molecules creates hydroxyl nearby cathode, and the randomized pores with accumulated hydroxyls can react with the Zn, which cause the localized of Zn with excess liquid electrolyte to form thick and nano-roughened ZnO upon Zn anode. Using the 7M KOH as electrolyte in the zinc/air fuel cell based reactor can provide faster formation process for transferring the Zn plate into the surface nano-roughered ZnO film. After reaction under different load, we found that when the load resistance became larger, the output voltage became larger and the output current became smaller. From the SEM micrograph, we found that the zinc oxide structure is tip-like with using a 7M KOH electrolyte, when the reaction current increase from 10.9 to 165 mA, the tip-like structure with tip diameter of 100 nm of zinc oxide assembled together and became more rod-like with rod diameter of 800 nm. In the saturated aqueous NaCl electrolyte, the zinc oxide structure is slice-like with slice size of diameter 1μm wide and thickness of ~125 nm, when the reaction current increase, the slice structure of ZnO become more obvious with slice size of 4~5μm wide and thickness of ~250 nm. From the PL spectra, we found that there are two peaks, one small peak at 380 nm and one large broaden peak at 550 nm(NaCl as electrolyte)/580 nm(KOH as electrolyte). This result is match with the PL peak of zinc oxide attributed to exciton recombination process, zinc vacancy defect, and oxygen interstitial defect. The red shift when using KOH as electrolyte may caused by when the reaction speed increase, the influence of oxygen interstitial defect become more powerful. When the reaction current increase, the large broaden PL peak has a red shift. This may cause by the same reason of the red shift when using KOH as electrolyte rather than using NaCl as electrolyte. Except the most intense XRD signal corresponding to (101 orientation) at 2θ of 31.6 degree, we can also observe the others at (002), (100), and (102) corresponding to 34.3 degree, 36.2 degree, and 47.4 degree, respectively. The XRD peak intensity increases by increasing the reaction current in the 7M KOH electrolyte, which leads to a thicker nano-roughened ZnO with higher crystallinity. In contrast, the saturated NaCl electrolyte contributes to a formation of amorphous ZnO nano-roughened surface with degraded XRD peak intensity when increasing the reaction current. A simplified zinc/electrolyte electrolysis fabricate the nano-roughered ZnO structure in shorter reaction time by using inverse electrolysis method at room temperature.
9:00 PM - H5.52
Wide Bandgap Zinc Oxide Schottky Diodes on Silicon and Metal Foil.
Bruce Willner 1 , Shangzhu Sun 1 , Gary Tompa 1
1 , Structured Materials Industries, Inc., Piscataway, New Jersey, United States
Show AbstractZinc oxide (ZnO) is an attractive material for high-speed high power devices. With a wide bandgap of 3.37eV and an exciton binding energy of 59 meV, ZnO has been found to have a high breakdown field of 5 * 106 V/cm, slightly higher than GaN and SiC, the materials that currently dominate the high performance power transistor field. ZnO also exhibits a carrier saturation velocity higher than SiC or GaN. Semiconducting thin films of ZnO have been produced by metal organic chemical vapor deposition (MOCVD). MOCVD is an ideal tool to produce these materials for performance devices, providing high quality material, and allowing well controlled deposition of more complex layer structures through parameter control. MOCVD is a highly scalable production technique which can provide uniform, pin-hole-free deposition over large areas. A chamber for ZnO deposition on up to 19 2” wafer has been built. ZnO deposition with thickness variation of under 3% across an 8”diameter substrate have been achieved. The material and fabrication costs for ZnO are estimated to be approximately 1/8 that of GaN or SiC.A MOCVD tool, designed specifically for ZnO deposition, was used to deposit high quality ZnO films for device fabrication. The lower deposition temperatures of ZnO (400-600°C), compared with other wide bandgap semiconductors, allows deposition on a greater variety of substrates. N-type ZnO films were deposited on n+ silicon wafers and on copper foil. The structures were processed and evaporated metal films were used for Ohmic and Schottky contacts to form Schottky diodes. The fabricated Schottky diodes were characterized electrically. These are the first reported wide bandgap semiconductor devices fabricated on metal foil substrates.The high breakdown field of ZnO allows thinner, and less resistive, high voltage diodes, which can reduce the forward operating losses. Substrate selection is very important for power devices, to control cost and to dissipate heat. Inexpensive, thermally conductive metal foils are an ideal substrate for high power Schottky diodes.
9:00 PM - H5.53
AFM Study of Smooth Zinc Oxide Thin Film via Liquid Phase Reaction with Al Ion Additives.
Takeyasu Saito 1 , Yoshihisa Hirata 1 , Mariko Oyanagi 1 , Naoki Okamoto 1 , Kazuo Kondo 1
1 Chemical engineering, Osaka Prefecture University, Sakai Japan
Show AbstractZinc oxide (ZnO) is a very attractive material as transparent, cheap, stabile and conductive thin films. ZnO thin films fabrication from liquid phase could offer a lot of advantages such as simplicity, low cost, large area growth and high throughput. Si (100) (10mm sq.) wafers were employed as substrates. Nucleation procedures were carried out by dipping in the solutions, SnCl2: 0.1 mol/l → AgCl: 0.1 mol/l → PdCl2: 0.1 mol/l for 10 minutes each. The substrates were hold in a solution of Zn(NO3)2: 0.1 mol/l and dimethylamine-borane (DMAB): 1E-2 to 0.1 mol/l in a range of 70°C for four hours. Al(NO3)3 in a range from 1E-4 to 5E-2 mol/l were added as a cation source. Surface and cross-section of ZnO films were observed by AFM, FE-SEM and crystallographic structure was determined by X-ray diffraction. FE-SEM results revealed that Al(NO3)3 addition improved surface roughness compared the conventional case. In this study, AFM study of smooth zinc oxide thin film prepared from Al(NO3)3 added solution were carried out and detailed surface structure will discussed.
9:00 PM - H5.54
ALD prepared Zinc Oxide for Thin Film Transistor Application.
Woon-Seop Choi 1
1 School of Display Engineering, Hoseo University, Asan-city Korea (the Republic of)
Show AbstractHigh quality ZnO thin films for transparent thin-film transistors are successfully prepared by injection type source delivery system of atomic layer deposition. By using this delivery system the source delivery pulse time can dramatically be reduced to 0.002s to minimize processing time. The growth of ZnO thin film at relatively low temperature at 150oC shows good characteristics. The process factors with the reactants for the film growth are characterized. The atomic ratios of [Zn]:[O] in the XPS spectrum are estimated to be 47.34:52.67. The bottom-contact and bottom-gate ZnO TFT was prepared and characterized. The threshold voltage (Vth) approximately 0.85 V and the saturation mobility about 0.43 cm2/Vs for a 10 μm X 10 μm device were obtained and the output curves measured at Vd from 0 to 40 V exhibit clear pinch-off and solid saturation
9:00 PM - H5.55
Realization of Stable p-type Behavior of ZnO Thin Films Deposited on InP.
Arjun Mandal 1 , Subhananda Chakrabarti 1
1 Electrical Engineering, Indian Institute of Technology Bombay, Mumbai, Maharashtra, India
Show AbstractIn recent times, ZnO has become a promising candidate in the field of optoelectronics because of its large band gap (3.37eV) and large exciton binding energy (60meV) at room temperature. The presence of oxygen vacancies and zinc interstitials make the intrinsically grown or deposited ZnO an n-type semiconductor. Herein lies the biggest challenge to achieve stable p-type behavior in a wide band gap material like ZnO. Both Group I elements (Li, Na, K) and Group V elements (N, P, As and Sb) are reported as the p-type dopants for ZnO. In this attempt, ZnO thin films were deposited on semi insulating <100>InP by Pulsed Laser Deposition (PLD) technique at 400°C in an oxygen ambient of 75mTorr. The thicknesses of the films were found to vary from 241.25nm to 301.56nm. The samples were subsequently subjected to Rapid Thermal Annealing (RTA) at 500°C, 550°C, 600°C and 650°C respectively. The RTA was performed with an expectancy to diffuse the Phosphorus from InP into the ZnO thin film and study the stable p-type behavior of ZnO by the substitution of oxygen atoms by Phosphorus atoms. The XRD results of all the samples revealed that ZnO <002> was grown. Furthermore, the full width at half maximum (FWHM) of the annealed samples were found to be much lower (~0.1428°) than that of as grown sample (~0.2652°). This improvement might be due to the better recrystallization of the annealed samples compared to the as-grown sample on account of the additional thermal energy supplied during annealing. Atomic force microscopy (AFM) images revealed growth of ZnO with roughness varying between 9.26nm to 22.28nm (root-mean-square) within the scanned area of 1µm2. UV/VIS/NIR spectroscopy in reflection mode was studied within the range of 200nm-1600nm and it was observed that reflectance had increased (from 33% to 98%) for the samples annealed at higher temperatures. It might be due to the formation of fused bigger grains over the surfaces which were reflecting much better. Temperature dependent Van Der Pauw Hall measurements were performed to study the electrical properties of the ZnO thin films within the range of 80K to 310K . The results revealed a transition of ZnO conductivity from n-type, for as-grown and 500°C annealed samples, to consistent p-type for the samples annealed at 550°C, 600°C and 650°C .The hole concentrations increased with the increase of annealing temperature from 1.9x1017cm-3 to 2.52x1018cm-3 at 80K and from 2.95x1019cm-3 to 5.41x1020cm-3 at 310K. Hall mobility was measured to be 397cm2/V-sec at 80K and 19.8cm2/V-sec at 310K for the p-type ZnO films of very low resistivity.This is probably one of the best reports of a combination of high hole concentrations and mobilities for any reported p-type ZnO thin films. Our future work includes fabrication and characterization of UV LEDs. We acknowledge Central SPM Facility of IIT Bombay for AFM study and DST for financial assistance.
9:00 PM - H5.6
Formation, Self-assembly, and Spontaneous Disassembly of ZnO Hexagonal Pyramids.
Ming Yang 1 , Kai Sun 2 , Nicholas Kotov 1 2 3
1 Chemical Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Material Sciences and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 3 Biomedical Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractZnO hexagonal pyramids with exposed polar surfaces were obtained in hydrophilic media without any additional surfactant present. Absence of a thick organic shell smearing anisotropic interactions makes possible observation and evaluation of the self-organization phenomena. Extensive studies of the transformation processes involving such pyramids were carried out, which reveal multiple unique features. The formation of pyramids involves not only simple nucleation and crystal growth of small clusters into the larger ones but a completely opposite process of disassembly of intermediate nanocrystals, which include pyramidal building elements. Crystallization process in this system is coupled to the evolution of basic nanoscale building blocks beginning from plates, to truncated pyramids, and finally to full pyramids. This is accompanied by spontaneous disassembly phenomena of the larger clusters, to chain assemblies, and to individual pyramids, which is associated with the decrease of anisotropy of the force field around the particles. As one of the components of interparticle interactions, the evolution of dipole moments of the nanocrystals is discussed and believed to be essential in understanding the behavior of the system. Well-defined hexagonal pyramids can be obtained after sufficient time and surface reconstruction. By utilizing the excluded volume interaction between colloids and polymers, the re-assembly of pyramids can be also achieved which gives the first example of a dynamic nanoparticle system with collective behavior of individual building blocks as well as distinct and experimentally controlled stages of assembly and disassembly.
9:00 PM - H5.7
An Investigations on the Structural, Electrical and Mechanical Properties of ZnO Nanowire/nanobelts.
Anjana Asthana 1 , Kasra Momeni 1 , Reza Shahbazian Yassar 1 , Yoke Yap 2
1 Mechanical Engineering and Engineering Mechanics, Michigan Technological University, Houghton, Michigan, United States, 2 Physics, Michigan Technological University, Houghton, Michigan, United States
Show AbstractIn recent years, one dimensional (1D) ZnO materials, such as nanowires and nanobelts have stimulated considerable interest for scientific research due to their importance for fundamental studies and in potential technological applications. We report here, an investigation of electrical, structural-microstructural and mechanical properties of an individual nanowire/nanobelt via in situ high resolution transmission electron microscopy (TEM) using an AFM-TEM system. The I-V characteristics of the ZnO nanobelt, just in contact with the AFM tip shows the insulating behaviour, however, it shows the semiconductor behaviour upon bending. The semiconducting parameters of the stressed ZnO nanobelt have been retrieved based on the experimental I-V curves. The analysis of the low magnification bright field images, high resolution lattice images and the corresponding electron diffraction patterns shows that each ZnO nanobelt is a single crystalline, having wurtzite hexagonal structure (a = 0.324nm, c = 0.52066 nm, P63mc) with a general growth direction of [0001]. A detailed and careful analysis of the bright field and dark field images of an individual ZnO nanobelt shows the presence of defect structure, pseudoperiodic structure, thickness fringes and bend contours.
9:00 PM - H5.9
A Luminescence Study of Electron-irradiated ZnO Crystals.
Manuel Avella 1 , Juan Jimenez 1 , Oscar Martinez 1 , Buguo Wang 2 , Peter Devrinsky 3 , David Bliss 3
1 GdS Optron lab, Universidad de Valladolid, Valladolid Spain, 2 , Solid State Scientific Corp., Hollis, New Hampshire, United States, 3 Sensors Directorate, Air Force Research Laboratory, Hanscom, Massachusetts, United States
Show AbstractThe understanding of point defects is essential for management of the electronic and optoelectronic applications of semiconductors. ZnO is a very promising semiconductor for UV optoelectronics, because of its large bandgap (3.3eV), and its large free exciton binding energy (60 meV). For device applications, the importance of high quality ZnO substrates is crucial to produce high quality homoepitaxial layers. However, the role of the native point defects is not well understood. High energy e-irradiation is an efficient method to create point defects, and luminescence techniques are very useful to study these defects. However, the analysis of e-irradiated ZnO has not reported relevant changes in the luminescence spectrum that could reveal the formation of point defects with a specific luminescence signature. This observation is supported by the strong radiation hardness of ZnO. Radiation induced defects have been mainly identified by magnetic resonance techniques and positron lifetime annihilation. The main product of the electron irradiation is a Zn vacancy defect. We present a photoluminescence and cathodoluminescence analysis of different ZnO crystals irradiated with high energy electrons. The luminescence signature of the irradiated samples depends on the nature of the sample and the surface preparation. The luminescence signature of the e-irradiation defects is located around 3.3 eV at low temperature. This band is overshadowed by another band usually observed in non-treated ZnO, and an additional band associated with mechanical damage. Because the defect introduction rate by e-irradiation is very small, the probability of observing the luminescence of radiation induced defects (probably VZn related defects) is very low, and most of times the e-irradiation luminescence print cannot be distinguished.
Symposium Organizers
Juergen Christen Otto-von-Guericke-Universität Magdeburg
Leonard J. Brillson Ohio State University
Hiroshi Fujioka University of Tokyo
H. Hoe Tan The Australian National University
H6: Growth
Session Chairs
Sukit Limpijumnong
Hoe Tan
Wednesday AM, December 02, 2009
Ballroom C (Hynes)
9:30 AM - H6.1
Growth and Characterization of Hetero- and Homoepitaxial A-plane ZnO: A Comparison between CVD and MBE.
Sebastian Eisermann 1 , Pascal Becker 1 , Martin Eickhoff 1 , Bruno Meyer 1 , Bernhard Laumer 2 , Martin Stutzmann 2
1 1. Physics Institute, Justus-Liebig-University Giessen, Giessen Germany, 2 Walter Schottky Institute, Technical University Munich, Garching Germany
Show AbstractWe report on the growth of non-polar a-plane ZnO by chemical vapour deposition (CVD) and plasma-assisted MBE. We used r-plane-sapphire, a-plane GaN-templates and a-plane ZnO-single-crystals as substrates. Only the homoepitaxial growth approach lead to a true Frank-van-der-Merwe growth mode, as shown by atomic force microscopy. X-Ray-diffraction spectra of the homoepitaxial thin films mirror the excellent crystalline quality of the epilayers.The optical properties are investigated by temperature dependent photoluminescence, and the differences between polar c-plane and nonpolar homoepitaxial ZnO films are presented.
9:45 AM - H6.2
Al2O3 as a Transition Layer for GaN and InGaN Growth on ZnO by MOCVD.
Nola Li 1 2 , Shen-Jie Wang 1 , William Fenwick 1 , Andrew Melton 1 , Chung-Lung Huang 2 , Zhe Chuan Feng 2 , Christopher Summers 3 , Muhammad Jamil 1 , Ian Ferguson 1 3
1 School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 Graduate Institute of Photonics and Optoelectronics, National Taiwan University , Taipei Taiwan, 3 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractLight emitting diodes for solid state lighting have made a big impact on the general market. However, current improvements are limited by the current substrate technology. This paper uses ZnO substrates for GaN and InGaN growth by MOCVD. ZnO offers many advantages for GaN and InGaN due to its closely matched lattice constant, similar thermal expansion coefficients, and its ability to be easily chemically etchable leaving only a thin film for the LED. However, H2 etching of the ZnO substrate and Zn diffusion out of the substrate still cause many issues during growth by MOCVD. Hence, a transition layer was grown on the substrate before MOCVD growth to prevent Zn diffusion, protect the ZnO substrate from H2 back etching, and promote high quality nitride growth. Al2O3, as the transition layer, was grown at various thicknesses for 20 and 50nm on ZnO substrates. Annealing was performed at various temperatures ranging from 1000-1300C for 5-60 minutes prior to nitride growth. High resolution x-ray diffraction (HRXRD) measurements revealed that the thin Al2O3 layers after optimal annealing were polycrystalline α-Al2O3 phase for the GaN and InGaN films grown epitaxially on ZnO. Wurtzite GaN is only seen on 20nm Al2O3/ZnO substrates with a mirror-like surface, no etched pits, and no peeling off as observed by HRXRD and field-emission scanning electron microscopy (FE-SEM). Room temperature photoluminescence (RT-PL) shows the near band-edge emission of GaN red-shifted, which might be from oxygen incorporation forming a shallow donor-related level in GaN. Raman scattering also indicated the presence of a well-crystallized GaN layer on the 20nm Al2O3/ZnO substrate. Optical transmission (OT) was performed to measure the bandgap energy of InGaN using Sigmoidal fitting. Auger Electron Spectroscopy (AES) atomic depth profile shows that there is a decrease in Zn in the InGaN layer when an Al2O3 layer is used. In addition, the diffusivity of Zn in GaN is calculated to be about 2x10-16atoms/sec2 at 700C.
10:00 AM - H6.3
Homoepitaxial MOVPE Growth of ZnO and ZnMgO Alloys.
Alexandre Ribeaud 1 , Patrice Gergaud 1 , Pascal Marotel 1 , Pierre Ferret 1 , Matthieu Lafossas 1 , Francois Levy 1 , Guy Feuillet 1
1 LETI Minatec, CEA, Grenoble France
Show AbstractAs a wide band gap semiconductor, Zinc Oxide (ZnO) is a potential alternative material to Gallium Nitride for Light Emitting Diodes (LEDs); indeed, one advantage related to the use of ZnO is the availability of high quality and large scale homo-substrates. To obtain high luminescence efficiency, one has to resort to ZnO quantum wells heterostructures with Zinc Magnesium Oxide (ZnMgO) alloys as barriers. The optical properties of these heterostructures are, to a large extent, dictated by the structural quality of the alloy barriers (dislocations and alloy stability). It is also closely linked to the abruptness of the quantum well/barrier interface, hence, to the structural perfection of the growth front.With a view to address this point, we report here on the results we have obtained concerning the growth of ZnMgO and ZnO layers by Metal Organic Vapor Phase Epitaxy (MOVPE) with N2O, Diethyl Zinc (DEZn) and Bis(cyclopentadienyl)magnesium (Cp2Mg) as respective sources of Oxygen, Zinc and Magnesium.The samples were characterized by High Resolution X-ray Diffraction (HXRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Interferometric Microscopy and Photoluminescence (PL). The influence of process pressure, temperature and oxygen/zinc ratio on the compared structural and optical characteristics of the grown layers will be presented. The best results were obtained at high temperature (950°C) and with a high oxygen to zinc ratio of 25,000. The grown layers are smooth (RMS Roughness of 2 nm) and with high crystalline quality (FWHM of 18 arcsecs on ZnO (002) rocking curves). We will discuss on the Mg content measurement using HXRD and PL.
10:15 AM - H6.4
Epitaxial p-NiO/n-MgxZn1-xO Heterojunction by Pulsed Laser Deposition.
Wei Wei 1 , Titas Dutta 1 , Chunming Jin 2 , Tsung-Han Yang 1 , Roger Narayan 2 , Jagdish Narayan 1
1 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 Joint Department of Biomedical Engineering, North Carolina State University and UNC Chapel Hill, Raleigh, North Carolina, United States
Show AbstractIn recent years, ZnO and ZnO-related materials have stimulated lot of research effort to seek possible optoelectronic and electronic device applications due to their excellent optical and electrical properties. However, the lack of p-type ZnO materials poses a huge challenge for device applications. As a result, alternative p-type materials such as NiO are used to form heterojunctions with ZnO and ZnO-related materials. In these heterojunctions, the quality of epitaxial grown films plays a very critical role in determining the electrical performance. On the other hand, MgxZn1-xO alloy is of particular interest among the ZnO-related materials since its bandgap can be tuned by controlling Mg content. With engineered bandgap, the optical transparency of MgxZn1-xO is tunable in the ultra-violet wavelength region and the conductivity of MgxZn1-xO can also be controlled. Therefore, it is of particular interest to investigate electrical properties of epitaxial grown p-NiO/n-MgxZn1-xO heterojunctions. In this study, epitaxial p-NiO/n-MgxZn1-xO heterojunctions with different Mg contents have been grown by pulsed laser deposition. The MgxZn1-xO was grown at 650oC and NiO was grown at 250oC. The epitaxial growth relationship was studied by X-ray diffraction and transmission electron microscope which showed that NiO [111] is parallel to MgxZn1-xO [0001] and NiO [011] is parallel to MgxZn1-xO [21-10] in the growth plane. The optical properties were studied by transmission and absorption spectrum, which showed that the heterojunction is highly transparent with transmittance of approximately 70%. The current-voltage (I-V) curves of p-n junctions exhibited typical rectifying behavior and the electrical performance of the heterojunctions could be effectively tuned by band gap engineering of n-MgxZn1-xO.
10:30 AM - H6.5
Growth and Characterization of Hetero- and Homoepitaxial ZnO/ZnMgO Quantum Wells.
Bernhard Laumer 1 , Thomas Wassner 1 , Martin Stutzmann 1 , Martin Eickhoff 1 2
1 Walter Schottky Institut, Technische Universität München, Garching Germany, 2 I. Physikalisches Institut, Justus-Liebig-Universität, Giessen Germany
Show AbstractIn this study, heteroepitaxial and homoepitaxial growth of ZnO by plasma assisted molecular beam epitaxy (PAMBE) is investigated. For heteroepitaxy on c-plane sapphire, a MgO/ZnO double buffer similar to [1] has been introduced to overcome the large lattice mismatch. Homoepitaxial growth was conducted on polar, c-plane (O-face and Zn-face) as well as unpolar, a-plane surfaces. The influence of the substrate temperature and the II/VI-ratio is studied for the different substrates leading to a more detailed understanding of the growth kinetics.The thus obtained results have been employed to fabricate single ZnO/Zn1-xMgxO quantum well (QW) structures with atomically abrupt interfaces. By fabricating specimens with different barrier height and well thickness, a deeper insight into basic material properties is attempted. Special attention was paid to polarization charges induced at polar heterointerfaces, since they reduce the radiative recombination efficiency by spatially separating electron and hole wave function in the QW (Quantum Confined Stark Effect).Reflection high energy electron diffraction (RHEED) was used for in-situ growth monitoring, atomic force microscopy (AFM) for ex-situ investigation of the sample morphology. The optical properties were investigated by photoluminescence spectroscopy (PL). PL spectra taken at 4.2 K of samples grown on c-plane sapphire are dominated by an emission line that is blue-shifted with respect to the ZnO emission and that is attributed to localized excitons in the QWs. By increasing the Mg-concentration in the barrier or decreasing the well width, a blue-shift of the QW emission is observed as expected from quantum confinement. As shown by temperature-dependent PL measurements, this emission line can be attributed to localised excitons at low temperatures, while for higher temperatures the free exciton contribution prevails.[1] Bakin et al., Journal of Crystal Growth 287, 7 (2006)
10:45 AM - H6.6
Interface Instability in ZnO/MgO/Al2O3.
Tsutomu MInegishi 1 , JIn-Sub Park 1 , Seunghwan Park 1 , K. Sumitani 2 , O. Sakata 3 , Takafumi Yao 1
1 Center for Interdisciplinary Research, Tohoku University, Sendai Japan, 2 , Saga Light Source, Tosu Japan, 3 , Spring 8, Hyogo Japan
Show AbstractMgO buffer has been deposited on sapphire substrates to control the crystal polarity of successively grown ZnO layers, ie, MgO buffer thicker than 2.7 nm helps grow Zn-polar ZnO, while MgO buffer thinner than 2.7 nm results in the growth of O-polar ZnO1,2). The underlying mechanism for such change in crystal polarity of the ZnO layers is that the crystal structure of the MgO buffer changes from Wurtzite to rock salt structure as the thickness exceeds the critical thickness1,2). Based on such results, periodically-polarity inverted structures suitable for nonlinear optical device applications have been fabricated3). Clear enhancement in second harmonic generation of light at around 400 nm is successfully achieved4). It should be noted, however, that alkaline earth metals such as Mg easily react with Al2O3 to form more stable compound of MgAl2O45). Those observations intrigue more investigation of the instabitlity of the MgO/Al2O3 interface and its effects on the growth of ZnO layers.We have evaluated the in-plane lattice parameter of MgO layers grown on Al2O3 at temperatures ranging from 500 to 800 oC by grazing incidence angle X-ray diffraction using a synchrotron light source, which indicates the formation of MgAl2O4 above 700 oC. Further in-situ investigation on the formation of MgAl2O4 is performed by using RHEED during MgO deposition. Both detailed observation of the evolution of the RHEED pattern and in-situ measurements of the in-plane lattice parameter reveals that the crystal structure of MgO changes from pseudomorphic WZ MgO to relaxed WZ MgO followed by the formation of MgAl2O4 spinel structure with <111> orientation normal to the substrate. In order to get insight into the formation process of MgAl2O4, we first grow relaxed WZ MgO at 800 oC followed by the further deposition of MgO at 550 oC. Interestingly, the deposition of MgO at 550 oC is governed by 3D growth accompanied by the transition of in-plane lattice parameter from WZ MgO to Rock-Salt MgO and eventually stabilized at that of MgAl2O4. Thus, the evolution of MgO during deposition can be summarized as follows: (1) the formation of WZ MgO in which Al is diffused; (2) the formation of islands with RS-MgO; (3) the formation of MgAl2O4 by Al diffusion.ZnO layers grown on such MgAl2O4 spinel buffer shows clear RHEED intensity oscillations indicative of two-dimensional growth as is the case of ZnO growth on MgO buffer. The crystal polarity of ZnO is O-polarity without 30o-rotational domains, which is again the same as ZnO growth on MgO buffer. However, the surface shows inhomogeneous features presumably due to inhomogeneous diffusion of Al atoms, which is different from ZnO on WZ-MgO.1) H Kato et al, Appl. Phys. Lett. 84, 4562 (2004).2) T Minegishi et al. J. Vac Sci. Technol.B23, 1286 (2005).3) T. Minegishi et al. J. Vac. Sci. Technol. B26, 1120 (2008).4) T. Minegishi et al. to be presented at II-VI-2009.5) I Ganesh and JMF Ferreira, Ceramics International 35, 259 (2009).
11:30 AM - **H6.7
Crystal Polarity Control of ZnO Films and Application in Second Harmonic Generation.
Jin Sub Park 1 , Takafumi Yao 1
1 , Center for Interdisciplinary Research, Sendai Japan
Show AbstractIn terms of crystal structures, ZnO is expected to have nonzero second-order susceptibility due to the lack of inversion symmetry in wurtzite crystal structures. Recently, ZnO has attracted interest as a nonlinear optical material with high nonlinear optical response. Despite of the progressive development of bulk materials as like the LiTiO3 and LiNbO3, in fact, there has been a continuous requirement of nonlinear optical materials for potential applications in integrated optics. This paper will introduce our recent work towards this end.We have established the selective growth technique of Zn-polar and O-polar ZnO layers on sapphire substrate using Cr-compound buffer layers. The crystal polarity of ZnO layers are successfully controlled by varying the surface structure of Cr-compound buffers. i.e., Rock-salt structured CrN and hexagonal structured Cr2O3. We have found that ZnO layers grown on CrN/sapphire show Zn polar, while those grown on Cr2O3/sapphire result in O-polar ZnO films. In order to verify the origin of nonlinear optical response of ZnO, the polarity-controlled ZnO thin films grown on different buffer layers were investigated as nonlinear optical materials for second harmonic generation (SHG). The effective nonlinear optical coefficient (deff) of ZnO grown on Cr-compound buffer layers showed a higher value than that of ZnO grown on MgO buffer layers. The correlations among the grain size and surface roughness with the values of deff were found to be strong. The deff of the ZnO film increased with decrease in the grain size and surface roughness.In addition, by combining suggested in-situ polarity control technique with photolithography technique, we have fabricated 1D and 2D periodically-polarity inverted (PPI) hetro-structures with periodicity ranging from 60 um to 0.5 um. Such PPI ZnO heterostructures show the enhancement of SHG intensity comparing with the ZnO films. Moreover, in the case of 2D PPI ZnO structures with sub micrometer scale period enable quasi-phase matching in nonlinear optical effects to achieve effective frequency conversion of the incident laser light. We have explored the nonlinear photonic crystal in the generation of the second harmonics. Detailed analysis with simple photonic bandgap model shows that quasi-phase matching is achieved.
12:00 PM - H6.8
MBE-grown NinMg1-nO and ZnzMg1-zO Thin Films for Deep Ultra-violet Optoelectronic Applications.
Jeremy Mares 1 , Ryan Boutwell 1 , Matthew Falanga 1 , Amber Scheurer 1 , Winston Schoenfeld 1
1 CREOL/The College of Optics & Photonics, University of Central Florida, Orlando, Florida, United States
Show AbstractWe report on the heteroepitaxial growth of high-quality single crystal cubic (B1) NinMg1-nO and ZnzMg1-zO thin films by radio frequency oxygen plasma-assisted molecular beam epitaxy (RF-MBE). Such films represent a novel semiconductor family which may enable p-i-n oxide structures for ultraviolet applications such as visible and solar-blind detection. Film compositions over the ranges n = 0 to n = 1 and z = 0 to z = 0.65 have been grown on lattice-matched MgO (100) and characterized optically, morphologically, compositionally, and electrically. Both of these ternary materials are shown to have bandgaps which vary directly as a function of Mg concentration. Optical transmission measurements of NinMg1-nO show the bandgap to shift continuously over the approximate range 3.5 eV (for NiO) to 6 eV (for MgO with trace Ni concentrations). Analogously, the bandgap of B1 ZnzMg1-zO can be shifted from about 4.9 eV (for z = 0.65) to above 6.4 eV (for z = 0.12). Films exhibit good morphological quality and typical roughness of NinMg1-nO films is less than 5 Å while that of ZnzMg1-zO is less than 15 Å, as measured by atomic force microscopy (AFM). Film compositions are interrogated by Rutherford Backscattering (RBS) and electrical characterization is made by room-temperature Hall measurement and transmission line measurements. Metal-semiconductor-metal (MSM) devices fabricated from both categories of films are presented along with photoresponse observations of the two films. Application of these materials to diverse electronic and optoelectronic technologies such as rewritable random access memory (Re-RAM) and solar blind detectors will be discussed. In particular, growth methodology for creating heteroepitaxial NinMg1-nO/ZnzMg1-zO devices will be discussed.
12:15 PM - H6.9
Anisotropic Strain Relaxation in Nonpolar a-plane ZnO on r-plane Sapphire.
Punam Pant 1 , J. Budai 2 , R. Aggarwal 1 , J. Narayan 1
1 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractWideband gap ZnO is of significant interest for use in optoelectronic devices such as light emitting diodes operating in blue and ultraviolet regions.ZnO semiconductors with wurtzite crystal structure are characterized by spontaneous and strain-induced electrical polarization. These polarization effects are detrimental to performance of (0001)oriented devices. Spontaneous polarization for ZnO is directed along c axis which is also growth orientation on (0001)sapphire. To eliminate effect of surface polarization charges, nonpolar growth orientations can be used. Strain-induced polarization which is caused by piezoelectric properties of material depends on nature of piezoelectric coupling and strain state. Strain in semiconductors is of significance as it may change crystal symmetry and electronic band structure. Lack of native substrate makes strain relevant for ZnO heterostructures,since growth on foreign substrates leads to built-in strain due to differences in lattice parameters and thermal expansion coefficients between film and substrates.ZnO heterostructures used in optoelectronic devices are grown on c-plane sapphire with c-plane as growth plane.Due to hexagonal symmetry of c-plane, films on (0001)sapphire experience isotropic biaxial in-plane strain. For growth on non-c-plane-oriented sapphire,films will have anisotropic biaxial strain as a consequence of anisotropy of growth surfaces. In this work, we report heteroepitaxial growth and strain relaxation in non-polar a-plane (11-20)ZnO films grown on r-plane (10-12)sapphire.The lattice misfit in the plane of film for this orientation varies from -1.26% in [0001] to -18.52% in [-1100] direction.X-ray diffraction θ-2θ scans and φ-scans revealed epitaxial relationship:(11-20)ZnO//(1-102)sap;[0001]ZnO//[-1101]sap;[-1100]ZnO//[-1-120]sap.High-resolution x-ray diffraction was used to determine out-of-plane and in-plane lattice parameters.Our results of strain measurements show complete relaxation of film in large misfit direction [-1100]ZnO and residual strain in small misfit direction [0001]ZnO.Consideration of activated slip systems along the in-plane [0001]ZnO and [1-100]ZnO shows that only prismatic and pyramidal plane slip systems can be activated along these directions. In conjunction with high-resolution transmission electron microscopy,observed anisotropic strain relaxation along [1-100] and [0001] ZnO stress directions and the fact that pyramidal plane slip systems can be activated along these directions,we have proposed misfit stress as driving force for plastic relaxation in films.Our results show that device quality ZnO films can be grown with manageable residual strains. Reference: (a)Structural characterization of two-step growth of epitaxial ZnO films on sapphire substrates at low temperatures,P Pant etal,J.Phys.D:Appl.Phys.42(2009)105409.(b)Thin Film Epitaxy and Structure Property Correlations for Nonpolar ZnO Films, P Pant etal accepted for publication in Acta Materialia.
12:30 PM - H6.10
Melt Grown ZnO Bulk Crystals.
Detlef Klimm 1 , Detlev Schulz 1 , Steffen Ganschow 1
1 , Leibniz Institute for Crystal Growth, Berlin Germany
Show AbstractCommercial zinc oxide wafers are fabricated almost exclusively from hydrothermally grown ZnO bulk crystals. The hydrothermal technology rests on decades of experience mainly with the growth of α-quartz crystals for piezoelectric applications. From alkaline hydrothermal solutions of zinc oxide, ZnO crystals sized up to 3 inch can be grown within several weeks. It is typical for crystal growth processes from solutions, however, that traces of the solvent (here mainly H, Li, K) are incorporated to the grown bulk. This is uncritical for a piezoelectric material, but can have substantial influence on a semiconductor.Crystal growth technologies that rely on pure melts without solvent (e.g. Czochralski, Bridgman) are desirable alternatives, and are used almost exclusively for the mass production of other electronic materials (Si, Ge, GaAs). For a long time it was assumed that the high melting point of ZnO (Tf = 1975°C), together with the high oxygen partial pressure that is needed to stabilize ZnO at Tf, would not allow to find any crucible material that can withstand molten ZnO. Recently we could show that this claim is wrong, instead a reactive atmosphere containing carbon dioxide can deliver by the equilibrium reaction CO2 ↔ CO + ½ O2 a "self adjusting" oxygen partial pressure which stabilizes ZnO melt in an iridium crucible [1,2]. This way zinc oxide bulk crystals can be grown from iridium crucibles e.g. by the Bridgman method.Now boules with 33 mm diameter and ca. 50 mm length can be grown that are suitable for the production of wafers that were successfully used for the deposition of (Zn,Mg)O and (Zn,Cd)O epilayers by MBE [3]. As expected, the concentration of most impurities is lower, compared with hydrothermal samples [4].Despite these encouraging results, two major problems are still waiting for a solution:a) Thermal stresses during crystallization and cooling of the ZnO boule are so large that grain boundaries or even cracks are often formed.b) Some trace impurities that are still found in the melt grown crystals (e.g. Fe), are presumably introduced from the crucible: The chemical purity of the iridium metal is typically only 99.9%.The contribution compares the status of ZnO melt growth with other technologies, and discusses prospects of future improvements.[1] Schulz et al., J. Crystal Growth 296 (2006) 27-30.[2] Klimm et al., J. Crystal Growth 311 (2009) 534-536.[3] Sadofev et al., Appl. Phys. Lett. 91 (2007) 201923.[4] Brauer et al., Phys. Rev. B79 (2009) 115212.
12:45 PM - H6.11
Chemical Vapour Transport: An Alternative Route for the Growth of Large Bulk ZnO Crystal.
Jean-Louis Santailler 1 , Claire Audoin 1
1 Optronic Department, CEA/LETI-MINATEC, Grenoble France
Show AbstractCommercially available substrates are up to now mainly grown by the hydrothermal method. This method delivers ZnO single crystal up to 3” in diameter with an excellent structural quality. Nevertheless only small dimensions are commercially available, putting a severe limitation for the use of this material. Furthermore, the purity of the crystal is hampered by the presence of Lithium, which is present as a growth agent.We have developed some alternative approaches based on Chemical Vapour Transport (CVT) with carbon monoxide as a chemical activator in order to yield a bulk crystal growth technology, compatible with a “full ZnO” LED process flow based on 2" wafers. Following basic thermodynamic computations based on Gemini (from Thermodata) and Factsage (from GTT) software’s, the ZnO Temperature - Pressure (T°-P) stability diagram was investigated and some T°-P domains well adapted to the growth of bulk ZnO single crystal were evidenced. The full potential of the method was confirmed and a specific patented process was developed. Along this process, zinc oxide powder (5N purity) decomposes around 1200°C; chemical species are transported under reduced pressure (Ar, N2) from the hot decomposition interface to the cold crystallization interface and condense close to 1000°C on both ZnO (0001) or on Sapphire seeds (0001).We could grow high purity ZnO single crystals with dimensions up to 68 mm in diameter on 3” sapphire substrates and 17 mm in diameter on ZnO seeds. The obtained crystals thicknesses are in the range of 10 mm, these being up to now only limited by the hot zone geometry. The growth rate can be tuned in the range 50-300 µm/h, depending on temperatures, pressure and CO flux. X Rays analysis confirmed that ZnO ingots are single crystals. The (0002) rocking curve FWHM is typically 100-200 arc.sec, while the (0002) 2θ/ω FWHM is 25 arc.sec. GDMS analysis gave a total amount of impurities not exceeding 3-7 at ppm that is to say 10 times lower than hydrothermal crystals (90 to 105 at ppm). SIMS measurements showed that Li concentrations (1015 atm.cm-3) are 2 orders of magnitude lower than in hydrothermal samples. Hall measurements confirmed that samples present good electrical mobility: µ= 200 cm-2/Vs @ 300K for a carrier density of nd=1015 - 1016 cm-3. Photoluminescence spectra showed intense light emission at 379 nm @10K (Near band edge) and a low defect band intensity. These are promising results showing the potential of the CVT method as an alternative ZnO crystal growth method.
H7: Nano Structures
Session Chairs
Hiroshi Fujioka
Axel Hoffmann
Wednesday PM, December 02, 2009
Ballroom C (Hynes)
2:30 PM - H7.1
Effects of Annealing on Optical and Electrical Properties of ZnO Nanowires.
Abhishek Prasad 1 , Archana Pandey 1 , Vamsi Kunapuli 2 , Paul Bergstrom 2 , Yoke Khin Yap 1
1 Physics, Michigan Technological University, Houghton, Michigan, United States, 2 Electrical and Computer Engineering , Michigan Technological University, Houghton, Michigan, United States
Show AbstractZnO nanostructures have been grown in variety forms such as nanotubes, nanowires and nanobelts etc. The nanostructures are attractive for electrical, optical, and piezo-electrical devices [1]. In order for ZnO nanowires (ZnONWs) to be more applicable in nanoscale electronic and optical devices, effective doping with either p-type or n-type dopants is important. However, it has been well known that Photoluminescence (PL) spectra of as-grown ZnONWs are always having a green band associated with native defects. These native defects have limited the use of ZnONWs from high-performance devices. The presence of these defects also leads to uncontrollable doping of ZnONWs. Here we present a series of study for effective elimination of native defects on ZnONWs. The optical and electrical properties of ZnONWs as well as their performance as field-effect transistors (FETs) will be discussed.Our ZnONWs are grown in a double-quartz tube thermal Chemical Vapor Deposition (CVD) system [2, 3]. The as grown samples we then subjected for a series of annealing in various gas ambient, durations, and temperatures. All annealed samples were then examined with field-emission scanning electron microscopy (FESEM), Micro-Raman spectroscopy, and PL spectroscopy. The acquired data were compared closely with the as-grown samples. Individual ZnONWs were also be connected with microelectrodes by photolithography and subjected for Current–Voltage (I-V) measurements. Results indicate that controlled annealing can totally removed the green PL band of ZnONWs. This means the native defects of the as-grown samples were successfully suppressed. More importantly, the morphology of these ZnONWs was maintained. This indicates that our annealing process is a non-destructive approach. I-V measurement on individual ZnONWs shows that the conductivity of ZnONWs was greatly improved. FETs fabricated by these NWs were also characterized with notable change in characters. We believe that our approach will further help in controlled doping of ZnONWs for nanoscale applications. Details of all these results will be discussed in the meeting.[1] A. Prasad et al, “Chapter 4 - Alternative Nanostructured Sensors: Nanowires, Nanobelts, and Novel Nanostructures,” in Sensors Based on Nanostructured Materials, Editor: Francisco J. Arregui, Springer, pg. 59-78 (2009).[2] S. L. Mensah et al, Appl. Phys. Letts. 90, 113108 (2007).[3] S. L. Mensah et al, J. Phys. Chem. C (Letter) 111, 16092 (2007).Yoke Khin Yap acknowledges support from the U.S. Army Research Laboratory and the Defense Advanced Research Projects Agency (Contract number DAAD17-03-C-0115).
2:45 PM - H7.2
Patterned Growth of Horizontally Aligned ZnO Nanowire Arrays for Alternative Current Nanogenerators.
Sheng Xu 1 , Yong Ding 1 , Yaguang Wei 1 , Hao Fang 1 , Yue Shen 1 , Zhong Lin Wang 1
1 School of Materials Science & Technology, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractAs an excellent member in the family of one-dimensional nanostructures, ZnO nanowire (NW) has found spectacular applications in fabricating ultraviolet (UV) lasers, light-emitting diodes, field emission devices, solar cells, as well as piezo-nanogenerators. In an effort to integrate the ZnO NWs into a more regular form to enhance the performance of the nanodevices, a variety of techniques have been employed to fabricate patterned vertically aligned ZnO NW arrays [1]. As a counterpart of the vertically aligned ZnO NW arrays, horizontally aligned ZnO NW arrays have also been grown by a few approaches. But the horizontal NWs were rather sparse and random in horizontal orientation, or of a poor alignment. Horizontal alignment of the ZnO NWs after growth was also achieved by dispersing the NWs into solvents then applying high frequency alternative electrical field. We report an approach to fabricating patterned horizontal ZnO nanowire arrays with a high degree control over their dimensionality, orientation and uniformity [2]. Our method combines electron beam lithography and a low temperature hydrothermal decomposition at a temperature lower than 100 °C. The horizontal ZnO NW arrays are epitaxially grown on single crystal ZnO (2-1-10) surfaces and are rather uniform in length and width. Although the NWs suffer from a lateral expansion once they grow out of the photoresist confinements, we can still adjust the dimensions of the NWs accordingly. Because of the anisotropic growth habits of the wurtzite ZnO NWs, we should also be insightful about the manipulation and control over orientation of the photoresist openings relative to the substrate for receiving specially designed and patterned ZnO NW arrays. This approach opens up possibilities to fabricate ZnO NW array-based strain and force sensors, two-dimensional photonic crystals, integrated circuit interconnects, and integrated high performance alternative current nanogenerators [3].[1] Sheng Xu, Yaguang Wei, Melanie, Kirkham, Jin Liu, Wenjie Mai, Dragomir Davidovic, Robert L. Snyder, Zhong Lin Wang, “Patterned Growth of Vertically Aligned ZnO Nanowire Arrays on Inorganic Substrates at Low temperature without Catalyst”, J. Am. Chem. Soc. 2008, 130, 14958-14959.[2] Sheng Xu, Yong Ding, Yaguang Wei, Hao Fang, Yue Shen, Ashok K. Sood, Dennis L. Polla, and Zhong Lin Wang, “Patterned Growth of Horizontal ZnO Nanowire Arrays”, J. Am. Chem. Soc. 2009, 131, 6670-6671.[3] Rusen Yang, Yong Qin, Limin Dai, and Zhong Lin Wang, “Power generation with laterally packaged piezoelectric fine wires”, Nature Nanotech. 2009, 4, 34-39.[4] Research supported by DARPA, DOE and NSE.[5] For more information: http://www.nanoscience.gatech.edu/zlwang/
3:00 PM - H7.3
ZnO Nanowires Grown by Chemical Bath Deposition in a Continuous Flow Microreactor.
Kevin McPeak 1 , Jason Baxter 1
1 Chemical & Biological Engineering, Drexel University, Philadelphia, Pennsylvania, United States
Show AbstractChemical bath deposition (CBD) is an inexpensive, low temperature, aqueous method for depositing oxide and chalcogenide thin films and nanowire arrays. However, the relationship between growth conditions and material properties and morphology is not well understood, often resulting in a tedious trial-and-error approach to optimizing bath chemistry. We report on a continuous flow microreactor for CBD of ZnO nanowires that allows direct correlation of material properties to growth conditions. The chemical bath flows through a sub-millimeter channel and material is deposited on a heated substrate which serves as one reactor wall. The microreactor operates in plug flow; bath composition changes as a function of distance down the reaction channel but the spatial concentration profile is time-invariant. Growth from a bath of spatially varied composition essentially creates a combinatorial library. Spatially-resolved characterization of the substrate enables rapid and direct correlation of material properties to growth conditions, which is not possible with a batch reactor where bath composition changes with time.We have used this microreactor to grow dense arrays of well-aligned, single-crystal ZnO nanowires from aqueous solutions of zinc nitrate and methanamine at 90 ○C with a series of different flow rates and inlet concentrations. Nanowires typically have diameters of 80-100 nm and lengths up to 8 μm. Room temperature photoluminescence measurements indicate that the nanowires are of high optical quality, with strong band edge emission around 3.28 eV. Slow flow rates result in nanowires whose lengths, growth mechanisms, and optical properties vary significantly along the length of the substrate; fast flow rates produce nanowires that are more spatially uniform. Flow-rate dependent morphology studies show that growth is limited by mass transfer to the substrate. Spatially-resolved characterization of a single substrate exposed to slow flow rates reveals that, along the direction of flow, nanowire lengths decreased, morphology changed from pyramidal tops to flat tops, growth mechanism transitioned from two-dimensional nuclei to spiral growth, and the band gap blue-shifted by 60 meV. X-ray diffraction confirms that the blue-shifted band gap is due to compressive strain along the ZnO c-axis, which is thought to be due to a shift to spiral growth mechanism at lower degrees of supersaturation. Annealing relieves the compressive strain and results in photoluminescence spectra that are uniform across the entire substrate. Such rapid characterization and detailed understanding of synthesis-structure-property relationships are not possible with conventional CBD reactor designs.
3:15 PM - H7.4
Converse Piezoelectric Effect Induced Transverse Deflection of a Free-Standing ZnO Microbelt.
Youfan Hu 1 , Zhong Lin Wang 1
1 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractZinc oxide is a semiconducting piezoelectric material that has been widely used in microelectromechanical systems (MEMS) as sensors and actuators. There are few examples about utilization of the converse piezoelectric effect for inducing transverse mechanical deflection of a nanowire (NW)/ nanobelt (NB), in which the mechanical action is perpendicular to the NW/NB. A common design is that such a device requires a nonsymmetric, side-by-side bilayer nanowire shape structure (such as ZnO/Al2O3 for example) that has distinctly different piezoelectric properties, so that the electrical response of the two would result in different degrees of axial deformation, which lead to the transverse deflection of the nanowire. This is similar to the design of a thermal meter based on two materials that have different thermal expansion coefficients. In this report [1], we demonstrate the first electric field induced transverse deflection of a single-crystal, free-standing ZnO microbelt as a result of converse piezoelectric effect without utilizing a nonsymmetric bilayer structure. First, by utilizing the finite element method (FEM), we calculate the response of a ZnO belt when an electric field is applied perpendicular to the ZnO belt that has a growth direction of the c-axis. The converse piezoelectric effect creates a shear stress in the a-plane, resulting in a bending of the free-standing belt, and transfers this tiny deformation at the root into a much larger transverse displacement due to the large aspect ratio of the belt. After an experimental approach was carefully designed and possible artifacts were ruled out, the experimentally observed degree of deflection of the microbelt agrees well with the theoretically expected result. Also we extrapolate our results to nanoscale by corresponding simulations. It showed that a much higher sensitivity of the nanobelt is obtained to converse piezoelectric effect. The device demonstrated has potential applications as transverse actuators/sensors/switches and electric field induced mechanical deflectors.[1] Youfan Hu, Yifan Gao, Srikanth Singamaneni, Vladimir V. Tsukruk and Zhong Lin Wang, “Converse Piezoelectric Effect Induced Transverse Deflection of a Free-Standing ZnO Microbelt” Nano. Lett. 2009, published online as Articles ASAP.[2] Research supported by DARPA, DOE and NSF.[3] For more information: http://www.nanoscience.gatech.edu/zlwang/
3:30 PM - H7.5
Ga Beam Induced Damages and Doping Effect in ZnO Nanowires.
Yao Cheng 1 , Yao Liang 2 , Ming Lei 1 , Sui Kong Hark 2 , Ning Wang 1
1 Physics Department, the William Mong Institute of Nano Science and Technology, the Hong Kong University of Science and Technology, Hong Kong China, 2 Physics Department, the Chinese University of Hong Kong, Hong Kong China
Show AbstractZinc oxide has attracted great attentions because of its interesting properties of wide band-gap (3.37eV) and large exciton binding energy (60meV). In recent years, novel morphologies of ZnO structures, such as nanocantilever arrays [1], nanohelixes [2-3], nanorings [4], nanosheets [5], have been synthesized. By controlling the sizes, shapes, crystal structures, and surface structures, these controlled ZnO nanomaterials exploited a wide range of technological application in chemical, optic, electronic and mechanical fields. However, most of these controllable nanostructures were directly grown from solution or vapor process resulting in their morphologies limited by their growth conditions. Hence finding a novel precise controllable method to orderly design nanostructures based on basic nanostructures (nanowires and nanobelts) represents a significant step in the field of controllable nanostructures fabrication process.In addition to ZnO nanostructures’ various controllable morphologies, there is intense interest in investigating its optical properties. Besides UV excitonic emission peak, ZnO nanostructure can exhibit many different peaks in the visible spectra region, which have been attributed to the defect emission. Green emission [6-14] is the most commonly observed defect emission in ZnO nanostructure. Many mechanisms have been proposed to understand the origin of green emission in the literature, such as vacancies and interstitials of oxygen and zinc [6-11], antisite oxygen [12], surface defects [13] and Cu impurities [14]. Unfortunately, the origin of green emission remains an open and controversial question and requires further study. In the same time, some methods have been proposed to influence the green emission. For example, coating ZnO nanostructures with a surfactant could suppressed green emission [13], which also provides helpful information to understand the origin of defect emissions.Using FIB technology (through an ultra fine Ga ion beam), nano-sized structures can be modified or processed. However, the Ga ion beam also causes damages during nanofabrication. In our work, the damages induced by FIB in different metal oxide nanowires have been investigated by high-resolution transmission electron microscopy (HRTEM). ZnO nanowires have been found to be very stable under Ga ion beam bombardment. Moreover, it has been observed for the first time by cathodoluminescence (CL) spectroscopy at liquid nitrogen temperature that the Ga-ion beam can largely suppress or eliminate the defect-induced green light emission. While, the Ga+ beam has nearly no impact to the peak position of UV emission from ZnO nanowires. Based on this interesting effect, ZnO nanowires with controllable periodically doped structures have been fabricated which have potential technological applications in nanodevice fabrication.
3:45 PM - H7.6
Synthesis of ZnO Nanorods on Metal Substrates as a Novel Adhesion Promoting Surface Treatment.
Ozlem Ozcan 1 , Patrick Keil 2 , Berkem Ozkaya 2 , Guido Grundmeier 1
1 Technical and Macromolecular Chemistry, University of Paderborn, Paderborn Germany, 2 Interface and Surface Chemistry, Max Planck Institut für Eisenforschung, Düsseldorf Germany
Show AbstractThe synthesis of ZnO nanocrystalline films has attracted great attention due to their great potential as functional materials in catalysis and microelectronics. From our point of view, these nanorods can also provide a perfect system for fundamental investigation of adhesion and corrosion processes. Technically, ZnO passive films govern the adhesive and corrosive properties of galvanically protected steels. With the high surface area the nanorod geometry can provide for adsorption of adhesion promoters, zinc oxide nanorod modification of metal substrates is a promising candidate for being used as a novel adhesion promoting surface treatment.
In this paper we are presenting the synthesis of ZnO nanorods on various metal substrates by a hydrothermal method. Films with different nanorod lengths were characterized by electrochemical impedance spectroscopy (EIS) for their electrochemical properties. Their morphology and crystallinity were examined using scanning electron microscopy (SEM) and X-Ray diffraction (XRD). Adsorption of different organofunctional adhesion promoters was studied to gain a better understanding of their binding mechanisms to zinc oxide.
SEM and XRD measurements have shown that nanorod films have a preferred growth along their c-axis and are densely and homogeneously coated on the metal substrates. The determination of the donor densities was done using the Mott-Schottky analysis technique based on the capacitance data obtained from EIS spectra collected at different potentials. The measured donor densities are on the order of 1019 – 1020 cm-3 for nanorod films with different heights and tend to increase with increasing rod height.
The combined microscopic and electrochemical results have shown that ZnO-nanorod growth form aqueous solutions is a promising method to functionalize metal substrates to achieve properties such as improved corrosion protection and high adhesion strength to adhesives.
4:30 PM - **H7.7
Multifunctional ZnO and Its Nanostructures for Novel Devices.
Yicheng Lu 1 , Jian Zhong 1 , Pavel Ivanoff Reyes 1 , Chiehjen Ku 1 , Ziqing Duan 1 , Yang Zhang 1 , Xiaotian Yang 1
1 Department of Electrical and Computer Engineering, Rutgers University, Piscataway, New Jersey, United States
Show AbstractZnO is a promising wide band gap (Eg ~ 3.3 eV at room temperature) semiconductor. ZnO can be alloyed with CdO and MgO to form the ternaries CdxZn1-xO and MgxZn1-xO, extending the direct energy band from 2.8 eV to 4.0 eV. ZnO has been also emerging as an important multifunctional oxide. Through proper doping, ZnO can be made transparent and conductive, piezoelectric, or ferromagnetic. ZnO based single crystal nanostructures can be grown on various substrates at relatively low temperature. We have integrated ZnO based multilayers and nanostructures to construct new devices. High quality epitaxial ZnO and MgxZn1-xO films are grown on r-plane sapphire substrates by metaloganic chemical vapor deposition (MOCVD). The non-polar a-plane ZnO-based structures are used to make various devices, including high speed UV detectors, Schottky diodes, and field-effect transistors (FETs). Semiconductor and piezoelectric ZnO layers are monolithically integrated to make the voltage controlled phase shifter and wireless UV detectors using the acousto-electric and acousto-optical interactions in the multilayer structures. Single crystal ZnO nanotips are grown on various substrates, including Si, GaN, glass, and metal. The nanotips are oriented along the c-axis, normal to the growth plane. The ZnO nanotips exhibit dominant free excitonic transition and enhanced luminescence efficiency with negligible deep-level emission. Such nanotips are used for new sensors and optoelectronic devices. ZnO nanotips are integrated with piezoelectric and semiconductor ZnO epilayers to form novel surface acoustic wave (SAW) and bulk acoustic wave (BAW) actuators for biochemical and biological sensing. Integration of ZnO nanostructures with a GaN LED enhances photon extraction. The recent progress on the resistive switching devices using in-situ Fe-doped ZnO nanostructures will also be discussed.ACKOWLEDGEMENTS: This work has been supported by AFOSR and NSF.
5:00 PM - H7.8
Synthesis and Electrical Characterization of Highly-doped ZnO Nanowires.
Alberto Salleo 1 , Rodrigo Noriega 1 , Ludwig Goris 1 , Jonathan Stebbins 2 , Linda Thompson 2 , Aaron Palke 2 , Hagen Klauk 3 , Daniel Kaelblein 3
1 Materials Science and Engineering, Stanford University, Stanford, California, United States, 2 Geology Department, Stanford University, Stanford, California, United States, 3 , Max Planck Institute for Solid State Research, Stuttgart Germany
Show AbstractThe use of ZnO nanowires has become a widespread topic of interest in optoelectronics. In order to correctly assess the quality, functionality, and possible applications of such nanostructures it is important to accurately understand their electrical and optical properties. Aluminum- and gallium-doped single-crystal ZnO nanowires were synthesized using a low-temperature (T<350°C) solution-based process, achieving dopant densities of the order of 10^20 cm-3. Such nanowire suspensions may provide a cheap alternative to ITO as transparent windows for solar cells.Photothermal deflection spectroscopy (PDS), is used to characterize ensembles of heavily doped ZnO nanowires. By modeling the free charge carrier absorption as a Drude metal, we calculate the free carrier density and mobility as a function of dopant density and processing conditions. Single-wire field-effect transistor characterization confirms the optical measurements. Carrier mobility varies from ~50 cm2/V.s in undoped wires to ~15 cm2/V.s in heavily doped wires.Dopant incorporation into ZnO is usually believed to occurr as substitution of the heteroatom into Zn2+ lattice sites. Solid-state NMR is used to distinguish between coordination environments of the dopant atom. The fraction of atoms occupying different lattice sites and the formation of new phases such as ZnAl2O4 spinel, can be quantitatively monitored as a function of dopant density and processing conditions. ESR spectroscopy in conjunction with annealing experiments sheds light on the charge trapping sites.
5:30 PM - H7.10
Theoretical Study of ZnO nanoparticles: Stable Onion-Like Configurations.
Rodion Belosludov 1 , Hiroshi Mizuseki 1 , Andriy Dmytruk 2 4 , Igor Dmitruk 3 4 , Atsuo Kasuya 4
1 Tohoku University, Institute for Materials Research, Sendai, Miyagi, Japan, 2 Institute of Physics , National Academy of Sciences of Ukraine, Kyiv Ukraine, 4 Center for Interdisciplinary Research, Tohoku University, Senday, Miyagi, Japan, 3 Physics Department, National Taras Shevchenko University of Kyiv, Kyiv Ukraine
Show AbstractDuring the last decade interest in ZnO semiconductor nanostructures has increased notably due to their great potential in various applications such as photovoltaic solar cells, optical sensitizers, photocatalysts or quantum devices. In order to enhance their applicability, it is important to decrease as well as control the size of semiconductor nanoparticles. From this point of view, the prediction and realization of the magic clusters are the key tasks both for experimental and theoretical researchers. Recently, ZnO clusters have been produced by laser ablation of bulk powder zinc peroxide in vacuum and investigated by time-of-flight mass spectroscopy [1]. The obtained experimental results revealed several magic (ZnO)n clusters (n = 34, 60 and 78). Here, using first-principles calculations, we study the structure and electronic properties of ZnO clusters with specific composition identified by mass spectrometry [1]. It has been found that the (ZnO)34 cluster has a onion-like structure, which is similar to (CdSe)34 one predicted and experimentally realized by our group [2]. This cluster does not adopt the crystal form of ZnO while resembling in shell more closely carbon fullerene and is remarkably stable. We also tried to develop the basic strategy for building a series of clusters of greater size with one basic design feature evident in current systems as in the case of (ZnO)34. We have found design rules for several cage systems that predict their chemical stochiometry and dimensions. Using this strategy, the shell-core particles have been proposed and their properties have been investigated at the atomistic level using highly accurate first-principles calculations. REFERENCES[1] A. Dmytruk et al. Microelectronics Journal Materials 40 (2009) 218.[2] A. Kasuya et al., Nature Materials 3, 99-102, 2004.
Symposium Organizers
Juergen Christen Otto-von-Guericke-Universität Magdeburg
Leonard J. Brillson Ohio State University
Hiroshi Fujioka University of Tokyo
H. Hoe Tan The Australian National University
H8: Optical Properties & Devices
Session Chairs
Burhan Bayraktaroglu
Yicheng Lu
Thursday AM, December 03, 2009
Ballroom C (Hynes)
9:30 AM - **H8.1
Polariton Lasing in a ZnO Microcavity.
Zhanghai Chen 1 , Liaoxin Sun 1 , Hongxing Dong 1 , Yanjing Ling 1 , Qijun Ren 1 , Lihui Bai 1 , Weihang Zhou 1 , Xuechu Shen 1
1 Department of Physics, Fudan University, Shanghai China
Show AbstractIn recent years, scientists have been devoting greatest efforts to find the condensation of quasi particles such as exciton polaritons in solid environment [1–3]. From the point of view of application, the buildup of sponaneous coherent polariton state would trigger an inversionless lasing of so-called polariton laser. Unlike the conventional laser, the polariton laser has no threshold limited. Significant progresses on the Bose coherent condensation and the polariton lasing have been made in 2-dimensional planar cavities [3 - 5]. One of the most exciting achievements is the realization of room temperature polariton lasing in a GaN planar microcavity [4]. Recently, polariton behavior in cavities with reduced dimensionality is attracting more attention [6, 7], since it is expected that the reduced dimensionality will greatly lift the wave vector conservation selection rules in polariton scattering, and it is an effective way to realize polariton quantum degeneracy under nonresonant pumping [6]. On the other hand, as one of the most commonly adopted cavity types for photon confinement, a whispering gallery (WG) resonator where photons are confined two-dimensionally has in deed its advantage for the light-matter interaction studies [7]. The overlap between exciton and cavity mode is greatly enhanced and can be close to unity due to the body of WG microcavity is itself an active medium for excitonic emission. However, despite the above advantage, polariton behavior in WG cavities is less understood and the polariton lasing (and condensation) in WG cavities has not yet been realized and remains challenging, though it is of fundamental importance [7]. Moreover, so far, polariton laser out of a planar cavity remains untunable in both exciton-photon coupling strength and emission wavelength. From both the fundamental physics understanding and the application points of view, it is highly desirable to fabrication a polariton laser with tunable coupling strength and therefore the emitting wavelength. In this talk, we demonstrate for the first time a tunable one-dimensional polariton laser operating at room temperature, basing on a taperd ZnO nano-rod with a hexagonal cross-section forming a whispering gallery cavity. A distinct pump power threshold for transition to a coherent state is observed at strong coupling regime. In addition, a polariton lasing dispersion with the detuning between the cavity mode and excitons is determined and discussed. Our work demonstrates that ZnO WG microcavity is an excellent system for developing polariton-based devices operated at room temperature.[1] S. O. Demokritov et al, Nature(London) 443, 430 (2006).[2] D. Snoke et al, Nature(London) 418, 754 (2002).[3] J. Kasprzak et al., Nature(London) 443, 409 (2006).[4] S. Christopoulos et al., Phys. Rev. Lett. 98, 126405 (2007).[5] R. Balili et al, Science, 316, 1007 (2007).[6] Daniele Bajoni et al, Phys. Rev. Lett. 100, 047401 (2008).
10:00 AM - H8.2
Cavity Modes of ZnO Nanowires and Nanobelts.
Xiulai Xu 1 , Frederic Brossard 1 , David Williams 1 , Daniel Collins 2 , Mark Holmes 2 , Robert Taylor 2 , Xitian Zhang 3
1 Hitachi Cambridge Laboratory, Hitachi Europe Ltd., Cambridge United Kingdom, 2 Clarendon Laboratory, University of Oxford, Oxford United Kingdom, 3 School of Physics and Electronic Engineering, Harbin Normal University, Harbin China
Show AbstractZnO nano-structures attract current interest because they have the potential to implement cavity quantum electrodynamics at room temperature, which is due to the excitonic binding energy of approximately 60 meV. Furthermore, this excitonic transition has a large oscillator strength, resulting in the possibility of achieving a strong exciton-photon coupling. Recently, exciton-polariton dispersion has been demonstrated in ZnO nanowires, where both Fabry-Perot (FP) and whispering gallery modes were observed in different wires. We report a photoluminescence mapping of ZnO nanowires/nanobelts both at room temperature and 4.2 K. Multi-cavity modes were observed all over the wire and belt surfaces, which were induced by Fabry-Perot interference. The emission from the surface is enhanced at both ends for nanowires and nanobelts. In addition, the emission from sides of the belt is also enhanced because of mode leakage. The emission is highly linearly polarized in a direction perpendicular to the long axis of the wires and the belts. The results correspond well to those simulated by a finite-difference time-domain calculation.
10:15 AM - H8.3
Photoluminescence of Vertically Aligned ZnO Nanorods Grown by Pulsed Laser Deposition.
Chakrapani Varanasi 1 2 , K. Leedy 2 , D. Tomich 2 , G. Subramanyam 3 , D. Look 4
1 , University of Dayton Research Institute, Dayton, Ohio, United States, 2 , Air Force Research Laboratory, WPAFB, Ohio, United States, 3 , University of Dayton, Dayton, Ohio, United States, 4 , Wright State University, Dayton, Ohio, United States
Show AbstractGrowth and characterization of ZnO nanorods (NRs) is an active area of research being pursued by several groups due to their potential technical importance in the development of various photonic, electronic and gas-sensor applications. Specifically for sensor applications, the increased surface area of nanostructures grown on the top metal electrode surface of a resonant device is expected to enhance the sensitivity of such devices. ZnO nanorods were grown on a variety of substrates such as Si, SiO2/Si, and sapphire with metal electrode (Pt/Au/Ti) layers in a large-area pulsed laser deposition chamber designed for sensor device fabrication. Processing conditions were optimized to grow ZnO nanorods with or without seed layers. Au, Cr, and BaSrTiO3 (BST) seed layers were investigated to compare their effects on the diameter and orientation of ZnO nanorods. A high resolution scanning electron microscope was used to image the ZnO NRs on fractured cross-sections to determine the diameter and length of the NRs. Aligned ZnO nanorods of 100- 300 nm diameter were observed to grow depending upon the substrate and suitable growth conditions. ZnO NRs were found to align better when grown on sapphire, Cr, or BST seed layers as compared to Au or Si layers. Photoluminescence spectra were obtained at 4.2K with a 1.25-m spectrometer having a resolution of about 0.01 meV in the near-band-edge region of ZnO. The highest quality nanorods were those grown on BST seed layers, as evidenced by 4-K photoluminescence donor-bound-exciton linewidths of only 0.5 meV. Processing, growth mechanisms of ZnO NRs, and characterization details will be presented.
10:30 AM - H8.4
Comprehensive Characterization of Optical Confinement in ZnO Nanorods.
Min Gao 1 , Wenliang Li 1 , Chengyao Li 1 , Xiaoxian Zhang 1 2
1 , Peking University, Beijing China, 2 , Institute of Physics, Chinese Academy of Sciences, Beijing China
Show AbstractIn addition to the attractive 60 meV exciton binding energy, ZnO also has a large oscillator strength indicating strong light-matter interaction. The optical confinement in one dimensional (1D) ZnO nanostructures further strongly enhances the light-matter interaction and makes ZnO nanostructures favorite candidates for various photonic applications, e.g., polariton lasers, resonators and waveguides at subwavelength scales. In this paper, we report our recent results on the optical confinement in ZnO nanorods by comprehensive luminescence techniques including the newly developed angular dependent photoluminescence (PL) and cathodoluminescence (CL) spectroscopy of individual 1D nanostructures. [1,2] The hexagonal-shaped ZnO nanorods used in this study are synthesized on silicon and sapphire substrates by Au-catalyst-assisted chemical vapor deposition at 910°C. Temperature dependent PL measurement shows continuous blueshift of the FX-nLO emissions with increasing temperature, indicating strong light-matter interaction and the formation of exciton-polaritons. To quantitatively characterize the optical confinement efficiency, we attach individual suspended ZnO nanorods to nanometer sized metal tips, thus angular resolved micro-PL measurements on individual nanostructures can be carried out. The luminescence detected along directions parallel and perpendicular to the axes of the same nanorods shows that >99% of the NBE emission can be confined inside the nanorods and emitted from the top end facets. Further measurements along the length of the nanorods show that the confinement is evidently influenced by the surface microstructure. By confocal micro-PL, we show that the local excitation position has strong influence on the intensity and shape of the guided near band edge (NBE) emission inside the ZnO nano-resonators. Especially, with the increasing distance between the excitation position and the nanorod end facet, pronounced blueshifts occur to the guided Fabry-Pérot (F-P) modes, which is not expected for classic cavity modes. In addition, the blueshifts exhibit local maxima at LO-related emissions. Our results support a polariton-related mechanism for the sub-wavelength waveguides, and emphasize the role of LO phonon in the relaxation and population dynamics of the guided exicton-polaritons along the nanorod axis. Above results have been further confirmed by angular dependent CL measurement which integrates fiber probe and metal nanoprobe techniques inside SEM and provides better spatial resolution. We will also discuss on the influences of nanorod diameter, excitation intensity and temperature on the optical confinement inside the ZnO nanorods. [1] W.L. Li, M. Gao, R. Cheng, X.X. Zhang, S.S. Xie, and L.-M. Peng, Appl. Phys. Lett. 93, 023117 (2008)[2] C.Y. Li, M. Gao, C. Ding, X.X. Zhang, L.H. Zhang, Q. Chen and L.-M. Peng, Nanotechnology 20, 175703 (2009)
10:45 AM - H8.5
Direct Imaging of the Visible Emission Bands from Individual ZnO Nanowires by Near-field Optical Spectroscopy.
Frank Guell 1 , J. Oriol Osso 2 , Alejandro R. Goni 3 , Albert Cornet 1 , Joan Ramon Morante 1 4
1 Departament d'Electrònica, Universitat de Barcelona, Barcelona Spain, 2 , MATGAS 2000 A.I.E, Bellaterra Spain, 3 , ICMAB-CSIC, Bellaterra Spain, 4 , IREC, Barcelona Spain
Show AbstractOne-dimensional nanostructures such as nanowires (NWs) have attracted much attention due to their unique properties. The reduction in size leads to novel electrical, mechanical, chemical, and optical properties. NWs are also expected to be important functional units for optoelectronic nanoscale applications, when being integrated in nanodevices. Zinc oxide (ZnO) is a semiconductor material of great importance for optoelectronics due to its wide direct band-gap of 3.37 eV and its extremely large exciton binding energy of 60 meV. Therefore, ZnO NWs have great potential for an advantageous use in devices. In this respect, it is crucial to have detailed information about the optical properties of single NWs and their dependence on wire dimensions. In this work, room-temperature photoluminescence (PL) measurements have been performed on single-crystal ZnO NWs grown on SiO2/Si and quartz substrates by the vapour transport method using Au as catalyst. Two emission bands are apparent, one in the UV spectral region around 380 nm (3.26 eV) associated to exciton recombination processes, and a much broader structure in the visible range from 420 to 700 nm, which exhibits two distinct peak-like features around 520 and 590 nm (2.38 and 2.10 eV). Spectrally resolved scanning near-field optical microscopy appears to be a very promising metrology tool for studies of nanostructured materials, in particular, ZnO NWs. Here we have successfully applied SNOM spectroscopy for obtaining a direct image of the PL emission of single ZnO NWs with spatial resolution below 100 nm. The measurements made in reflection-collection mode reveal that the green and yellow PL bands at 520 and 590 nm, respectively, are unique properties of the ZnO NWs. Furthermore, the results of transmission-collection experiments not only support the interpretation that the yellow band arises most likely from the radiative recombination of donor-acceptor-pairs related to Au impurities introduced during their catalyst-assisted growth, but also give incipient evidence of wave-guiding effects by the ZnO NWs.
11:15 AM - H8.6
The Quantum Confined Stark Effect in Square and Graded Barrier ZnO/ZnMgO Quantum Wells.
Christopher Hall 1 , Jeffery Davis 1 , Lap Dao 1 , Peter Hannaford 1 , Hoe Tan 2 , Chennupati Jagadish 2 , Kazuto Koike 3 , Shigehiko Sasa 3 , Masataka Inoue 3 , Mitsuaki Yano 3
1 Centre for Atom Optics and Ultrafast Spectroscopy, Swinburne University of Technology, Hawthorn, Victoria, Australia, 2 Electronic Materials Engineering, The Australian National University, Canberra, Australian Capital Territory, Australia, 3 Nanomaterials Microdevices Reseach Centre, Osaka Institute of Technology, Osaka Japan
Show AbstractWhen designing optoelectronic devices it is common practice to form the active material into quantum structures to take advantage of quantum confinement effects, such as tunable transitions, enhanced oscillator strength and enhanced exciton and biexciton binding energies. ZnO growth is preferentially along the c-axis, along which a large spontaneous polarization exists. The magnitude of this polarization is larger in ZnMgO than ZnO, and thus at the barrier-well interface within ZnO/ZnMgO quantum wells there is a charge imbalance. The sign of the excess charge is opposite at each side of the quantum well, thereby inducing an electric field resulting in the Quantum Confined Stark Effects (QCSE). Where quantum wells would typically enhance the optical properties, the QCSE makes them worse than the corresponding values in bulk ZnO, reducing the electron-hole wavefunction overlap, reducing the oscillator strength, increasing the radiative lifetime and reducing the exciton and biexciton binding energies.The QCSE is well known, but within the ZnO materials system the investigation of this effect is essentially limited to a number of photoluminescence measurements and fewer time-resolved measurements which do not look at early time dynamics. We have investigated the QCSE within a 6 nm ZnO/Zn0.7Mg0.3O multiple quantum well on short to long time scales, and observe the optical properties as they evolve with shielding of the internal electric field. Intensity dependent photoluminescence reveals a blue shift of 66 meV, much larger than the expected blue shift due to Mott transition. Intensity dependent pump-probe experiments reveal a continuously varying, carrier density dependent decay rate. Lifetimes shifting from 100 ps to 4 µs are measured. At higher probe photon energies strong initial transient absorption is observed due to it becoming resonant with the blue-shifted exciton transition following shielding of the internal electric field. We have designed, grown and tested a range of graded barrier quantum wells intended to counter the QCSE. The designs include symmetric triangular shaped barriers and two different asymmetric gradings, one graded so that the conduction band energy slopes in the opposite direction of the internal electric field, and one so that the valence band slopes in the opposite direction to the internal field. We compare the properties of these samples with equivalent square wells through photoluminescence and time-resolved optical experiments, identifying a mitigation of the QCSE, in agreement with simulations.
11:30 AM - H8.7
Time-Resolved Photoluminescence on ZnO-Based Single and Double Quantum Wells.
Alexander Mueller 1 , Marko Stoelzel 1 , Jan Zippel 1 , Martin Lange 1 , Matthias Brandt 1 , Gabriele Benndorf 1 , Holger Hochmuth 1 , Michael Lorenz 1 , Marius Grundmann 1
1 Halbleiterphysik, Universität Leipzig, Leipzig Germany
Show AbstractZnO/MgZnO is a promising material system for optoelectronic applications. To increase the performance of future devices, low-dimensional structures such as quantum wells are required. Using time-resolved photoluminescence, we investigated ZnO based single and double quantum well structures, which have been grown by pulsed laser deposition on a-sapphire and ZnO substrates. As excitation source served a frequency doubled/tripled titanium:sapphire laser, which could be tuned for excitation above and below the band gap of the MgZnO barrier as well as between the transition energy levels of the double quantum wells. For resonant excitation, the luminescence of the single quantum wells exhibits a mono-exponential decay, which was investigated in dependence on well width and substrate. In contrast, for excitation above the band gap of the barrier material, a slow, non-exponential decay due to diffusion of excitons and reabsorption of light from the barrier was observed. To understand the underlying mechanisms, the MgZnO luminescence decay was fitted. For the double quantum wells, the barrier width between the wells was varied. We observed an indirect exciton transition, which also exhibits a slow luminescence decay due to the spatial separation of electron and hole.
11:45 AM - H8.8
Effect of Disorders on Excitons in ZnO/MgZnO QWs.
Almamun Ashrafi 1 , Hark Tan 1 , C. Jagadish 1
1 , Electronic Materials Engineering Department, Research School of Physics and Engineering, The Australian National University, Canberra, Australian Capital Territory, Australia
Show AbstractWide bandgap Oxide semiconductors have received considerable attention due to their potential applications in optoelectronics. ZnO has a direct bandgap energy of 3.37 eV at room temperature with its largest exciton and biexciton binding energies of 60 and 15 meV, respectively, and these are enhanced further in ZnO/ZnMgO QWs, which offer the possibility to produce the efficient light emitting devices operated at elevated temperatures [1-2]. Toward these goals, there are few reports in ZnO/ZnMgO QWs which demonstrate the excitonic recombination and polarization effects. A square well of ZnO/MgZnO QWs grown along the (0001) axis experiences a huge internal electric field induced by the difference in spontaneous and piezoelectric polarizations between the well and the barrier leading to a modified potential profile [1-3]. As a consequence, the resulting built-in electric field spatially separates both the electrons and holes wave functions to each side of the QWs, i.e., the electrons and holes are more sensitive to the well width fluctuation and/or alloy broadening. Furthermore, owing to the very small exciton lateral extension, quantum confinement should extremely be sensitive to the nanoscale interface roughness and alloy broadening. These disorders break the in-plane translational symmetry and lead to a dramatic change in the physical behavior of excitons, producing multiple scattering and localization. In this abstract, we have addressed these effects in ZnO/MgZnO QWs grown on Al2O3(0001) substrates by pulsed laser deposition. ZnO well width was varied from 0.6 to 8.0 nm by keeping the constant MgZnO barrier layer thickness at 50 nm. X-ray diffraction and photoluminescence (PL) measurements indicated that the Mg composition in MgZnO barrier is to be 0.24. A significant blueshift was observed in ZnO/MgZnO QWs. This has been fitted with the simple triangular potential well model by E = Eg + ΔEc + ΔEv - Eex - qLF, where L is the ZnO well width and F is the internal electric field. PL data indicated weak polarization effects resulting in the internal electric field of 0.3~0.7 MV/cm. Interface broadening was observed due to the alloy broadening and interface roughness. The alloy broadening was calculated to be 11.77 meV, while the interface roughness is 7~18 meV. Even with these inhomogeneous broadening, enhanced luminescence properties were observed in the temperature-dependent PL measurements. Carrier/exciton localization with activation energy of 4−16 meV was measured which have been attributed to potential fluctuations at the well-barrier interfaces. The built-in electric field will also be investigated in the ZnO/MgZnO MQWs. The quantized energy, band offset, potential asymmetry, exciton dynamics and the development of the concurrent ZnO/MgZnO QWs will be presented. Reference[1] J. A. Davis and C. Jagadish, Laser Photon Rev. 3(2009)85[2] T. Makino, etal, Superlatt. Micrstruc. 42(2007)206[3] M. Leroux, etal, Phys. Rev. B 58(1998)R13371
12:00 PM - H8.9
Zinc Oxide Based Hydrogel for Bio-Applications using Two-photon Excitation.
Ben Urban 1 , Sween John 1 , Sreekar Marpu 1 , Yasuhisa Fujita 2 , Zhibing Hu 1 , A. Neogi 1
1 , University of North Texas, Denton, Texas, United States, 2 , Shimane University, Matsue Japan
Show AbstractCell imaging generally uses conventional fluorescent dyes or organic fluorophore like green fluorescent protein. The drawbacks for these routine tags are: 1) poor photo stability, narrow absorption band and broad emission spectra. Colloidal quantum dots (QDs) such as CdSe,CdTe are efficient fluorophore for biological application but it may be toxic in nature. Therefore, ZnO can be elected as a potential replacement for conventional dyes and toxic quantum dots in bio-medical applications. This study presents a novel ZnO-Hydrogel based fluorescent colloidal semiconductor nanomaterials system for bio-medical applications such as cell imaging. The preparation of ZnO nanoparticles and their surface modification to make a biocompatible material with enhanced optical properties is discussed. High quality rod shaped ZnO particles of 200~300nm in diameter with narrow UV emission is produced compared to the surface defect induced green emitting ZnO particles prepared by chemical methods. Semiconductor materials including ZnO are insoluble in water. Since biological applications require water soluble materials, ZnO nanoparticles are first dispersed in water by ball milling method, and their aqueous stability and fluorescence properties are enhanced by incorporating them in bio-compatible Poly N-isopropylacrylamide (PNIPAM) based hydrogel polymer matrix. These microgel particles measured by Dynamic Light Scattering method were found to be ~300 nm in diameter. The optical properties of ZnO-Hydrogel colloidal dispersion versus ZnO-Water dispersion were analyzed. Photoluminescence Spectroscopy at room temperature indicates approximately 10 times enhancement of fluorescence in ZnO-Hydrogel colloidal system compared to ZnO-Water system, confirming the surface modification of ZnO nanoparticles by hydrogel polymer matrix. For comparison purposes, concentration of ZnO in both the samples was maintained constant. The fluorescence spectra and lifetime decay of ZnO excited by single and two photon excitation in hydrogel was compared to ZnO in water at room temperature. Excitation wavelength used for single photon spectroscopy was 350 nm where as for two-photon spectroscopy it was 720 nm, using a 80 MHz Mai-Tai Laser. The single photon emission is at 376 nm where as the emission due to two photon excitation is at 420 nm , which corresponds to the band-edge emission of ZnO. The enhancement of fluorescence in ZnO-hydrogel system is likely due to the modification of ZnO surface by polymer gel. Enhancement in trap induced blue emission by the modification of ZnO using PMMA has been reported.We postulate that the surface of a ZnO particle is modified by the adsorption of microgel particles, thereby increasing the scattering centers per unit area of cross-section, and hence increasing the luminescence from the ZnO-gel samples.
12:15 PM - **H8.10
A Large-area, Touch-sensitive Surface Comprising of a Zinc Oxide Nanowire Composite for Display Applications.
Sieglinde Pfaendler 1 , Michael Swanwick 1 , Aron Rachamim 1 , Paul Beecher 2 , Sophie Machin 1 , Piers Andrew 2 , William Milne 1 , Andrew Flewitt 1
1 Engineering (Electrical division), University of Cambridge, Cambridge United Kingdom, 2 , Nokia Research Centre, Cambridge United Kingdom
Show AbstractFuture generation of mobile devices will be both fully flexible and integrate tactile control, as has been proposed in Nokia’s ‘morph’ concept device. This will require simultaneous multi-touch sensing capability in which the force applied can also be determined and distinguished from flexing. Commercial touch sensors based on capacitive or resistive technologies lend themselves poorly to such an application. In this work, a novel touch sensing system is presented in which vertically aligned zinc oxide nanowires are embedded in a polymer matrix to produce an engineered composite material. The purpose of the polymer matrix is to provide a soft mechanical support to the nanowires allowing their deformation when a vertical force is applied whilst resisting fracture. When the substrate is simply flexed, the polymer will take up the strain induced in preference to the nanowires, which should therefore be unaffected by this type of deformation. Such a piezo system has the added benefit of promising optical transparency and biocompatiblity.Nanowire deformation due to touch is detected using a two-terminal electrical measurement system comprising of large-area, parallel electrodes on the top and bottom of the composite layer ~1 mm^2 in area. When a force is applied to the nanowires in the region of the electrodes, a small electrical signal is detected. A suggested mode of operation involves charge generation due to compression of the piezoelectric ZnO nanowires. In this case, the fact that the nanowires are connected in parallel means that the capacitance of the system is sufficiently high for a signal to be measured, which counters the argument that a single wire could not generate sufficient charge.
12:45 PM - H8.11
Room Temperature Ferromagnetism in Al-doped/Al2O3-doped ZnO Film.
Yuwei Ma 1 2 , Jun Ding 1 , Lap Chan 2 , Chee Mang Ng 2 , Jiabao Yi 1 , Nina Bao 1
1 Department of Materials Science and Engineering, National university of Singapore, Singapore, Singapore, Singapore, 2 , Chartered Semiconductor Manufacturing Ltd, Singapore, Singapore, Singapore
Show AbstractZnO doped with transition metals (TMs) has been theoretically predicted to have room temperature ferromagnetism (RTF). Therefore, ZnO has received extensive attentions in spintronics applications. Room temperature ferromagnetism has been reported experimentally by many research groups for 3d TMs doped ZnO. The origin of ferromagnetism in ZnO films is still under debate. Recently, our research group also found that carbon-doped ZnO is magnetic at room temperature via p-p interaction of C and O sp orbitals.In this manuscript, we study the magnetic property of ZnO films doped with Al or Al2O3. The room temperature ferromagnetism is found in ZnO doped with metallic Al elements while there is no ferromagnetism found in ZnO doped with Al2O3 (Al <5 mol%). We report two physical ways to introduce Al metallic elements into ZnO films, either by diffusion or co-deposition. In the diffusion method, Al top layer (10nm) was grown on ZnO underlayer (120nm) by pulse layer deposition (PLD) at 800oC. The film was then underwent the vacuum heat treatment at 700oC. SQUID measurement shows that the film possesses room temperature ferromagnetism (saturation magnetization=1.79emu/cm3). The ferromagnetism disappears when the film was further annealed in air atmosphere at 700oC. In the co-deposition method, both metallic Al and ZnO were deposited onto quartz substrate (x-cut) with Al concentration of 5 mol%. The composition of the film was measured by both EDS and XPS. We cannot observe can impurity (especially magnetic particles) in the film with the detection limit of XPS. The saturation magnetization is recorded as 3.4emu/cm3 with Curie temperature above 300K. The detailed XPS study shows that the ferromagnetism may be related to the charge transfer between Al and ZnO as Zn2+ is reduced into Zn1+/Zn0. This is quite expected as metallic Al is very easy to be oxidized in ZnO matrix. We further study the magnetic property in Al3+ doped ZnO. The film was deposited from (Zn, Al)O ceramic target onto quartz substrate by PLD at 400oC under an oxygen partial pressure of 10-4 torr. There is no ferromagnetism found in Al2O3 doped ZnO with Al<5 mol%. In this sample, there is no charge transfer between Zn and Al observed. In addition, when Al molar concentration increases to 30 mol%, high resolution TEM shows that ZnO nanocrystals (NCs) are observed in ZnO-Al2O3 amorphous matrix. SQUID measurement shows the film has room temperature ferromagnetism with saturation magnetization of 3.6 emu/cm3. The ferromagnetism in the film is ZnO NCs size dependence. The optimum size of ZnO NCs to give the highest magnetic moment is around 4.5 nm. The mechanism of room temperature ferromagnetism may be explained by the interaction of ZnO NCs and surface defects of ZnO NCs.
H9: TCO and TFT
Session Chairs
Steve Durbin
Sieglinde Pfaendler
Thursday PM, December 03, 2009
Ballroom C (Hynes)
2:30 PM - H9.1
Effect Of Substrate Temperature and Distance on RF-Sputtered Ga-doped ZnO Thin Films.
Christopher Gorrie 2 1 , John Perkins 1 , Ajaya Sigdel 3 1 , Brandon Reese 4 1 , Joseph Berry 1 , Paul Holloway 2 , David Ginley 1
2 , University of Florida, Gainesville, Florida, United States, 1 , National Renewable Energy Lab., Golden, Colorado, United States, 3 , Denver University, Denver, Colorado, United States, 4 , University of Colorado, Boulder, Colorado, United States
Show AbstractDue to their potential for improved electronic and environmental stability, Ga-doped zinc oxide (GZO) thin films are being developed as an alternative to Al-doped zinc oxide for transparent electrode applications. In this work, we report the coupled effects of substrate distance and substrate temperature (TS) on the optical, electrical and structural properties of GZO. In particular, GZO thin films were deposited on glass substrates by radio frequency (RF) magnetron sputtering using a ZnO:Ga2O3 ceramic target (4 at.% Ga vs. Zn). The highest conductivity of 3200 S/cm was obtained at a temperature of 250 °C and at a target-substrate distance of 2". This sample also had the highest carrier concentration in this study, 9.6 x 1020/cm3 corresponding to a doping efficiency of ~ 0.6 electrons per Ga. Our results, which find a strong maximum in the conductivity with deposition temperature at TS ≈ 250 °C, contrast published work which reports monotonically increasing conductivity with increasing temperature up to 500 °C. Optically, all of the films in this study were clear, with an optical transmittance of ∼ 90% in the visible range, but show strong free carrier (Drude) absorption peaked between 1300 and 1600 nm. The grain size (determined from the 002 XRD peak width) of the most conducting film was ~ 40 nm whereas for less conducting films grown with a shorter distance, the grain size was as large as 60 nm. In addition, for all the (002) textured films, there was no correlation in the spread of (002) grain angle orientation from pseudo rocking curve measurements with differences in the conductivity. In summary, TS was found to have more pronounced effect on conductivity than did the substrate-target distance and collectively, these results indicate that Ga donor activation varies more than carrier mobility for the films grown in this work. Work is underway to determine if RF superimposed DC sputtering will yield further improvements.
2:45 PM - H9.2
Optical Properties of Indium Zinc Oxide (IZO) Micro- and Nanostructures Grown by Thermal Treatments of InN, In2S3 and ZnO Powders.
Javier Bartolome 1 , David Maestre 1 , Ana Cremades 1 , Javier Piqueras 1
1 , Universidad Complutense de Madrid, Madrid Spain
Show AbstractSemiconductor nanowires represent an important and broad class of one-dimensional structures at the forefront of nanoscience and nanotechnology. In particular an intense activity has been carried out on oxide semiconductors such as In2O3, SnO2,or ZnO. Other fields of research are emerging due to the interesting properties of Indium Zinc Oxide (IZO), as for instance in organic related technology. Our work report on the study of the structural and optical properties of IZO micro- and nanostructures grown by an evaporation-deposition method, as a function of their dimensions, morphology or composition. The analysis of the morphology, as well as the effect of the doping concentration on the luminescence of these IZO micro- and nanostructures, have been performed by means of imaging and spectral cathodoluminescence and EDX-SEM techniques. The waveguiding behavior of the structures for exciting light has been investigated by illumination with green light.Two different materials have been used as precursors in the fabrication of IZO structures, in order to analyze their effect in the resulting morphology and properties of the structures. When using a mixture of InN- ZnO (5%wt) powders, rods and arrow-shape structures are obtained at different temperatures between 700 and 1000 C. However when using In2S3 – ZnO (5% wt) mixtures as precursors, wires and complex branched structures of IZO are grown at temperatures around 1000C. Luminescence properties measured by cathodoluminescence in the SEM show a dependence on the Zn concentration and the growth temperature. EDS-SEM analysis has been applied to visualized the incorporation of Zn in the different structures. Under illumination, intense green spots are observed at the ends of the structures showing the waveguiding of the green light. The effect of the Zn doping on this behavior is analyzed.[1] D. A. Magdas, A. Cremades, J. Piqueras, Appl. Phys. Lett. 88 (2006) 113107[2] D.Maestre, A.Cremades, J.Piqueras, J.Appl. Phys. 97 (2005) 044316
3:00 PM - H9.3
Transport Properties and Electronic States of Anatase Ti1-xWxO2 Epitaxial Thin Films.
Akira Chikamatsu 1 , Utahito Takeuchi 1 , Taro Hitosugi 2 3 , Hiroshi Kumigashira 4 , Masaharu Oshima 4 , Yasushi Hirose 1 3 , Toshihiro Shimada 1 3 , Tetsuya Hasegawa 1 3
1 Depertment of Chemistry, The University of Tokyo, Tokyo Japan, 2 Advanced Institute for Materials Research, Tohoku University, Sendai Japan, 3 , Kanagawa Academy of Science and Technology, Kawasaki Japan, 4 Department of Applied Chemistry, The University of Tokyo, Tokyo Japan
Show AbstractTi1-xNbxO2 and Ti1-xTaxO2 are members of d-electron based transparent conducting oxides with excellent resistiivity and transmittance comparable to ITO. In these materials, each Nb or Ta ion substituting for Ti is almost completely ionized to Nb5+ or Ta5+ and release one conduction electron. Doping of hexavalent ions into TiO2 might provide more carriers and thus further improve resistivity. Here, we report on transparent conducting properties of W-doped anatase TiO2 (Ti1-xWxO2) fabricated by pulsed laser deposition (PLD). Based on transport and XPS measurements, we discuss the conduction mechanism of Ti1-xWxO2, focusing on the valence state of W. Comparison will also be made between Ti1-xNbxO2 and Ti1-xWxO2 in terms of the electronic structure near the Fermi level as influenced by impurity doping.Ti1-xWxO2 (x = 0.01 - 0.09) films were grown on LSAT (100) substrates by PLD method. Crystal structures were characterized by an X-ray diffraction (XRD) spectrometer. Transport properties were measured in the six-terminal geometry. Electronic states were investigated by soft-X-ray photoemission spectroscopy (PES) at BL-2C of the Photon Factory, KEK. XRD measurements confirmed epitaxial growth of anatase without any secondary phases. We also found that the lattice constant of anatase systematically varies along with W-doping, proving W substitution for Ti. The Ti0.95W0.05O2 film deposited under an optimized condition showed resistivity of 2 x 10-3 Ωcm at room temperature, which is approximately 10 times higher than that of Ti0.94Nb0.06O2. This is mainly due to lower carrier density in Ti1-xWxO2 films. Based on the core-level PES results that doped W exists as W6+ in the films, we evaluated the activation efficiency as ~ 10 %, which is substantially low compared to that of Ti0.94Nb0.06O2 film, ~ 100 %. These results suggest that doped W atoms might form impurity states trapping carriers. From PES measurements of Ti0.91W0.09O2 films, indeed, we observed the finite density of states originating from O 2p hybridized with W near the top of the valence band. On the other hand, we could not find any states at the same position in the Ti0.94Nb0.06O2 films. In addition, the density of states near the Fermi level, N(EF), is quite low in the Ti0.91W0.09O2 films, in sharp contrast to Ti0.94Nb0.06O2 indicating remarkably high N(EF) assigned to the bottom of the Ti 3d conduction band. These observations are consistent with the results of transport measurements. Consequently, the difference in transport properties between Ti1-xWxO2 and Ti1-xNbxO2 is attributed to the difference of electronic states near EF associated with W or Nb doping.
3:15 PM - H9.4
Growth and Characterization of p-NiO Thin Films and Heterostructure of (NiOx / ZnGa0.05O) Thin Films for Transparent Electrode Application.
Titas Dutta 1 , Pranav Gupta 1 , Alok Gupta 1 , Jagdish Narayan 1
1 Material sci. and Engg., North Carolina state university, Raleigh, North Carolina, United States
Show AbstractNiO (a cubic rock salt structure with lattice constant, a = 0.417 nm) is a promising candidate for transparent conducting oxides (TCOs) as it is a p-type semiconductor with band gap energy from 3.6 to 4.0 eV, and it has excellent chemical stability. We report here the epitaxial growth of cubic NiO thin film on c-plane sapphire (α-Al2O3) by pulsed laser deposition technique. Our results show that NiO (111) grows along the c-axis on α-Al2O3. To meet the required physical properties of NiO for TCO applications, the growth parameters like Ts (substrate temperature), oxygen partial pressure and the placement of oxygen nozzle were optimized. It was found that p-type semiconducting behavior was entirely dependent on these growth conditions. The X-ray photoelectron spectroscopy (XPS) results show that Nickel exists in multiple oxidation states, mainly (Ni2+ and Ni3+). The presence of defects (localized Ni3+) in the non-stoichiometric NiOx are the suggestive cause of hole conductivity in the NiOx. Additionally, NiOx provides higher work function, and is envisaged to improve the transport of the carriers across the heterojunction in the organic photovoltaic devices, thus resulting in increased device efficiency. We have demonstrated in our earlier work that highly conducting and transparent Ga:ZnO can be deposited on single crystalline sapphire substrates as well as on glass substrates using pulsed laser deposition (PLD) [1,2]. To overcome the low work function problem of ZnO based TCO’s, we have also reported the growth of novel alternative high work function TCO consisting of thin buffer layer of MoOx (2.0 < x < 2.75) on Ga doped ZnO (GZO) by PLD. Using this approach, we have created bilayerd TCO consisting of thin overlayer of p-NiOx on Ga doped ZnO (GZO) by PLD [3]. Incorporation of p-NiO facilitates hole conduction in the device. It was found from the Ultraviolet Photoelectron Spectroscopy (UPS) measurements that the ZnGa0.05O films with ultra thin NiOx (~ 3 - 6 nm) overlayer showed a higher work function (~ 5.3 eV) as compared to the single layer ZnGa0.05O film work function (~ 4.5 eV). An attempt has been made to establish correlations between the processing parameters, the structure and properties of the bilayer films (NiOx /ZnGa0.05O) for transparent electrode applications. It is shown that by growing a thin NiOx buffer layer on ZnGa0.05O, a higher work function can be achieved without affecting the overall transmittance and sheet resistance.1. V. Bhosle, A. Tiwari, and J. Narayan, Appl. Phys. Lett. 88, 32106 (2006).2. V. Bhosle, A. Tiwari, and J. Narayan, J. Appl. Phys. 100, 033713 (2006).3. Titas Dutta, P. Gupta, V. Bhosle, J. Narayan, J. Appl. Phys. 100 (5) (2009).
3:30 PM - H9.5
First-principles DFT Study of Dopants at Interfaces in the TCO Material ZnO.
Wolfgang Koerner 1 , Christian Elsaesser 1
1 , Fraunhofer-Institut fuer Werkstoffmechanik IWM, Freiburg Germany
Show AbstractA first-principles density-functional-theory (DFT) study of ZnO with focus on its use as a transparent conducting oxide (TCO) material is presented. The impact of grain boundaries in a doped polycrystal on thermodynamic and electronic properties of atomic defects is investigated by DFT pseudopotential calculations for supercell models containing both interfaces and dopants. For oxygen vacancies, cation dopants Al and Ga substituting Zn, or anion dopants N and P substituting O, energies of defect formation and interface segregation are determined in the local density approximation (LDA). Defect levels in the electronic band structure of ZnO are analysed in terms of densities of states, which are calculated by means of the LDA with a self-interaction-correction (SIC). This SIC-LDA approach yields a theoretical band structure of bulk ZnO with sufficiently accurate band widths and band gaps, as compared to experiment, and it is applicable to large supercells needed for interfaces with no more computational effort than LDA. The important outcome of this study is a detailed microscopic information on how much positions and shapes of electronic defect levels can be altered at grain boundaries with respect to a doped single crystal.
3:45 PM - H9.6
Choice of Gate Dielectric on the Performance of Thin Film Transistors based on Indium Zinc Oxide Deposited at Low Temperature.
Mark Mann 1 , Flora Li 1 , Ahmed Kiani 1 , Andrew Flewitt 1 , James Dutson 2 , Steve Wakeham 2 , Mike Thwaites 2 , William Milne 1
1 Department of Engineering, University of Cambridge, Cambridge, Cambridgeshire, United Kingdom, 2 , Plasma Quest Ltd., Hook, Hampshire, United Kingdom
Show AbstractWide-band gap, high mobility, amorphous ionic oxide semiconductors, such as Indium Zinc Oxide (IZO), have received significant attention as the channel layer for transparent thin film transistors (TFTs). These can be used in applications such as active matrix organic light emitting diode displays on glass or plastic substrates. However, in order to realise a high performance TFT, it is necessary to identify both a compatible dielectric and a means of controlling the defect density at the resulting interface. In the case of amorphous silicon transistors, it is known that bottom gate TFTs with a silicon nitride dielectric have acceptable device characteristics, but no consensus yet exists for IZO TFTs.In this study, hafnium oxide, aluminium oxide, thermal silicon oxide and PECVD silicon nitride have all been investigated as gate dielectrics for TFTs incorporating an IZO channel. The deposition of the metal oxide materials is performed at room temperature using a high target utilisation sputtering (HiTUS) system at very high growth rates. This process, in which plasma generation is separated from the substrate, results in reduced ion bombardment of the substrate and allows for more precise control over deposition parameters and thus the film properties. TFTs fabricated using amorphous IZO as the channel layer have a high switching ratio of over 107, and exhibit a field effect mobility of 10 cm2V-1s-1, which is an order of magnitude higher than amorphous silicon technology. The relative advantages of each gate dielectric will be discussed, together with the results of bias-stressing of the metal-oxide thin film transistors to allow assessment of the device stability. A strong dependence of device performance and stability under prolonged bias stress on the choice of gate dielectric material will be reported.
4:30 PM - **H9.7
ZnO Thin Film Transistors for RF Applications.
Burhan Bayraktaroglu 1 , Kevin Leedy 1 , Robert Neidhard 1
1 , Air Force Research Laboratory, Wpafb, Ohio, United States
Show AbstractAlthough thin film transistors (TFT) have been in use as long as the conventional transistors, they are still regarded as poorly performing devices that are only suitable for low speed applications. Very low electron mobilities (0.1-1 cm2/V.s) typically associated with conventional TFT’s based on amorphous Si and organic semiconductors coupled with threshold instabilities due to poor grain boundary and interface charge control issues have prevented this technology from advancing to more demanding applications. Recently, alternative TFT technologies based on heavy metal oxide compounds such as ZnO have been investigated to overcome these limitations. High speed operation of nanocrystalline ZnO TFTs was demonstrated on GaAs substrates. Devices with 2um gate lengths produced record figure-of-merit numbers such as on/off ratio of 10^12, current density of >400mA/mm and field effect mobility of 110 cm2/V.s. Building on these early results, we have refined material deposition and device design approaches to demonstrate that microwave ZnO TFTs can also be fabricated on Si substrates for potential future integration of high speed TFTs on Si integrated circuits. We have achieved fT and fmax values of 2.45GHz and 7.45GHz, respectively with 1.2um gate length devices demonstrating that ZnO-based TFTs are suitable for RF applications.ZnO films were deposited in a Pulsed Laser Deposition (PLD) system optimized for uniform films over large area (up to 100mm) high resistivity (>2000 ohm-cm) Si substrates. The gate insulator was SiO2 prepared by Plasma Enhanced Chemical Vapor Deposition (PECVD). ZnO grain sizes were typically in the 20-40nm range depending on the growth conditions. TEM studies have shown that grains are arranged as nanocolumns with only vertical grain boundaries present. Such an arrangement is preserved even over conformal surfaces. The device stability was examined by storing devices under constant gate and drain bias conditions at room temperature and then re-measuring the transfer characteristics. No shift in threshold voltage was observed within experimental limitations. Microwave measurements were made using on-wafer coplanar microwave probes in an Agilent 8364B PNA. From the measured S-parameters, the current and power gain cut-off frequencies , fT and fmax were determined. The results indicate that ZnO TFTs are suitable for RF applications even with moderate gate lengths. Microwave performance above 10GHz is expected with shorter gate devices.1] B. Bayraktaroglu, K. Leedy and R. Neidhard, “Microwave ZnO Thin-Film Transistors”, IEEE Electron Dev. Lett., p. 1024 (2008)
5:00 PM - H9.8
Fabrication of IGZO-channel Ferroelectric-gate TFTs with Organic P(VDF-TrFE) Film.
GwangGeun Lee 1 , SungMin Yoon 2 , JooWon Yoon 3 , Yoshihisa Fujisaki 4 , Hiroshi Ishiwara 3 , Eisuke Tokumitsu 1
1 Precision and Intelligence Lab, Tokyo Inst of Tech, Yokohama Japan, 2 Electronics And Telecommunications Res. Inst. , ETRI, Deajeon Korea (the Republic of), 3 Central Research Laboratory, Hitachi Ltd, Tokyo Japan, 4 Interdisciplinary Graduate School of Science and Engineering, Tokyo Inst of Tech, Yokohama Japan
Show Abstract Recently, thin film transistors (TFTs) using transparent oxide semiconductor (TOS) as a channel material have been extensively studied for flat panel displays and system-on-panel applications. Among the various TOSs, amorphous oxide InGaZnO (IGZO) has relatively high electron mobility, good uniformity, low processing temperature and stability. In particular, a combination of IGZO and organic ferroelectric materials will produce the nonvolatile memories which can be realized on plastic or glass substrate. In this work, we fabricated ferroelectric-gate TFTs using IGZO as a channel and P(VDF-TrFE) as a gate insulator. We also investigated the effect of thermal annealing of the IGZO thin films on electrical properties of ferroelectric gate TFTs. We fabricated two kinds of TFTs which have top-gate-type coplanar structure with different IGZO fabrication conditions, with thermal annealing and without thermal treatment. First, 10-nm-thick IGZO layers were deposited as a channel layer by RF sputtering at room temperature, which was performed in an argon atmosphere with oxygen partial pressure of 1.9%. Then, one sample was annealed at 300 °C for 15 min in O2 and the other sample was fabricated without thermal treatment. Next, ITO films (80-nm-thick) were sputtered as the source and drain electrodes and then, the P(VDF-TrFE) films (210-nm-thick) were prepared by spin-coating method and annealed on a hot plate at 140 °C for 1 h to enhance the crystallinity. The contact holes for the source and drain regions were formed by RIE. Finally, the gate electrode was formed with thermally evaporated Al and the pattering of the Al was performed by wet chemical etching method. The channel length and width of the fabricated Fe-FETs were 25 µm and 100 µm, respectively. The drain current and gate voltage (ID-VG) characteristics of both fabricated TFTs showed counterclockwise hysteresis loop due to the ferroelectric nature of P(VDF-TrFE) films. When the gate voltage was varied from – 20 V to 20 V and then returned to - 20 V at a constant drain voltage of 5 V, the obtained on/off ratio and memory window were more than 10^6 and 4 V, respectively. TFT with the thermal annealing IGZO film showed the shift of the threshold voltage (Vth) to positive direction compared with the sample fabricated without thermal treatment. The Vth values of TFTs fabricated with and without thermal treatment were about -2 V and -4 V, respectively. It means that thermal annealing IGZO film affect the carrier concentration and electrical resistivity of the IGZO channel layer. In summary, we fabricated the nonvolatile TFTs using IGZO films and organic ferroelectric P(VDF-TrFF) films. Fabricated TFTs showed good ferroelectric properties and the thermal annealing of IGZO film affected the Vth of the device.This work was partly supported by Grants-in-Aid for Scientific Research (A) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
H10: Poster Session II
Session Chairs
Leonard Brillson
Jürgen Christen
Hiroshi Fujioka
Hoe Tan
Friday AM, December 04, 2009
Exhibit Hall D (Hynes)
9:00 PM - H10.1
Study of Light-induced Electron Spin Resonance in Transition Metal Doped TiO2.
Kun Xu 1 , Mitsuru Izumi 1 , Osami Yanagisawa 2 , Tetsuya Ida 3
1 Labotorary of Applied Physics, Tokyo University of Marine Science and Technology, Tokyo Japan, 2 Maritime Technology Department, Yuge National College of Maritime Technology, Ochi-gun Japan, 3 Department of Electronic Control Engineering, Hiroshima National College of Maritime Technology, Hiroshima Japan
Show AbstractThe 3d transition-metal doped TiO2 has been the subject of interest for its room temperature ferromagnetic properties. However, the intrinsic mechanisms of the ferromagnetism appearance and magneto-optical properties still need further investigations. Due to the small value of the magnetization, whether the transition-metal impurities are the origin or not is under debate. Besides, different conclusions have been drawn by using different preparation methods.We study a series of transition-metal (TM = Cr, Cu, Co, Mn etc.) doped either rutile or anatase TiO2 thin films prepared by the rf -magnetron sputtering. X-ray diffraction results show no remarkable intensity for possible hkl diffraction with a trace of oxide of doped metals. Both the dc magnetization and electric properties are exhibited together with the M-H hysteresis loops measurements. The results show a ferromagnetic behavior and the Curie temperature TC is much higher than room temperature.The X-band electron spin resonance measurement was carried out at room temperature. The resonance field and linewidth vary slightly with different transition metals and doping rates. Furthermore, the angular dependence of both resonance field and linewidth is analyzed with a fitting procedure based on Landau-Lifshitz-Gilbert equation. In the next step, we report the effect of light illumination on the present paramagnetic resonance study performed by using a 160 mW Nd-YAG laser with photon energy of 1.17 eV. The observed resonance linewidth and intensity profile during the light illumination are discussed. This phenomenon could be attributed to the enhancement of carrier mobility. This experiment result may give a direct evidence that the ferromagnetism in transition-metal doped TiO2 is induced by charge carrier.
9:00 PM - H10.10
Synthesis and Characterization of Fe Doped ZnO Nanoparticles.
Lydia Johnson 1 , Aaron Thurber 1 , Joshua Anghel 1 , Dmitri Tenne 1 , Alex Punnoose 1
1 Physics, Boise State University, Boise, Idaho, United States
Show AbstractTransition metal doped oxides have a wide range of potential applications, from microelectronics to novel cancer treatments. In this study, ZnO nanoparticles doped with Fe over a range of increasing percentages from 0 to 20 percent have been synthesized via a forced hydrolysis method. Resultant particle sizes are estimated at 7.47 ± 0.34 nm by applying the Scherrer relation to x-ray powder diffraction (XRD) data. The XRD data was also used to calculate the lattice parameters a and c for the ZnO hexagonal lattice. Graphical analysis of the lattice parameter ratio (c/a) versus Fe doping percent indicates a steady decrease in c/a to a minimum at 7.5%, followed by little change at higher percents. Further characterization was performed via vibrating sample magnetometry, transmission electron microscopy and x-ray photoelectron spectroscopy, as well as Raman and photoluminescence spectroscopy. Analysis of photoluminescence spectra taken at 10 K indicates that the bandgap energy of the Fe:ZnO nanoparticles increases with Fe percent, from 3.34 eV for 0.1% Fe:ZnO, to 3.49 eV for 20% Fe:ZnO. Resonant Raman scattering by ZnO phonons up to sixth order has been observed in the Fe:ZnO nanoparticles as the bandgap energy approaches the excitation laser energy of 3.8 eV. Supported in part by NSF (CAREER award DMR-0449639, MRI grant 0722699), DoE EPSCoR (grant DE-FG02-04ER46142), and ARO.
9:00 PM - H10.11
Nanoscale Co-Assembly of Zinc Oxide and Conjugated Organic Molecules in Macroscopic Films via Electrodeposition.
David Herman 1 , Joshua Goldberger 2 , Haoming Zhang 1 , Carson Bruns 2 , Samuel Stupp 1 2 3
1 Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States, 2 Chemistry, Northwestern University, Evanston, Illinois, United States, 3 Feinberg School of Medicine, Northwestern University, Chicago, Illinois, United States
Show AbstractThe nanoscale assembly of zinc oxide with organic materials is important for the use of these hybrid materials in many applications, including photovoltaics and photodetection. The current challenge is to effectively control the periodicity and macroscopic architecture of zinc oxide and organic phases across scales in order to optimize charge transport in devices. Through a one-step electrodeposition process, we have produced films that exhibit nanoscale lamellar periodicity of zinc oxide and assemblies of the conjugated organic molecule pyrene butyric acid. While maintaining this nanoscale periodicity, we demonstrate control over the macroscopic film growth by varying the supporting electrolyte and applied potentials. We also show how this method of co-assembly can be extended to other conjugated organic molecules.
9:00 PM - H10.12
On the Properties of Indium-doped Zinc Oxide Films prepared by Atomic Layer Deposition using Metalorganics and Nitrous Oxide.
Chi-Ying Hsiao 1 , Jing-Hsung Yang 1 , Jyh-Rong Gong 1 , Dong-Yuan Lyu 2 , Tai-Yuan Lin 2 , Cheng-Tao Lu 3 , Der-Yuh Lin 3
1 Department of Physics, National Chung Hsing University, Taichung Taiwan, 2 Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung Taiwan, 3 Department of Electronic Engineering, National Changhua University of Education, Changhua Taiwan
Show AbstractIndium-doped zinc oxide (IZO) films were successfully grown on (11-20) sapphire substrates at 300°C by atomic layer deposition (ALD) using diethyl-zinc (DEZn), trimethyl-indium (TMIn) and nitrous oxide (N2O) as precursors. Conductive and transparent IZO films were achieved at a gas phase composition ratio [TMIn/(TMIn+DEZn)] of 0.01 with resistivities being at high 10^-4 Ω-cm and optical transmittances being higher than 90% in the visible spectra. The effect of Burstein-Moss shift was also observed in the high In-incorporated IZO films.
9:00 PM - H10.13
Polarity Engineering in Polycrystalline ZnO by Inversion Boundaries.
Jong-Lo Park 1 , Chan Park 1 2 , Wook Jo 3 , Chul-Jae Park 4 , Sang-Yun Jeon 4 , Jong-Sook Lee 4
1 Department of Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of), 2 Research Institute of Advanced Materials (RIAM), Seoul National University, Seoul Korea (the Republic of), 3 Institute of Materials Science, Technische Universität Darmstadt, Darmstadt Germany, 4 School of Materials Science and Engineering, Chonnam National University, Gwangju Korea (the Republic of)
Show Abstract A ZnO varistor is used to protect circuits from surge current. Some dopants are added into the varistor to control the electrical property and microstructure of the varistor. An inversion boundary (IB) which affects the polarity of the grain boundary, can exist in the grains of varistor. The non-linear current-voltage property of ZnO is influenced by a Schottky barrier formed at the grain boundaries. Single-junction measurements of ZnO varistor have suggested that IBs are not of direct responsibility for the varistor properties, compared with the grain boundaries. However, it is essential to understand the role of IB which affects the crystallographic property of the grain boundary. To investigate the effect of IB on varistor properties, polarity-engineered polycrystalline ZnO samples have been prepared via uniform formation of inversion boundaries in individual grains. The presence of the head-to-head IB induced by Sb dopant and the tail-to-tail IB by Ti dopant was directly confirmed by the geometry of chemical etch pits. The electrical properties of the specimens were characterized by impedance analysis and I-V measurements at room temperature. From the electrical characterization, ZnO with Sb has varistor property which is superior to ZnO doped with Ti. The polarities of the grain boundary planes which are affected by those of IBs, can be closely related to the height of the grain boundary Schottky barrier in ZnO, which also explains the superior performance of ZnO varistors containing IBs.
9:00 PM - H10.14
Polarity Determination of ZnO Films by X-ray Diffraction using Anomalous Dispersion.
Yutaka Adachi 1 , Naoki Ohashi 1 , Isao Sakaguchi 1 , Hajime Haneda 1
1 , National Institute for Materials Science, Tsukuba, Ibaraki, Japan
Show AbstractIn this study, we attempted the determination of the polarity of c-axis oriented ZnO films on SiO2 glass substrates by XRD using anomalous scattering at an absorption edge. ZnO films with a thickness of 1100 nm were grown on SiO2 glass substrates by pulsed laser deposition (PLD) with a backing pressure of 2×10-9 Torr. Nominally undoped ZnO and 1 mol% Al-doped ZnO ceramics were prepared by solid state reaction method. A pulsed laser of the fourth harmonic generation (FHG) of neodymium-doped yttrium gallium garnet (Nd:YAG, wavelength=266 nm, repetition rate=5 Hz) was focused on the targets. The films were deposited at a temperature of 700 oC in O2 pressure of 1×10-5 Torr. The intensity of {0002} reflections from ZnO was obtained as a function of wavelength from the continuous spectrum generated by a rotating copper x-ray anode tube. The sample was placed on the sample holder with {0002} planes satisfying the Bragg relation. Scans were taken along the 2theta/theta axis. As 2theta was scanned continuously, wavelength varied as well. 2theta/theta scans were carried out near the K absorption edge of ZnThe diffraction curves from the continuous spectrum produced by {0002} planes of undoped and Al-doped ZnO films on glass substrates showed a clear differential absorption edge effect due to anomalous dispersion at the Zn K-edge. The normalized pre- and post-Zn K-edge diffraction intensity ratio of the {0002} diffraction peak was obviously smaller for the Al-doped ZnO film than for the undoped ZnO film. For comparison, we measured the diffraction curves from the continuous spectrum produced by (0002) and (000-2) planes of a ZnO single crystal. The pre- and post-Zn K-edge diffraction intensity ratio for the (0002) plane of the single crystal was smaller than that for the (000-2) plane. These results indicate that the Al-doped ZnO on SiO2 glass substrates has a (0001) face, whereas the undoped ZnO film has a (000-1) face. More details will be presented at the conference site.
9:00 PM - H10.15
P-type Conductivity at the Surface of Polymer Embedded ZnO Nanowires: A Scanning Probe Microscopy Investigation.
Eddy Latu-Romain 1 , Philippe Gilet 1 , Nicolas Chevalier 1 , Denis Mariolle 1 , Francois Bertin 1 , Pierre Ferret 1 , Francois Levy 1 , Alexei Tchelnokov 1 , Guy Feuillet 1
1 LETI, CEA, Grenoble France
Show AbstractIn order to circumvent the difficulties associated with p type doping in ZnO, alternative processes are being sought to try and induce hole conductivity. Besides, ZnO surfaces are known to be electrically active, leading, for instance to the development of applications such as gas sensors based on ZnO nanowires to take advantage of the increased surface to volume ratio. In particular, Fermi level pinning at surface states is expected which would result in band bending and depleted regions in the vicinity of surfaces and/or interfaces.Along these lines, we demonstrate here that p-type conductivities can be achieved at ZnO nanowire surfaces when these nanowires are embedded in a polymer matrix, leading to a core-shell type of p-n junction. The non intentionnaly doped ZnO NWs were grown on (0001) Al2O3 substrates by catalyst-free metal-organic vapour phase epitaxy (MOVPE) in a horizontal hot-wall reactor and further integrated using a novolac-based polymer. The investigations of transport properties in the vertically integrated ZnO NWs were carried out using Scanning Probe Microscopy (SPM). Two different modes of the SPM were used, namely Scanning Capacitance Microscopy (SCM) and Scanning Spreading Resistance Microscopy (SSRM). For nanowires with large enough diameter, SCM revealed local p-type space charge regions spreading over the outer shell of the polymer-coated ZnO NW, while the inner part of the nanowire remained n-type. On the other hand, smaller diameter nanowires exhibited full depletion. Correlatively, different electrical behaviours were found for the core and shell parts of the nanowires using Scanning Spreading Resistance Microscopy (SSRM). A self-consistent electrical transport model involving surface states arising from the surrounding environment was developed to account for the observed sub-surface p-type layer. From this a critical diameter under which full p-type conductivity occurs was demonstrated in clear correlation with the experimental observations. Electrostatic contributions to Poisson’s equation were also discussed.
9:00 PM - H10.16
ZnO Epitaxy on Si Substrates with Bixbyite Oxide Buffer Layers-Defects and Interface Properties.
Wei Guo 1 , Christopher Nelson 1 , Michael Katz 1 , Yi Zhang 1 , Tassilo Heeg 2 , Darrell Schlom 2 , Bing Liu 3 , Yong Che 3 , Xiaoqing Pan 1
1 Material Science and Engineering, University of Michigan-Ann Arbor, Ann Arbor, Michigan, United States, 2 Materials Science and Engineering, Cornell University, Ithaca, New York, United States, 3 Functional Materials Group, IMRA America, Inc., Ann Arbor, Michigan, United States
Show AbstractThe integration of ZnO epitaxy and high-κ bixbyite oxides M2O3 (M: Sc, Lu, or Gd) on Si substrates opens up new prospects of developing ZnO-based multi-functional devices operating with the ultimate-scaled Si electronics. The ZnO films grown on (111) Si substrates with M2O3 buffer layers has narrow widths of x-ray diffraction (XRD) rocking curves, high Hall moblities at room temperature, and photoluminescence characteristics comparable to that of ZnO single crystal. Interfacial defects and related strain relaxation properties of these high quality epitaxial films were investigated by high resolution XRD and transmission electron microscopy. Strong deep level and weak near-band-edge emissions were observed in electroluminescence measurements from n-ZnO/M2O3/p-Si heterojunction structures.
9:00 PM - H10.17
Structure Evolution of ZnO Nanostructures Synthesized by Chemical Solutions.
Jack Luo 1 , Zhu Liu 2 , Wei Guo 2 , William Milne 3
1 Centre for Material Research & Innovation, University of Bolton, Bolton United Kingdom, 2 School of Material, University of Manchester, Manchester United Kingdom, 3 Department of Engineering, University of Cambridge, Cambridge United Kingdom
Show AbstractZnO has been widely used in electronics, optics and piezoelectronics due to its remarkable properties: a wide energy gap, large exciton-binding energy, near-UV emission and excellent piezoelectricity. With the discovery of ZnO nanostructures, its application has been widened dramatically owing to the enhanced properties at the nanoscale. Solution-based synthesis is a simple method to produce ZnO nanostructures at low temperature, suitable for fabrication of nanostructures on glass and even plastics. Various ZnO nanostructures such as nanorods and nanowires have been synthesized in solutions. Although there are many solution–based methods to grow ZnO nanostructures, it is still difficult to control the shapes of ZnO nanostructures, and the results are often not re-producible, probably due to the minor variation of synthesis conditions. We report a systematic investigation on the effect of chemicals, pH value and temperature on the ZnO nanostructure.Zn(NO3)2.6H2O (hereafter Zn(NO3)2 in short), NH4OH, C6H12N4 (HTM) and Zinc acetate were used for the synthesis of ZnO nanostructures. HNO3 and NH4OH were used to adjust the pH value of the solution. The synthesis of the ZnO nanostructures was conducted on a 50nm ZnO seed layer deposited by rf sputtering on Si-substrates. Various combinations of chemicals were used to synthesis the nanostructures to identify the effect of the chemicals and growth conditions on the shapes. The results are summarized as follows;1.Chemicals used in the synthesis have profound effects on the shape of the nanostructures. A zinc acetate and C6H12N4 combination produces thin disk-like ZnO nanostructures with hexagonal shapes of 0.2~0.5um in diameter. The nanostructures typically form a film covering the whole area. 2.Zn(NO3)2 and NH4OH produces needle-like nanostructures with diameters from a few nm to a few tens of nanometers. The nano-needles have a rounded shape with the needle being sharper at the tip.3.Zn(NO3)2 and HTM solution produces hexagonal nanostructures roughly perpendicular to the substrate. The diameter of the hexagonal structures varies from a few nm to a few hundreds of nm, depending on the mole ratio of the chemical concentration, the pH value and the growth temperature. At low concentration and mole ratio of Zn(NO3)2 to HTM (<1:4), the nanostructure typically is a needle like shape, and gradually changes to a hexagonal structure. 4.Under optimal conditions, a Zn(NO3)2 and HTM solution can produce a thick continuous film which consists of hexagonal nanostructures and has a smooth surface suitable for fabrication of acoustic wave devices and large area opto-electronic devices such as LEDs and lasers. 5.Adjustment of the pH value of the Zn(NO3)2 and HTM solution leads to dramatic change of the shape of the ZnO nanostructures. At pH < 7.5, the ZnO has a hexagonal rod structure, and the nanorod becomes rounded and thinner as the pH value increases, and eventually stops growing at a pH > 11.
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Tilted Epitaxial ZnO Nanospears on Si(001) by Chemical Bath Deposition.
Jay Switzer 1 , Guojun Mu 1 , Rakesh Gudavarthy 1 , Elizabeth Kulp 1
1 Department of Chemistry, Missouri University of Science and Technology, Rolla, Missouri, United States
Show AbstractEpitaxial ZnO nanospears are deposited onto degenerate p-type Si(001) from an alkaline supersaturated solution of Zn(II) at 70 C using chemical bath deposition. The lattice mismatch between the hexagonal ZnO and diamond cubic Si is reduced from -40.16% for an un-tilted structure to -0.25% in the ZnO[1-210] direction and -0.99% in the ZnO[25-7-6] direction by tilting the nanospears 51 degrees relative to the surface normal. The tilted nanostructure brings the (20-23) planes of ZnO into coincidence with the (001) planes of Si. The photoluminescence (PL) spectrum of the film shows a sharp exciton emission at 364 nm and a broad green emission at 500-700 nm. The ZnO/Si heterojunction leads the way to integration of a large bandgap oxide semiconductor with traditional semiconductor devices.
9:00 PM - H10.19
Symmetrical Memristive Switching Properties of SnO2:Fe Transparent Thin Films.
Sinny Trivedi 1 , Komal Bhavsar 1 , Urvi Chhaya 2 , Uday Trivedi 3 , V. Joshi 4 , U. Joshi 1
1 Department of Physics, School of Sciences, Ahmedabad, Gujarat, India, 2 Physics Department, St. Xavier's College, Ahmedabad, Gujarat, India, 3 Department of Instruments & Control Engineering, Vishwakarma Government Engineering College, Gandhinagar, Gujarat, India, 4 Department of Physics, Veer Narmad South Gujarat University, Surat, Gujarat, India
Show AbstractRecently, many binary and perovskite oxides have shown a non-volatile retention behavior and large endurances for resistive random access memory (RRAM), which is one of the candidate technologies for the promising next generation non-volatile memories with fast switching speed, low power consumption and nondestructive read out [1]. SnO2 is a transparent wide-band-gap n-type oxide semiconductor because of the existence of intrinsic defects (oxygen vacancies and/or metal interstitials) [2]. Fe2+,/3+ substitution for Sn4+ introduces holes in the system and annihilate part of the intrinsic n-type carriers and decreases the carrier density [2]. Here we report on the electric pulse induced resistive (EPIR) switching properties of Fe doped SnO2 transparent thin films grown by low cost chemical solution deposition on quartz substrates for non-volatile memory applications. Nominal 5 at. wt% Fe doped SnO2 films were synthesized by sol gel spin coating technique [3]. High purity metal salts (> 99.9%) were dissolved in 2-mithoxyethanol and monoethanolamine to yield clear and transparent 0.3 M solution, which was spun coated on sonicated SiO2 substrates at 4000 rpm. All the peaks of grazing incidence X-ray diffractograph GIXRD correspond to rutile-type phase SnO2 nanostructure (space group P42/mnm) and the unit cell parameters were found to be a=b= 0.51 nm and c = 0.946 nm. Cross sectional SEM revealed uniform thickness of ~ 170 nm with a clear Ag/Fe0.05Sn0.95O2 interface. Reproducible hysteresis in the I-V curves with symmetrical resistance switching ratio of more than 4 x 10e3 has been demonstrated for the Ag/SnO2:Fe/Ag planar device cell. Nearly 430% “ON” state to “OFF” state resistance ratio was observed in electric pulse induced resistance (EPIR) switching behavior, at a small current compliance of 200 mA. Detailed I-V analysis of the SnO2:Fe film indicate that space charge limited current (SCLC) combined with Schottky conduction mechanism plays role in the resistance switching phenomena. A mechanism of the formation and rupture of conducting filament is proposed based on the Joule heating effect with external electron injection at the Ag/SnO2 interface. A possible role of gross defect states in the oxide band gap on EPIR switching will be highlighted [3]. Acknowledgements : USJ thank DAE-BRNS, India for Young Scientist Research Award.References : 1.A. Sawa, Materials Today, 11, 28 (2008)2.J. Ni, X. Zhao, X. Zheng, J. Zhao, B. Liu, Acta Mater. 57, 278 (2009)3.U.S. Joshi, S.J. Trivedi, K.H. Bhavsar, U.N. Trivedi, S.A. Khan, D.K. Avasthi, J. Appl. Phys., 105, 73704 (2009)
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Self-assembled Formation of ZnO Hexagonal Micropyramids and their Photolumiescence Properties.
DaeGwi Kim 1 , Masaaki Nakayama 1
1 , Osaka City University, Osaka Japan
Show AbstractZnO has been intensively investigated from the interest in applications to new optoelectronic devices in an ultraviolet region and from the viewpoint of excitonic properties owing to a large exciton-binding energy. In the application to photonic devices, it is very important to enhance and to control photoluminescence (PL) properties using cavity structures. We have found the self-assembled formation of ZnO hexagonal micropyramids after the growth of a sufficiently thick ZnO layer on a (0001) Al2O3 substrate by the rf-magnetron sputtering deposition. In the hexagonal micropyramids, the extremely strong enhancement of the luminescence is realized, which originates from a cavity effect peculiar to hexagonal micro-resonators. Furthermore, we have succeeded in controlling the size of hexagonal micropyramids by changing the growth temperature and have investigated the PL properties of ZnO thin films including micropyramids. Thin films of ZnO with the layer thickness of 2.5 μm were grown on the (0001) Al2O3 substrate by the rf-magnetron sputtering method. A mixed gas of oxygen and argon with a ratio of 2:1 was used as the sputtering gas with a total pressure of 1.3 Pa. The growth temperature was changed from 550 C to 700 C in order to control the size of the micropyramids. We have confirmed with scanning electron microscopy that the mean side length of the bottom hexagon is changed from 2.5 μm to 2.9 μm, where the higher growth temperature results in the larger micropyramids. In a PL spectrum, a defect-related PL band, a so-called green band, was negligibly weak, and the free-exciton and bound-exciton bands were observed with sharp bandwidths that are comparable to those in samples prepared by plasma-assisted molecular-beam epitaxy and laser-molecular-beam epitaxy. Thus, the PL spectrum demonstrates the very high crystalline quality of the micropyramids and base layer of ZnO. Under intense-excitation conditions, a PL band originated from an exciton-exciton scattering process, leading to stimulated emission, is observed.
9:00 PM - H10.20
Modulation Doping in ZnO Nanorods for Electronic Nanodevice Applications.
Jinkyoung Yoo 1 , Chul-Ho Lee 1 , Yong-Joo Doh 1 , Hye Seong Jung 1 , Gyu-Chul Yi 2
1 Department of Materials Sci. and Eng., POSTECH, Pohang Korea (the Republic of), 2 National CRI center for Semiconductor Nanorods, Department of Physics and Astronomy, Seoul National University, Seoul Korea (the Republic of)
Show AbstractRecent progress in the growth of one-dimensional (1D) semiconductor nanostructures has provided significant opportunities for electronic and photonic nanodevice applications. For the device fabrications, the electrical characteristics of semiconductor nanomaterials must be precisely controlled by impurity doping. Recent reports have shown that conductivity control by impurity doping in semiconductor nanomaterials is more problematic than in thin film technology. Dopant incorporation during conventional homogeneous doping process causes a change in the nanowire growth direction, resulting in structural deformation. Delta-doping process, which generate dopant atoms that are confined within a few atomic layers, have been widely used for fabricating modulation-doped thin film heterostructures. For 1D semiconductor nanostructures, the modulation-doping process forms dopant layers on the surfaces of the 1D nanostructures, which can show a more significantly enhanced modulation-doping effect due to their high surface to volume ratio. Here we present a modulation-doping method to control electrical characteristics of ZnO nanorods. Compared with a conventional homogeneous doping method, the modulation-doping method generates localized doping layers along the circumference in ZnO nanorods, useful for many device applications. In this study, we investigated electrical, structural, and optical characteristics of Ga-doped ZnO nanorods with the dopant modulation layers. Electrical conductivity of ZnO nanorods was controlled by changing either dopant mole fraction or the number of modulation-doped layers. Furthermore, the modulation-doped nanorod field effect transistors exhibited precisely controlled conductance in the order of magnitude without degradation of electron mobility. The effects of the doping on structural and optical characteristics of the nanorods are also discussed.
9:00 PM - H10.21
The Properties of Transparent Conductive Titanium-Doped Indium Oxide Films by Radio-Frequency Sputtering.
Lei Li 1 , Yiding Wang 1 , Menglong Cong 1 , Feng Cao 1 , Zhenyu Song 1
1 State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin university, Changchun, Jilin, China
Show AbstractAmong Transparent conductive oxide (TCO) thin, Indium Oxide has attracted much focus because of its high transmission; low resistivity; mechanical strength and chemical properties. The TCO films are widely applicated to transparent electrodes of flat panel displays, solar cells, and IR reflectors. Among of them, ITO (In2O3: Sn) thin films are deposited onto glass substrates, as are received extensively and profoundly develop.In2O3 -based TCO thin film could be deposited with chemical-vapor deposition (CVD), sol–gel, spray pyrolysis techniques, and so on. The film properties can be adjusted by controlling the doping materials and ratios, deposition parameters, and post treatment. However, the ration-frequency sputtering technique owing to its many advantages makes it a good candidate for the thin film preparation applications.In this paper, highly transparent and conductive In2O3: Ti thin films are prepared onto glass substrates by RF sputtering at 3000C with different O2/Ar ratios. The results indicate that In2O3: Ti can be used to fabricate high performance TCO thin films.
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Systematic Study of Physical Properties of Epitaxial ZnO Thin Films and the Role of Defects.
Siddhartha Mal 1 , Sudhakar Nori 1 , Jagdish Narayan 1 , John Prater 2 1
1 Materials Science & Engineering Department, North Carolina State University, Raleigh, North Carolina, United States, 2 Materials Science Division, Army Research Office, Durham , North Carolina, United States
Show AbstractDefects play a very important role not only in determining the macroscopic physical properties, but also in controlling the optical, magnetic and electrical properties of any oxide system in general. In particular, ZnO has been a subject of intense investigation, recently, in search for the want of spintronics based materials for technological applications. A typical spintronics device exploits both the fundamental attributes of an electron, ie, the charge and also the spin degrees of freedom. Practical realization of such devices then certainly becomes a challenging task and a correlation of structural properties such as morphology, size or domain and crystal structure with the corresponding physical properties is very essential. Fine tuning and control of these properties are important in order to realize any of the next generation spintronics devices. ZnO based materials have been found to be very good candidates for spintronics application by displaying room temperature ferromagnetism (RTFM). The physical properties of zinc oxides are highly sensitive to the growth parameters such as oxygen stoichiometry and substrate temperature1. We have grown several ZnO single crystalline thin films on c-axis oriented sapphire by pulsed laser deposition at various substrate temperatures in range 350-7500 C and in a vacuum of 10-6 Torr. The c-axis (002) orientation of the films has been established by both θ-2θ as well as φ-scans of X-ray diffraction study. The characteristic excitonic peaks were observed at 364 and 371 nm in the optical absorption spectra of the films. Higher intensities in photoluminescence (PL) spectra were observed in the case of samples grown at higher temperatures. The optical quality of the films was degraded in samples grown at lower temperatures while the magnetic properties were enhanced when compared to the films grown at higher temperatures. The transmission and PL spectra were used to correlate RTFM and point defects for a possible mechanism of 3-D ferromagnetic ordering. The observed optical and magnetic properties in undoped ZnO films are attributed to intrinsic defects. We have devised methods to control defect concentrations and hence tune electrical, optical and magnetic properties. Preliminary results indicated that ZnO is a promising system for spintronics applications, although the growth and processing parameters need to be optimized to get the most desirable set of properties.
9:00 PM - H10.24
Photoluminescence of Pb-Implanted Bulk ZnO.
Rueben Mendelsberg 1 2 , John Kennedy 4 2 , Steven Durbin 3 2 , Roger Reeves 1 2
1 Physics and Astronomy, University of Canterbury, Christchurch New Zealand, 2 , The MacDiarmid Institute for Advanced Materials and Nanotechnology, Wellington New Zealand, 4 National Isotope Center, GNS Science, Lower Hutt New Zealand, 3 Electrical and Computer Engineering, University of Canterbury, Christchurch New Zealand
Show AbstractThe excitonic photoluminescence (PL) spectrum of ZnO is quite complex, with more than 10 reported recombinations near the band edge at low temperatures. After nearly 50 years of research, several of these excitonic features have been traced back to their chemical and structural origins but many remain mysterious. Now that high quality bulk crystals are commercially available there is more opportunity to observe and identify more of these sharp features through controlled experiments. Once the features are identified, PL becomes a powerful tool for trace impurity detection at ppm or even ppb concentrations.Pb is a common impurity in many material deposition systems, commonly showing up in Zn sputter targets. To see what effect Pb impurities have on the excitonic spectrum of ZnO, hydrothermally grown bulk ZnO was implanted with Pb ions. After implantation the samples were annealed in oxygen at 600 °C for one hour to repair the damage caused by the ion beam. PL was then recorded as a function of temperature, excitation intensity, and as a function of the intensity of a second, below bandgap laser. Two sharp and previously unreported features were observed in the near band edge region of the low temperature PL in the Pb implanted crystals. From the PL behaviour, these peaks were identified as neutral donor bound excitons. One was assigned to interstitial Pb while the other was assigned to Pb substituted on the Zn site. Pb plays a complex role when incorporated into ZnO and understanding the behaviour of this common impurity may prove vital for future progress in ZnO applications.
9:00 PM - H10.25
Study on Wet Chemical Etching of IGZO and HfO2 for the Fabrication of Transparent TFT Application.
Jae-Kwan Kim 1 , Jun Young Kim 1 , Seung-Cheol Han 1 , Joon Seop Kwak 1 , Ji-Myon Lee 1
1 Materials Science and Metallurgical Engineering, Sunchon National University, Sunchon, Chonnam, Korea (the Republic of)
Show AbstractZnO-based oxide thin film transistors (TFTs) have attracted a great deal of attention as promising large-area backplane electronics devices for flat panel displays, such as liquid crystal displays and organic light emitted displays. Recently, much effort has been made to reduce the power consumption for the purpose of realizing flexible and mobile applications using oxide-based TFTs with a high-k gate dielectric. Since the InGaZnO and HfO2 are might be used as a channel and gate dielectric material, respectively, it is very important to develop well-defined etching process for various transparent oxide. In this study, etch-rates and etched surface morphology of InGaZnO (IGZO) and HfOx (HfO) were investigated by using various acidic solutions. IGZO and HfO with thickness of 600 nm, respectively, were grown on Si(001) substrate by radio-frequency magnetron sputtering method. Wet-etching experiments were performed by various acidic solutions such as sulfuric acid, buffered oxide etchants (BOE), 85 % formic acid, oxalic acid and mixed solution of BOE and formic (BF) acid. Etch rates were determined by using surface profilometer by averaging each 10 points in the samples. The surface morphologies and compositions were characterized by FE-SEM, AFM and EDX systems.IGZO can be etched easily by the all acids tested in study such as sulfuric, BOE, formic, oxalic, and BF acids. It was found that the etch rates were dependent on the tested solution and that the highest etch rate was measured to be about 125 nm/min by using BF mixed acid at room temperature. By using sulfuric acids, the etch rate was observed to be lower than that of BF acid and was drastically increased with increasing the concentration of sulfuric acid over the 0.5 mole. Although, IGZO was not etched by oxalic acid under the concentration of 0.05 mole, the etch rate of IGZO by using oxalic acid over 0.05 mole was measured to be about 100 nm/min, which is similar to that of BOE. HfO was found to be etched by BOE and BF acid and highest etch rates of 100 nm/min and 23 nm/min was achieved by using BOE and BF, respectively, indicating that the BF solution is highly promising for the use as selective etchant of IGZO over HfO. Furthermore, we also found that the etch selectivity of IGZO to the HfO by using sulfuric acid and oxalic acid are very high. In this presentation, we will also report the characteristics of etched surface morphology as well as etching mechanism.
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Luminescence Properties of ZnO/Zn1-xCdxO Double Heterostructures.
Martin Lange 1 , Christof Dietrich 1 , Gabriele Benndorf 1 , Christian Czekalla 1 , Michael Lorenz 1 , Marius Grundmann 1
1 Institut für Experimentelle Physik II, Universität Leipzig, Leipzig, Saxony, Germany
Show AbstractWe present detailed investigations of the luminescence properties of ZnO/Zn1-xCdxO double heterostructures (Cd-DHSs). Zn1-xCdxO is a promising ZnO–based alloy with a smaller bandgap relative to that of ZnO. It has entered the spotlight of ZnO alloys, especially as a part of heterostructures together with ZnO or MgyZn1-yO. For our samples the Zn1-xCdxO–luminescence could at most be shifted 350 meV below the ZnO bandgap. With annealing the thermal stability of the Cd-DHSs and the Cd-diffusion were investigated.
The Cd-DHSs were grown by pulsed-laser deposition on a-plane sapphire substrates. The oxygen partial pressure and the substrate temperature were varied to fabricate samples with high cadmium content and small bandgap maintaining high luminescence yield.
We have studied the as-grown samples regarding their luminescence properties. Temperature dependent photoluminescence (PL) in the temperature range from 2 to 295 K and spatially resolved PL measurements were therefore performed. Parts of the samples were annealed for 30 minutes at 620°C to 970°C in air. The annealing was carried out to study the thermal stability of the samples and to investigate a possible Cd-Diffusion. Subsequent PL measurements at T=2 K were performed.
For the as-grown samples an intense luminescence signal was observed from the Zn1-xCdxO for the entire temperature range. A maximal shift of 350 meV below the ZnO bandgap was achieved. An S-shape behaviour for the peak energy vs. temperature of the Zn1-xCdxO–luminescence signal was observed and the standard deviation of the inhomogeneous broadening of the confinement potential σ was estimated with values between 15 and 18 meV. A large number of phonon replicas indicate a strong electron-phonon coupling. Using the Huang-Rhys factor and the fraction of strongly localized excitons the depth of the localization potentials was estimated in good agreement with σ determined from the S-shape [1]. Using the intensity of the Zn1-xCdxO–luminescence band as a function of temperature the thermal activation energy of non-radiative processes has been determined with values of 14 to 15 meV.
After the annealing step the Zn1-xCdxO–luminescence band was still observable if the annealing temperature did not exceed 920°C. A shift of the Zn1-xCdxO peak to higher energies occurs already for temperatures as low as 720°C proving Cd diffusion in this temperature range. The intensity of the Zn1-xCdxO–luminescence could just slightly be enhanced due to the annealing.
[1] M. Lange, J. Zippel, G. Benndorf, H. Hochmuth, M. Lorenz, and M. Grundmann, J. Vac. Sci. Technol., B 27, 1741 (2009)
9:00 PM - H10.27
ZnO Thin Film Transistors Fabricated by Atomic Layer Deposition Method.
Yumi Kawamura 1 , Yukiharu Uraoka 1 2
1 Graduate School of Materials Science, Nara Institute of Science and Technology, Nara Japan, 2 CREST, Japan Science and Technology Agency, Saitama Japan
Show Abstract In recent years, the application of zinc oxide (ZnO) thin film as an active channel layer in thin film transistors (TFT) has become of great interest owing to its specific characteristics. ZnO is wide band gap (~3.37eV), transparent to visible wavelengths, and the ability to fabricate good quality films over large areas at low temperature suggests compatibility with plastic or flexible substrates. Field-effect mobility of ZnO TFTs has recently been demonstrated as higher than that of a-Si:H TFTs. However, there is one of the serious problems is remained. That is the issue of stability. An atomic layer deposition (ALD) method is one of the thin film preparation technologies, which attracts much attention in LSI industry. The ALD thin film is deposited with alternating exposures of a source gas and an oxidant. The ALD film has additional features of accurate thickness control, high conformity, and uniformity over large areas, because of the alternating gas supply. Further, it is reported that the TFTs with ALD thin film as the channel layer demonstrated high mobility. In this study, we fabricated TFTs using ZnO thin film as the channel layer deposited by ALD, and evaluated their reliability. The fabricated TFTs were annealed at 100~500[oC], 1hour, oxigen (O2=20%, N2=80%) atmosphere, and their electrical properties were measured using semiconductor parameter analyser. The Id on increased with a rise in anneal temperature, and the mobility also increased. The measurement of electrical properties in TFT annealed in O2 ambient suggested that the oxygen deficiencies were reduced by the introduction of oxygen during the annealing. In addition, we applied bias stress on fabricated TFTs, and measured their bias stress stability. We measured transfer characteristics of the fabricated TFTs annealed at 300[oC] and 400[oC] after 1, 10, 100, 1000, 10000 seconds application of Vg/Vd=20/20 V. The Vth was positively shifted approximately 3.5 V after 10000 seconds stressing in the TFT annealed at 300[oC]. However, the Vth shift was remarkably reduced by annealing at 400[oC]. We suppose that the decrease of Vth shift was attributable to the compensated deficiencies in ZnO thin film. Further, we examined the depth profiles of the films using secondary ion mass spectrometry (SIMS). SIMS measurements were performed for the ZnO film annealed in O2 ambient at 300[oC] and 400[oC]. In the sample annealed at 400[oC], excellent decrease of hydrogen was observed. It is considered that the remaining hydrogen was released by the annealing. These results suggest that the hydrogen plays an important role for the improvement of reliability. Through this study, we found that electrical performances were dependent on the annealing temperature, and threshold voltage shift by the stress are caused by the remaining hydrogen.
9:00 PM - H10.28
Transparent Conductive ZnO:Al Films Prepared by Radio Frequency Magnetron Sputtering.
Huang Tien-Heng 1 , Yi-Wen Kao 1 , Kuo-Chuang Chiu 1
1 , Industrial Technology Research Institute, Hsinchu Taiwan
Show AbstractAluminum doped zinc oxide polycrystalline films (AZO) were prepared by rf magnetron sputtering on glass using specifically designed AZO targets prepared by soft-chemical route process. The aluminum content varied from 1 to 5 mol%. The best conductors were obtained for the ZnO containing 2 mol% of aluminum. The structural, electrical and optical properties of the AZO films prepared with ZnO:2%Al target were investigated in terms of the deposition conditions, such as rf power, substrate temperature and working pressure. The minimum resistivity of 4.9*10-4 ohm-cm in AZO films was obtained from the target prepared by soft-chemical route process, and was lower than that prepared by solid state reaction process. The optical transmittance of ZnO:Al films at 550nm is ~90%. Both AZO films structure were characterized by X-ray diffraction (XRD).
9:00 PM - H10.3
Hydrothermal Growth and Photoluminescence of Indium, Gallium, and Aluminum-doped ZnO Crystals.
B. Wang 1 , M. Callahan 1 , L. Bouthillette 1 , D. Bliss 2 , M. Suscavage 2
1 , SSSC/AFRL, Burlington, Massachusetts, United States, 2 , AFRL, Hanscom AFB, Massachusetts, United States
Show AbstractGroup-III (indium, gallium and aluminum) doped ZnO crystals are promising candidates to produce electrically conducting substrates that are needed for light-emitting devices. We have grown ZnO bulk crystals in the presence of group-III ions by the hydrothermal technique, to obtain indium, gallium, and aluminum-doped ZnO crystals. ZnO crystals doped with GaN were also grown hydrothermally. Photoluminescence spectra of indium, gallium and aluminum-doped ZnO, as well as Ga/N co-doped ZnO crystals have been measured from room temperature to 4K. In this paper, we will compare the various growth characteristics of hydrothermal ZnO crystals in the presence of In3+, Ga3+ and Al3+, and report the PL peaks related to different doped elements, supported by impurity analysis by SIMS and GDMS in more detail. This research will not only have an impact on the hydrothermal preparation of ZnO conducting substrates, but also contribute to the understanding of defect chemistry in ZnO crystals.
9:00 PM - H10.30
Structure, Morphology and Optical Properties Arising From Thickness Variation in ZnO:Al Thin Films Grown by 30°- Incident RF Magnetron Sputtering.
Bhaskar Chandra Mohanty 1 , Yeon Hwa Jo 1 , Deuk Ho Yeon 1 , Ik Jin Choi 1 , Yong Soo Cho 1
1 Materials Science & Engineering, Yonsei University , Seoul Korea (the Republic of)
Show AbstractThickness-dependent structure, morphology and optical properties of Al-doped ZnO thin films have been investigated. Films of varying thicknesses in the range of 80 to 700 nm were grown by 30°-incident magnetron sputtering of a ZnO:Al (2 wt %) compound target at RF power of 200 W in pure Ar ambient (2 mtorr) at room temperature. Surface roughness scaling study of the films revealed a cross over in scaling behavior with transition thickness ~350 nm. Both the regimes exhibited anomalous scaling of surface roughness, on the lines of dynamic scaling theory. X-ray diffraction (XRD) patterns of the films of thickness less than 350 nm contained a lone peak corresponding to (002) plane of wurtzite phase. As the thickness of the films increased further, several other peaks corresponding to the wurtzite phase appeared, suggesting increased polycrystallinity. Although, the peaks in the XRD patterns indicated wurtzite phase, the peaks were asymmetric and shifted to lower 2θ values. The in-plane stress in the films was calculated using the bi-axial stress model. The thinnest film in this work (~84 nm) had a stress of -8.39×109 Nm-2 and with increase in thickness, magnitude of the stress decreased. Average visible transmittance was typically more than 80 % for all films. However, a red shift of fundamental absorption band edge with increasing thickness was observed. A linear dependence of optical band gap on stress in the films with a coefficient of 54.6 meV/GPa has been estimated.
9:00 PM - H10.31
Raman Scattering of CdO Thin Films Grown by Metalorganic Vapor-Phase Epitaxy on Sapphire Substrates.
Jordi Ibanez 1 , Ramon Cusco 1 , Esther Alarcon-Llado 1 , Jesus Zuniga-Perez 2 , Vicente Munoz-Sanjose 2 , Luis Artus 1
1 , Inst. Jaume Almera (C.S.I.C.), Barcelona Spain, 2 , Department of Fisica Aplicada, Universitat de Valencia, Burjassot Spain
Show AbstractCadmium oxide (CdO) has recently attracted substantial attention due to its potential application as a transparent conducting oxide for solar cells or flat-panel displays. CdO is also interesting because alloying of CdO with ZnO allows one to extend to higher wavelengths the emission of ZnO-based optoelectronic devices. In spite of its technological importance, the vibrational properties of CdO have been scarcely investigated so far.Here we present a lattice-dynamics study of CdO. We perform Raman scattering measurements on a series of high-quality CdO thin films grown by metal-organic vapour phase epitaxy (MOVPE) on a-plane (11-20), r-plane (01-12), and m-plane (10-10) sapphire substrates. Owing to its rock-salt structure, first-order Raman modes are forbidden and thus only second-order bands are expected in the Raman spectrum of CdO. We find that the room-temperature spectra are dominated by a very broad band centered at ≈390 cm-1 and a weaker, narrower band located at ≈260 cm-1. In order to interpret the experimental data, we have carried out an ab initio calculation of the phonon dispersion in CdO. While the broader band reflects the high two-phonon density of states in the 300-450 cm-1 frequency region, the feature at ≈260 cm-1 can be tentatively assigned to a TO+TA second-order mode. The dependence of the Raman spectra on substrate orientation, temperature and excitation wavelength is discussed.Raman scattering is a standard tool widely used to characterize the crystal quality of semiconductor compounds and structures. In the case of rock-salt crystals, the use of Raman scattering is however hindered by the absence of first-order peaks. Here, with the aim of exploring the usefulness of Raman scattering for the routine characterization of CdO crystals, we investigate CdO thin films that were implanted after growth with different doses of O ions. The analysis of the Raman spectra shows that the width of the TO+TA mode is sensitive to the degree of crystal disorder introduced by the ion bombardment. Our results demonstrate that the second-order Raman spectra may provide valuable information about the crystal quality of rock-salt crystals like CdO.
9:00 PM - H10.32
Space-charge Regions in ZnO-based Metal-semiconductor-metal Photodetectors and Metal-semiconductor Field-effect Transistors.
Zhipeng Zhang 1 , Michael Lorenz 1 , Lucie Behnke 1 , Christian Czekalla 1 , Heiko Frenzel 1 , Matthias Schmidt 1 , Alexander Lajn 1 , Gisela Biehne 1 , Holger Hochmuth 1 , Michael Lorenz 1 , Holger von Wenckstern 1 , Marius Grundmann 1
1 Fakultät für Physik und Geowissenschaften, Universität Leipzig, Leipzig, Sachsen, Germany
Show AbstractDue to the wide band gap of ZnO (approximately 3.3 eV) at room temperature, ZnO-based devices are promising candidates for the realization of transparent electronic (e.g. active-matrix liquid crystal displays [1]) and optoelectronic (e.g. UV-photodetectors) devices. We present Light Beam Induced Current (LBIC) investigations of interdigital metal-semiconductor-metal (MSM) Schottky barrier photodetectors and metal-semiconductor field-effect transistors (MESFETs). The Schottky contacts (SCs) were fabricated by reactive dc sputtering in an Ar/O2 atmosphere of Ag and Pd on nominally undoped n-ZnO thin films heteroepitaxially grown by pulsed-laser deposition (PLD). Besides the reactively sputtered metal layer, the SCs consist of an additional metal layer, assuring an equal surface on top of reactively sputtered contacts [2].LBIC measurements applied to ZnO SCs enable us to characterise the geometric formation of the space-charge region (SCR) and the lateral homogeneity of the carrier distribution in the samples. In general, the SCs exhibit low ideality factors <1.5 and high effective barrier heights >0.8 eV.The electrical and photoelectrical properties of the Pd-MSM photodetectors were studied by current-voltage (IV) and LBIC measurements. The photocurrent was measured with different external biases. The LBIC line scan profiles show that the photocurrent was generated only in the vicinity of the reverse biased MSM fingers. The uniformity of the photocurrent distribution was observed in two-dimensional LBIC-scans. Additionally, results from MgxZn1-xO (0<x<0.087) will be presented, tuning the detectable photon energy to higher values compared to the ZnO band gap energy. The dependence of the quantum efficiency on the photon energy and the Mg-content will be shown. The individual properties and the influence of different gate metals on the performance of ZnO-based MESFETs were investigated [3]. Normally-off Ag-gate MESFETs show an on/off-ratio of 106 and low off-currents in the range of 10-13 A. Normally-on Pd-gate MESFETs have on/off-ratio of 103 and low off-currents in the range of 10-8 A. The leakage current for Pd-MESFETs is 2 orders of magnitude higher than for Ag. The channel mobilities for Ag und Pd are 1.3 and 0.3 cm2/Vs, respectively. The photocurrent was measured under constant reverse gate-voltage VG with different drain-voltages VD. LBIC line scans proved the existence of the SCR under the gate contact and exhibit a continuous shift of the photocurrent maximum towards the drain for increasing VD. The two-dimensional LBIC measurements indicate that the SCR crosses the entire channel depth.[1] J. F. Wager, Science 300, 1245-1246 (2003)[2] A. Lajn et al., J. Vac. Sci. Technol. B 27, 1769-1773 (2009)[3] H. Frenzel et al., Thin Solid Films (2009), in press
9:00 PM - H10.33
Photocurrent Measurements of PAMBE-grown MgxZn1-xO Epitaxial Layers of Different x.
Thomas Sander 1 , Richard Thoet 1 , Peter Klar 1 , Martin Eickhoff 1 , Thomas Wassner 2
1 I. Physics Institute, Justus-Liebig University of Giessen, Giessen Germany, 2 Walter Schottky Institut, Technische Universität München, Garching Germany
Show AbstractThe MgxZn1-xO alloy can be grown in wurtzite structure with Mg contents x up to 0.4 and the band gap of the alloy increases significantly with x. Furthermore, ZnO/MgxZn1-xO quantum well structures are of type I and thus are of interest for the active region of opto-electronic devices.We report on in-plane photocurrent measurements of MgxZn1-xO epitaxial layers with x up to 0.37 in the temperature range from 10 K to 300 K. The epitaxial films are grown by plasma-assisted molecular beam epitaxy on c-plane sapphire substrates with a thin MgZnO buffer layer. We map the evolution of the band gap transitions as a function of temperature and Mg composition. The contributions of A, B and C excitons to the band gap signals are analysed and discussed in terms of their intensities and line widths.
9:00 PM - H10.34
Structural and Magnetic Properties of Zn1-xCoxO Nanoparticles Prepared by a Simple Sol-Gel Method at Low Temperature.
Segundo Jauregui-Rosas 1 , Oscar Perales-Perez 2 , Silvana Urcia-Romero 3 , Maharaj Tomar 3
1 Department of Physics, National University of Trujillo, Trujillo Peru, 2 Department of Engineering Science and Materials, University of Puerto Rico, Mayaguez Campus, Mayaguez, Puerto Rico, United States, 3 Department of Physics, University of Puerto Rico, Mayaguez Campus, Mayaguez, Puerto Rico, United States
Show AbstractPure and Co-doped ZnO nanoparticles have been prepared by a simple sol-gel method at low temperature, where neither any chelating agent nor additional annealing treatment were used. The effect of Co ions (x=0.01 - 0.0625) concentration on the structural and magnetic properties has been evaluated. XRD patterns show that single-phase samples with the wurtzite structure were obtained and no trace of cobalt metal, oxides or any other zinc cobalt phases were detected in the powders for all the dopant levels, which was confirmed by FT-IR. The cell parameters a and c vary linearly with ‘x’, which would indicate the replacement of Zn by Co. Micro Raman spectroscopy analysis shown that, in addition to the normal modes associated with wurtzite ZnO, an additional band centered at 534cm-1 was detected and it can be assigned to a local vibrational mode related to Co. The intensity and broadening of this band increased as Co content increases but no any shift was observed. In turn, a blue shift of the bands corresponding to A1 (E2, E1) and E2high, modes were observed in the Co-doped samples. Magnetic measurements, carried out by VSM at room temperature, shown that while the ZnO has a diamagnetic behavior, the Co-doped ZnO nanoparticles were paramagnetic in nature.
9:00 PM - H10.36
Seedless Synthesis and Field Emission of ZnO Nanoneedles on Metal Electrodes.
Gregory Wrobel 1 , Martin Piech 2 , Puxian Gao 1 , Sameh Dardona 2
1 Department of Chemical, Materials and Biomolecular Engineering & Institute of Materials Science, University of Connecticut, Storrs, Connecticut, United States, 2 Department of Physical Sciences, United Technologies Research Center, East Hartford, Connecticut, United States
Show AbstractHigh aspect-ratio zinc oxide (ZnO) nanoneedles were grown hydrothermally from metal electrode surfaces. A facile, one-step synthesis employing diaminopropane (DAP) growth modifier bypassed the need for substrate pre-seeding. Nanoneedles grown on Cu surfaces were of smaller size and higher aerial density compared to those grown from Fe surfaces. While metal etching was observed during the synthesis on both Cu and Fe surfaces, Auger analysis revealed dopant-free nanoneedles. Electrical and thermal annealing in vacuum significantly improved nanoneedles field emission characteristics by enhancing the emission current and lowering the turn-on field
9:00 PM - H10.37
Sputtered Glancing Angle Deposition of Metal Oxides: Controlling Micropost Morphology with Substrate Rotation.
Joshua LaForge 1 , Michael Brett 1 2
1 Electrical and Computering Engineering, University of Alberta, Edmonton, Alberta, Canada, 2 , NRC National Institute for Nanotechnology, Edmonton, Alberta, Canada
Show AbstractStructured thin films of metal oxides, such as zinc oxide[1], are desirable for several applications, including sensors, catalysts, and photovoltaic cells[2]. In the case of ZnO, large areas of isolated vertical posts should be useful in piezoelectric energy harvesting devices[3] or as birefringent, transparent electrodes for liquid crystal displays. We present a study of porous columnar ZnO fabricated by Glancing angle deposition (GLAD), a physical vapour deposition technique that uses an oblique flux direction, self shadowing of incident flux, and low adatom mobility to create a film structure of isolated vertical posts. Due to the shadowing requirements of collimated flux, evaporation techniques are typically used for GLAD, since they exhibit working pressures between 10-5 and 10-7 Torr. In comparison, sputter deposition, while proven for large-scale thin film fabrication, suffers from a relatively high working pressure, which leads to poorly defined structures. Recent work with sputtered ZnO has shown that substrate rotation can produce columnar or vertical post morphologies that are not present without rotation[4]. To further improve the morphology of sputtered ZnO films, a hollow-cathode has been used to reduce the minimum working pressure. Depositions at a variety of rotation speeds have been performed to study the transition to post morphology of ZnO. Preliminary results, at 1.7 mTorr, a deposition angle of 85°, and a 11 cm throw distance produce films 300-400 nm thick, with post diameters ranging from 50-100 nm for films deposited at a rotation speed of 69 RPM, and diameters of 100-250 nm at 1 RPM. We present details from our survey of the morphological phase space for ZnO and other oxides, including SEM data and crystallinity data from X-ray diffraction.[1] Wang, Z. L. (2004), 'Zinc oxide nanostructures: growth, properties and applications', Journal of Physics: Condensed Matter 16(25), R829--R858.[2] Peiro, A. M.; Ravirajan, P.; Govender, K.; Boyle, D. S.; O'Brien, P.; Bradley, D. D. C.; Nelson, J. & Durrant, J. R. (2006), 'Hybrid polymer/metal oxide solar cells based on ZnO columnar structures', Journal of Materials Chemistry 16(21), 2088--96.[3] Wang, Z.L. and Song, J. (2006), ‘Piezoelectric Nanogenerators Based on Zinc Oxide Nanowire Arrays’, Science 312, 242.[4] Teki, R.; Parker, T.C.; Li, H.; Koratkar, N.; Lu, T.M. and Lee, S. (2008), ‘Low temperature synthesis of single crystalline ZnO nanorods by oblique angle deposition’, Thin Solid Films 516, 4993-4996
9:00 PM - H10.38
Synthesis and Impedance Spectroscopy Studies of Electrical Properties of Hexagonal ZnO Nanoparticles.
Apisak Meesrisom 1 , Siu-Wai Chan 1
1 Materials Science and Engineering, Columbia University, New York, New York, United States
Show AbstractZinc oxide (ZnO) particles in nanometer size regimes have been synthesized using co-precipitation of zinc nitrate and hexamethylenetetramine (HMT) at 80 oC. The shape of synthesized nanoparticles of ZnO was hexagonal prisms. The combinations of ratio of concentration of HMT to that of zinc nitrate and synthesis time were employed for size control of ZnO nanoparticles. ZnO nanoparticles width and length decreases as concentration of HMT increases. Synthesis time also plays a vital role in nanoparticles growth rate. As synthesis time increases, nanoparticles sizes were increased. The resultant nanoparticles range from 2-85 nm in diameter and 11-95 nm in length. The smallest size of 2 nm was achieved by using the highest concentration of HMT and lowest synthesis time of 30 seconds. Then, particles with selected grain size diameter of 15, 60 and 85 nm were pelletized by using a uniaxial press die with applied pressure of 0.4GPa at room temperature without binder for impedance spectroscopy tests. Sample pellets were sintered at 400 oC for 15 minutes prior to testing. ZnO sample pellets were measured at temperature range of 0 – 400 oC with frequency span from 32 MHz – 0.1 Hz with 0.5 V AC. The main interest in nano size ZnO is to understand the effect of grain size on its electrical properties when temperature is elevated. Also, the impedance spectroscopy measurements of pre-sintered compacts of ZnO were carried out with the aim of characterizing lattice and grain boundary electrical behaviors. The result obtained in these experiments is the demonstration that the conductivity increased as particle size gets smaller. At around 70 oC, all of nano size ZnO started to increase in their conductivity noticeably. Also, activation energy was decreased significantly as particle size increased but there was not much difference in lattice case. This mainly was due to obstruction of the electrical flow at the grain boundaries. The conductivities of lattice were as high as 100 to 50 times larger than corresponding grain boundary conductivities.
9:00 PM - H10.39
Reaction Anisotropy and Size Resolved Oxidation Kinetics of Zinc Nanocrystals.
Xiaofei Ma 1 , Michael Zachariah 2
1 Department of Mechanical Engineering, University of Maryland-College Park, College Park, Maryland, United States, 2 Department of Mechanical Engineering and Department of Chemistry and Biochemistry, University of Maryland-College Park, College Park, Maryland, United States
Show AbstractZn and ZnO nanostructures have attracted considerable interest for their various potential applications. Zn can act as both the fuel, and a working catalyst in the Zn/ZnO thermo-chemical solar water splitting cycle for hydrogen generation. ZnO is an important semiconductor also exhibits piezoelectric properties. In this work, size-classified substrate-free Zn nanocrystals (NCs) are prepared and investigated for their oxidation kinetics using an in-flight tandem ion-mobility method. The first mobility characterization size selects the NCs, while the second mobility characterization measures changes in mass resulting from a controlled oxidation of the NCs. This method allows for a direct measurement of mass change of individual particles and thus enables us to explore the intrinsic reactivity of NCs while minimizing the sampling error introduced by mass and heat transfer. Two reaction regimes were observed for Zn NC oxidation. A shrinking core model is used to extract the size-dependent oxidation activation energies. We also observed a strong anisotropy effect in the oxidation process as imaged by electron microscopy. An oxidation mechanism is proposed that qualitatively explains the oxidation anisotropy and its relationship to the surface energy of the Zn NCs.
9:00 PM - H10.4
Optical Spectroscopy Studies of Energy Transfer in Well-Aligned Eu-Doped ZnO Nanowires.
Tom Wu 1 , Dandan Wang 1
1 , Nanyang Technological University, Singapore Singapore
Show AbstractRear-earth (RE) doped semiconductors have attracted extensive attentions as a prime candidate for full color or monochromatic displays because of their potential applications in electronics and photonics. It is reported that the luminescence of RE ions incorporated in the semiconductor matrix could be sensitized significantly by exciton recombination in the host. Typically, with a wide band gap (3.37 eV) and large exciton binding energy (60 meV) at room temperature, ZnO is an exceptionally important semiconductor and a good candidate as a host material for RE ions. In this work, well-aligned Eu3+ doped ZnO nanowires were synthesized by a facile vapor transport method, and their physical properties have been measured systematically. It is desirable to obtain an efficient energy transfer from excited ZnO nanowires to Eu3+ ions which could result in strong red emission. The time resolved photoluminescence results further confirmed the energy transfer from the ZnO host to the Eu3+ ions. This research could be applied to develop rational synthesis for advanced flat panel displays and field emission display devices.
9:00 PM - H10.40
Micro-structural, Optical, and Ferromagnetic Properties of Zn1-xCuxO Thin Films.
Kousik Samanta 1 , Ram Katiyar 1 , Pijush Bhattacharya 2
1 Physics, University of Puerto Rico, San Juan, Puerto Rico, United States, 2 Physics, Fisk University, Nashville, Tennessee, United States
Show AbstractThe 3d-transition metal doped ZnO based dilute magnetic semiconductors (DMS) attract an enormous research interests for their applications in the field of optoelectronics and spintronics. We have investigated the structural, optical, and magnetic properties of Zn1-xCuxO (x=0.01, 0.03, and 0.05) thin films grown on Al2O3 substrates by pulsed laser deposition technique. Raman scattering and high resolution transmission electron microscopy (HRTEM) analysis confirms the substitution of Cu up to 3% in ZnO host lattice; moreover, the films are epitaxial, defects free, and nearly single crystalline. Optical transmission and photoluminescence spectrum analysis provide evidence of sp-d exchange interaction in Zn1-xCuxO thin films. The near band edge transition (NBE) was found to be decreases due to the Cu doping. The green emission (~ 2.60 eV) peak was observed in Zn1-xCuxO (x > 1%) thin films and this peak broadened at higher Cu concentrations. We have deconvoluted the broad green emission band of 5% Cu doped thin film (~ 2.6 eV) and found two emission peaks 2.506 and 2.697 eV. Room temperature ferromagnetism was observed in of Zn1-xCuxO thin films with maximum saturation magnetization of 0.76 μB/Cu in 3% Cu doped sample. The saturation magnetization decreased in further increase of Cu concentration. The coercive fields (Hc) of 1, 3, and 5% Cu doped samples are 171, 185, and 145 Oe respectively.
9:00 PM - H10.41
Plasma Treatment Effects on the Electrical Properties of Aluminum Doped Zinc Oxide Films.
Lixi Yuan 1 , Dever Norman 1 , Quark Chen 2 , Priya Chinta 3 , Xuemei Wang 3 , Wei-Kan Chu 3 , Hom Kandel 1 , Tar-Pin Chen 1 , Hye-Won Seo 1
1 Physics and Astronomy, University of Arkansas at Little Rock, Little Rock, Arkansas, United States, 2 Physics, National Sun Yat-Sen University, Kaohsiung Taiwan, 3 Physics, University of Houston, Houston, Texas, United States
Show AbstractAluminum doped zinc oxide (AZO) is a promising candidate as the transparent electrodes for III-V and II-VI optoelectronic devices due to its good optical, electrical properties and the structural compatibility. Using a pulsed laser deposition technique, we have successfully grown single-crystalline AZO thin films on sapphire substrate. X-ray diffraction, atomic force microscopy and Rutherford back-scatter spectroscopy methods were used to characterize the crystal quality, morphology, composition and thickness of the thin AZO films. Their transport properties were also studied by temperature dependent resistivity and Hall effect measurements. Especially we have focused on the effects of post growth treatment of the AZO films under nitrogen and hydrogen plasma of various powers and temperatures. The mechanism of the changes in electrical carrier behaviors of the AZO films due to the plasma treatment will be discussed in details.
9:00 PM - H10.42
Microstructures and Magnetic Properties of Fe-doped Zn-TiO2 Rutile Nanoparticles.
Imaddin Al-Omari 1 , S. Al-Harthi 1 , M. Al-Saadi 1 , K. Melghit 1
1 , Sultan Qaboos University, Muscat Oman
Show AbstractOxide duiluted magnetic semiconductors, in which nonmagnetic oxide semiconductors are doped with low percent magnetic element have an important role in the development and the applications of semiconductor spintronics [1, 2], gas sensors, …etc. Since the discovery of the ferromagnetic state in the Co-doped TiO2 semiconductors at room temperature and with a curie temperature of 400 K [3] the attention of many researchers have been drawn to understand the effect of doping a transition metal on the magnetotransport properties of these systems. Among the well-known three crystalline structures (Rutile, Anatase, and Brookite) of TiO2 Rutile is the most thermodynamically stable phase. In this study we prepared Fe (x wt. %)-doped Zn-TiO2 rutile nanoparticles, with x= between 0 and 10 wt. % using the wet-gel stirring method. We found that the powder samples exhibit nanorods oriented in different directions and accompanied by an amorphous Zn on the surface. Each sample in this system has three components: Zn surface layer which acts as electron trapping component, Fe ion dopants which narrows the energy gap and enhance the magnetization, and TiO2 nanoparticles which operate as enhancers of the photocatalytic efficiency due to their large surface area. The x-ray diffraction patterns of the different samples show that all the samples crystalline in the rutile phase, which is characterized by the (101) diffraction peak. The shift in the peaks, with increasing x, toward lower angles can be attributed to the iron doping. The magnetic properties of the samples were studied using a vibrating sample magnetometer (VSM) in magnetic filed up to 13.5 kOe and in the temperature range of 100 K to 300 K. We found that the magnetization of the samples does not saturate in the maximum available field. The magnetization (M) at an applied magnetic field of 13.5 kOe is found to increase with increasing the Fe percentage at room temperature and at 100 K. We also found that the magnetization of each sample increases by about 10-times when we reduce the temperature from 300 K to 100 K. Transmission electron microscopy (TEM) images for two representative samples undoped (x = 0) and Fe-doped with (x = 4%) showed that in the undoped sample there are many nanorods with an average length of 60 nm and an average diameter of 7 nm while in the doped sample we found that the density of the nanorods decrease due to the breakdown to sheets, which is due to the increase of the acidity of Fe(NO3)39H2O solution while sample preparation. AFM images confirms the coexistence of the nanorods and nanosheets and the increase of the number of nanosheets compared with the number of nanorods for the doped sample. [1] W.T. Geng, Kwang S. Kim, Phys. Rev. B 68, 125203 (2003).[2] S.V. Chong, K. Kadowaki, J. Xia, and H. Idriss, Appl. Phys. Lett. 92, 232502 (2008).[3] W.K. Park, R.J. Ortega-Hertogs, J.S. Moodera, A. Punnoose, and M. Seehra, J. Appl. Phys. 91, 8093 (2002).
9:00 PM - H10.43
Zinc Oxide Transparent Thin-film Transistors on a Plastic Substrate Fabricated by a Microwave Heating Process.
Chanwoo Yang 1 , Tae An 1 , Jaeyoung Jang 1 , Dae Chung 1 , Chan Park 1
1 , Pohang University of Science and Technology (POSTECH), Pohang Korea (the Republic of)
Show AbstractWe report solution-processed flexible zinc oxide (ZnO) transparent thin-film transistors (TFTs) with a polymer gate dielectric using a commercially available microwave oven. The ZnO active layer fabricated by microwave heating showed a highly uniform and densely packed array of the large crystal size (~58 nm) in the (002) direction of ZnO nanorods on the plasma-treated polymer gate dielectric. The flexible ZnO TFTs with the plasma-treated polymer dielectric exhibited a mobility of 1.1 cm^2/Vs, an on-off ratio of 10^3 whereas the mobility of TFTs with the bare polymer dielectric showed 0.13 cm^2/Vs.
9:00 PM - H10.44
Low Temperature Electrical Resistivity of Ga-Doped MgxZn1-xO.
Wei Wei 1 , Chunming Jin 2 , Nori Sudhakar 1 , Roger Narayan 2 , Jagdish Narayan 1
1 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 Joint Department of Biomedical Engineering, North Carolina State University and UNC Chapel Hill, Raleigh, North Carolina, United States
Show AbstractThe growing demand for transparent conducting oxides (TCO) for optoelectronic devices such as flat panel displays and solar cells has been the primary motivation for research on these materials. Among the TCOs, ZnO based materials has stimulated a lot of research effort in recent years, due to their high optical transparency and low resistivity. Such low resistivity has attracted considerable research interest to study the fundamental electrical properties in ZnO based materials with an aim of better understanding of transport properties such as the carrier generation and carrier scattering mechanisms, which are very critical to improve the electrical conductivity of ZnO based materials in the future.In this work, we studied electrical conduction of Ga-doped MgxZn1-xO thin films in the temperature range from 300 K down to 50 mK. Ga-doped MgxZn1-xO thin films were grown epitaxially on c-sapphire substrates using pulsed laser deposition at a substrate temperature of 650oC. The Ga contents were 0.05 at.%, 0.1 at.%, 0.5 at.% and 1 at.%, respectively. These samples were characterized by X-ray diffraction which showed that all the samples were homogeneous and no other phases and segregations were present. The carrier concentration and Hall mobility of these samples were measured at room temperature. The temperature-dependent resistivity showed a typical Mott transition which was found to be a function of Ga content. The resistivity of samples with 0.05 at.% and 0.1 at.% Ga showed an insulator-like temperature dependence, while the samples with 0.5 at.% and 1 at.% Ga showed finite resistivity values at the lowest temperature, which were a typical metallic behavior. The temperature dependent resistivity was also analyzed in order to understand the electron-electron and electron-phonon interactions in the high electron density system.
9:00 PM - H10.46
Hyperfine Interactions in the EPR Spectra of Neutral Nitrogen Acceptors in ZnO.
Adam Brant 1 , Shan Yang 1 , Sean Evans 1 , Larry Halliburton 1 , Nancy Giles 2
1 Physics Department, West Virginia University, Morgantown, West Virginia, United States, 2 Engineering Physics Department, Air Force Institute of Technology, Wright-Patterson AFB, Ohio, United States
Show AbstractDevelopment of p-type zinc oxide (ZnO) continues to be a challenge for the materials community. Nitrogen is the most promising shallow acceptor (due to its similar size to oxygen), but routine nitrogen doping at high levels has not yet been achieved. Nitrogen can be incorporated at low levels in zinc oxide [1], and we experimentally investigate the electronic structure of the ground state of nitrogen (e.g., the nature and extent of its wave function) in order to obtain insight to methods for increasing the concentration of nitrogen in thin films and bulk crystals. In the present study, electron paramagnetic resonance (EPR) is used to characterize the 14N (I = 1) and 67Zn (I = 5/2) hyperfine associated with the neutral substitutional nitrogen acceptor in ZnO. Data were acquired from a bulk crystal grown at Eagle-Picher (Miami, OK) by the seeded chemical vapor transport method with N2 added to the gas stream to serve as a doping source. Room temperature Hall results showed that the sample was n type, with a free carrier concentration of 2 x 1016 cm-3. Sets of EPR angular dependence data were taken within the basal plane and from the c axis to the basal plane, and were analyzed with a spin Hamiltonian containing electron Zeeman, hyperfine, nuclear electric quadrupole, and nuclear Zeeman terms. Forbidden transitions appear in the 14N EPR hyperfine spectra when the magnetic field is aligned nearly perpendicular to the c axis, and their intensities provide a value for the magnitude of the nuclear electric quadrupole interaction. Spectra from the one 67Zn neighbor along the c axis and the three 67Zn neighbors in the basal plane demonstrate the axial nature of the acceptor wave function. These 67Zn interactions also describe the amount of delocalization of the nitrogen’s wave function. This work was supported by the National Science Foundation (Grant Number DMR-0804352). [1] N. Y. Garces et al., Applied Physics Letters 80, 1334 (2002).
9:00 PM - H10.47
Spectroscopic Ellipsometric Study of Amorphous InGaZnO Thin Films.
Mohammad Ebdah 1 , Martin Kordesch 1 , David Ingram 1
1 Dept. of Physics & Astronomy, Ohio University, Athens, Ohio, United States
Show AbstractAmorphous InGaZnO thin films were successfully fabricated using radio frequency (RF) sputtering technique and were deposited onto Si substrates. Sputtering was achieved at a rate of 0.3–0.4 Å/s using a sputtering target of liquefied Ga, and pure In and Zn metals in an Ar/O2 atmosphere. The films have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), Rutherford Back Scattering (RBS) and ellipsometric spectroscopy techniques. The XRD patterns reveal that the grown films are amorphous. For all grown films, the index of refraction and extinction coefficient were obtained by the analysis of the measured ellipsometric spectra of Ψ and Δ with the Tauc–Lorentz model over the wavelength range (200-1200) nm. The optical absorption edge, Eg, and the absorption coefficient were analyzed using the modified Tauc linear extrapolation.
9:00 PM - H10.48
Synthesis, Transfer, and Characterization of Zinc Oxide Nanowires on Zinc Oxide Sputtered Substrate and Indium Tin Oxide Substrate.
Ebraheem Azhar 1 , Hongbin Yu 1 , Tej Belagodu 1
1 Electrical Engineering, Arizona State University, Phoenix, Arizona, United States
Show AbstractThe prevalence of Zinc Oxide (ZnO) as a primary material for nano-electronic development is evidenced by the almost exponential growth of literature on the subject since 2005. Its benefits arise electrically from its high bandgap and electron mobility, and physically from its tetrahedral structure contributing to its piezoelectric nature. Though ZnO is a versatile solution to many applications, it is the aim of this research to explore the vertical growth of ZnO nanowires (NW) manganese doped ZnO sputtered on silicon substrate as well as Indium Tin Oxide (ITO) substrate. The NWs are then transferred using a mechanical sliding process which will also be expanded upon.The ZnO NWs were grown under a high temperature growth condition in a furnace that housed glass tubing. The ZnO mixed with graphite at 1:1 molar ratio was placed at the hottest (930C) temperature region, the center of the furnace. Following a temperature profile having been taken of inside the furnace, the ZnO coated substrate was placed 6’’ from the center (800C) with a flow rate of 110 sccm argon and 3 sccm for oxygen. The result under electron microscopy (SEM) was an array of vertically aligned nanowires about 3 microns in length exhibiting hexagonal ends when viewed from above.The ZnO film coated Si substrate provides an opportune setting for ZnO powder to form NWs in a vertical highly ordered manner. This occurs because the pores formed on the surface of the substrate match the unit cell structure of the ZnO source material allowing it to set into the pore and linearly promote growth perpendicular to the surface. Because the orientation of the substrate is [002], there is indeed a tilt reflected upon the NW array. The use of an ITO substrate furthermore changes the morphology of the resulting NWs. The NWs grown on ITO exhibit a thicker and denser collective.The transfer process consists of aligning the growth substrate and sticking out NWs above a receiver substrate coated with lithographically patterned resist. The growth substrate is then slid across and the wires then detach while reattaching to the new substrate by the van der Waals force. A combination of octane and mineral oil serve as lubrication to minimize friction in the process and also reduces the rupturing the NWs themselves. The end result is a collection of NWs on a patterned contact that can be used as a device.
9:00 PM - H10.49
MOCVD of Zinc Oxide Transparent Conductive Oxide for Contacts.
Shangzhu Sun 1 , Bruce Willner 1 , Gary Tompa 1
1 , Structured Materials Industries, Inc., Piscataway, New Jersey, United States
Show AbstractZinc Oxide (ZnO) and its alloys are wide bandgap semiconductors that can serve very effectively as transparent contact layers on electronic and optoelectronic devices such as solar cells and LEDs. ZnO films have very high transparency at visible and UV wavelengths, good lattice match to GaN and good high temperature stability. Highly conductive transparent ZnO can be produced by doping with Al, In, or Ga. Despite degenerate doping, 2 at.% or more, the material crystalline quality, as shown by photoluminescence and other characterizations, and optical transparency remains very good. The alloying of ZnO with magnesium or cadmium oxide provides the ability to tune the energy bandgap and index of refraction without a large change in the lattice constant.Metal-organic chemical vapor deposition (MOCVD) of ZnO thin films can produce high quality films of varied and well-controlled composition, uniformly over large areas. MOCVD deposited ZnO demonstrates significantly higher material quality than sputtered ZnO, and at much lower cost than MBE. We have optimized a reactor design with a high rotation speed susceptor, performance scaled up to 16” diameter deposition planes and with modeling scaled to greater diameters for single and multi-layer ZnO structures. MOCVD offers greater composition control to the deposited layers, including the ability to vary the composition during deposition. Doping densities lower than 1015 cm-3 and as high as 1022 cm-3 are possible, allowing precise control of the electrical and optical properties of the material. Deposited film structures exhibit pin-hole-free, smooth morphology and demonstrate 3% thickness uniformity across an 8” wafer. Deposition can be tuned to exhibit amorphous, polycrystalline and single crystalline films at temperatures from 300°C to 1000°C. High quality, conductive films can be produced below 500°C. We demonstrate ZnO TCO films as transparent contacts for GaN LEDs, providing a clear emission window and improved operating efficiency. Annealing studies were performed to optimize ZnO electrical contact with GaN.
9:00 PM - H10.5
Interface Structure and Chemical States of Pt-based Metal Contact on Polar-ZnO Single Crystal.
Takahiro Nagata 1 , Janos Volk 2 , Yoshiyuki Yamashita 1 3 , Hideki Yoshikawa 3 , Masamitsu Haemori 1 , Ryoma Hayakawa 1 , Michiko Yoshitake 1 , Shigenori Ueda 3 , Keisuke Kobayashi 3 , Toyohiro Chikyow 1
1 Advanced Electric Materials Center, National Institute for Material Science, Tsukuba Japan, 2 , Research Institute for Technical Physics and Materials Science, Budapest Hungary, 3 NIMS Beamline Station at SPring-8, National Institute for Materials Science, Hyogo Japan
Show Abstract Zinc oxide (ZnO) is a wide band-gap semiconductor that has recently been shown to have major potential for use in optical devices in the UV wavelength region. Although similar devices based on gallium nitride (GaN) have been put to practical use, ZnO has a number of advantages over GaN, including higher quantum efficiency and greater resistance to high-energy radiation. To apply ZnO to electric devices, a high-quality metal/ZnO interface is required. In the case of Schottky contacts, several groups reported that metal contact on ZnO shows changes in electrical properties with time or after annealing. It has also been reported that the oxidization at the metal/ZnO interface affects its electrical properties. Thus, understanding the interface structure and the chemical states at the metal/ZnO interface is the key to the formation of a high-quality metal/ZnO interface. ZnO is a polar material due to the lack of inversion symmetry and the ionic nature of the Zn-O bond, so two distinct {0001} planes along the c axis, which are the Zn-polar face and the O-polar face, show some different electrical properties. The characteristics (crystal structure, chemical state, etc.) of the metal/ZnO interface should be affected by the polarity of ZnO but have not yet been sufficiently elucidated. In this study, Pt-based Schottky contact on a polar ZnO single crystal was systemically investigated by the combinatorial metal-alloy film synthesis technique and synchrotron XPS measurements at BL15XU in SPring-8. By employing these methods, we have successfully controlled the Schottky barrier height of Pt-based metal contacts on ZnO and revealed the structure and the chemical states at a metal/ZnO interface. For the Zn polar face, Zn diffused into the metal layer, forming several oxidation states at the interface. By contrast, for the O-polar face, a metal oxide species is predominantly formed around the interface and show thick, where the structure appears to be inhomogeneous. These phenomena are not depended on kinds of metal we employed. Thus, the Zn-polar face exhibits better crystallinity in the metal layer around an interface than the O-polar face. Additionally comparing interface structure with the chemical states at an interface revealed electric properties in detail. The authors are grateful to HiSOR, Hiroshima Univ. and JAEA/SPring-8 for the development of HX-PES at BL15XU of SPring-8.
9:00 PM - H10.50
Optimising the Low Temperature Growth of Uniform ZnO Nanowires.
Nare Gabrielyan 1 , Shashi Paul 1 , Richard Cross 1
1 Emerging Technologies Research Centre, De Montfort University, Leicester United Kingdom
Show AbstractRecently, much work has been done on the synthesis and application of many different types of nanoscale materials in an effort to exploit the novel physical, chemical and electrical properties. Zinc Oxide (ZnO) nanostructures have been studied extensively and have found their application in different devices, such as gas sensors [1], transistors [2], etc. A number of different methods of synthesising one-dimensional (1-D) ZnO nanostructures have been utilised, but these often include high temperature and/or metal catalysts to facilitate the growth, which limits their use for certain applications or can result in unwanted doping of nanostructures, thus affecting their physical properties.Recently, a new method for ZnO nanowire synthesis has been suggested [3]. This involves a two-step process: firstly a thin ZnO nucleation layer is deposited onto a substrate at room temperature by Radio Frequency magnetron sputtering; then a hydrothermal growth step takes place using equimolar aqueous solutions (0.01M?) of hexamine and zinc nitrate. However, an extensive optimisation of the nanowire arrays produced by the aforementioned method has yet to be carried out. In this regard, this paper describes the results of optimisation experiments for the large-area, low temperature uniform growth of ZnO nanowires The nucleation layer thickness was varied between 5-200 nm and the temperature of the growth solution varied between 40-90oC . The structural properties of the resultant nanostructures were analysed using Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD). And the morphology of the nucleation layers were investigated using Atomic Force Microscopy (AFM) in order to understand the influence of this layer on the structure/distribution of the as-grown nanowires arrays.[1] Z. Y. Fan and J. G. Lu, J. Nanosci. Nanotechnol. 5, 1561 (2005)[2] Z. Y. Fan and J. G. Lu, Appl. Phys. Lett. 86, 123510 (2005)[3] R. B. M. Cross, M. M. De Souza and E. M. Sankara Narayanan, Nanotechnology 16 (2005) 2188–2192
9:00 PM - H10.52
Self-Template-Directed Formation of Coordination-Polymer Hexagonal Tubes and Rings, and their Calcination to ZnO Rings.
Hee Jung Lee 1 , Won Cho 1 , Soyoung Jung 1 , Moonhyun Oh 1
1 Department of Chemistry, Yonsei University , Seoul Korea (the Republic of)
Show AbstractMicro- and nano-scale materials made from atomic or molecular building blocks have received a great deal of attention because of their useful properties, which allow for many applications, and because of their fundamental interests in unique structures. The size, morphology, and composition of these materials are important factors that dictate their characteristic properties. In this context, the morphology and size of particles have been manipulated to obtain the desired properties for specific applications. In addition, an understanding of the particle-growth mechanism is necessary for fine tuning of a given particle’s morphology and size. Recently, a synthetic strategy for the preparation of micro- and nanoparticles made from infinite coordination polymers has been demonstrated. Here we wish to report the preparation of hexagonal tubes and rings. A unique particle growth mechanism named self-template-directed growth mechanism has been proposed and verified. This growth mechanism involves the initial formation of hexagonal lumps or disks and the concomitant dual processes of growing new coordination polymers on the surfaces of the initially-formed particle acted as template and dissolving the initial template to afford final hexagonal tubes or rings. Furthermore, we also show that calcination of these unusually shaped coordination polymer particles (CPPs) results in the spontaneous formation of ZnO particles while the unique shape is maintained. Because CPPs can be prepared with a wide range of compositions and morphologies, this CPP calcination method will provide a novel paradigm in the manufacture of customized metal oxide nanoparticles.
9:00 PM - H10.53
Rapid Deposition of Adherent Oxide Thin Films on Semiconducting and Dielectric Substrates via Microwave Irradiation-assisted Chemical Synthesis.
Sanjaya Brahma 1 , S. Shivashankar 1
1 Material Research Centre, Indian Institute of Science, Bangalore, Karnataka, India
Show AbstractThe deposition of oxide thin films by the sol-gel method has been based on spin coating or dip coating, followed usually by prolonged thermal treatment at elevated temperatures. While the spin coating technique is well developed and has been applied to various oxides and to large area substrates, it is not applicable to non-flat substrates, or to polymer (low temperature) substrates. In the present work, a variant of both the sol-gel and dip-coating methods, which uses microwave irradiation to form oxides from metalorganic precursors has been developed, to provide a process for coating metal oxides on to semiconducting and insulating substrates, flat or flexible. The method involves preparing a solution of a coordination complex of the metal in question in a suitable solvent, together with an appropriate surfactant, and subjecting the solution to irradiation in a domestic microwave oven. The substrate to be coated is suspended in the solution during the irradiation, leading to the formation of a metal oxide coating, within minutes. Any residual surfactant present in the coating may be removed by a brief exposure to a temperature of about 400 degree centigrade, in air. The method is illustrated by obtaining coatings of ZnO on Si(100), Ge(100), and glass. A dilute solution of zinc acetylacetonate in ethanol, with a small molar proportion of a surfactant, is irradiated with 800 W of microwave power at 2.45 GHz. On a Si(100) substrate measuring 20 mm x 15 mm, suspended in the solution during irradiation, a uniform coating of well-crystallized ZnO (würtzite) is formed. By preparing the reactant solution appropriately, a dense, uniform coating as thick as several μm can be obtained (reproducibly) in about five minutes. The coating adheres well to Si(100), as revealed by the peel tape test. The microstructure and preferred orientation of the ZnO coating may be controlled by varying process parameters: for example, a coating comprised of tapered hexagonal crystallites of ZnO can be obtained on Si(100), or a coating made of smaller, irregularly shaped crystallites. Such ZnO coatings have been characterized in detail by X-ray and electron diffraction, and electron microscopy. The method has been extended to obtain well-crystallised and adherent coatings of α-Fe2O3, Gd2O3 and other oxides on Si(100), fused quartz, and glass, at high rates of formation. In each case, film microstructure may be controlled by choosing the metalorganic precursor, the surfactant, duration of irradiation, and other process parameters. Oxide coatings have also been formed on polymer substrates such as PMMA. The chemical reactions involved and the concomitant deposition process may be understood by invoking the dielectric response of the precursor/surfactant/solvent/-substrate quartet to the microwave field. The present method is scalable to larger substrates, and is promising as a low temperature technique for coating dielectric substrates, including flexible polymers.
9:00 PM - H10.54
Rapid Growth of ZnO Nanostructures via Microwave Irradiation-assisted Chemical Synthesis.
Sanjaya Brahma 1 , Srinivasrao Shivashankar 1
1 Material Research Centre, Indian Institute of Science, Bangalore, Karnataka, India
Show AbstractWe report synthesis of large quantities of single-crystalline ZnO nanostructures i.e, nanoparticles, nanorods and nanotubes by the microwave irradiation of metalorganic complex of zinc, in the presence of a surfactant. The method is simple, inexpensive, and yields pure nanostructures of the hexagonal würtzite phase of ZnO in minutes, and requires no conventional templating. The synthesis procedure involves a solution of the metal organic complex of zinc, zinc acetylacetonate, of high purity (prepared in our laboratory), in an appropriate amount of ethanol and polyvinyl pyrrolidone (PVP) as surfactant. This solution is then subjected to microwave irradiation in a domestic microwave oven operating at 2.45 GHz frequency with power varying from 160-800 W, leading to the formation of zinc oxide nanostructures within a few seconds to a few minutes. The spherical ZnO nanoparticles have a diameter of ~10 nm. The ZnO nanotubes formed have a hollow core with inner diameter varying from 140-160 nm and a wall thickness of 40-50 nm, their lengths varying in the narrow range of 500-600 nm. There is a systematic variation in the morphology of the ZnO nanostructures with variation of process parameters, such as microwave power, microwave irradiation time, surfactant type, and its concentration. There is also an appreciable change in the size and shape of ZnO nanostructures with the variation of molecular structures of the metalorganic complex, e.g., sharp-tipped, tapered ZnO nanorods or branched structures have been obtained by using an adduct of the complex zinc acetylacetonate, also synthesised in the laboratory. The microstructure of ZnO nanostructures obtained by the present method is found to depend significantly on the nature of the surfactant. For example, flower-like structures are obtained when a nonionic surfactant called Triton X-100 is used, and nearly monodisperse nanorods ~1 µm long and 80 nm in diameter can be obtained using sodium dodecyl sulphate, an anionic surfactant. The evolution in the morphology when employing surfactants such as CTAB, which is a cationic surfactant, Brij 35 (nonionic surfactant), and with the molecular weight of PVP, will be reported. The as-prepared powder sample may either need a very brief exposure to heat to remove the surfactant or no post-synthesis processing, and is found to be very pure and well-crystallised ZnO. These nanostructures have been characterized by x-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and selected area electron diffraction (SAED). Each ZnO nanorod and nanotube is found by SAED to be a single crystal. Study of the optical properties of the nanostructures, such as through photoluminescence, has provided some surprising results. The tapered nanorods are found to be excellent electron emitters. A model is proposed to explain the formation of highly ordered nanostructures of ZnO in the absence of any templating.
9:00 PM - H10.6
Low Resistivity and High Mobility Lithium Doped Zinc Oxide Materials.
Yi-Wen Kao 1 , Shan-Haw Chiou 1 , Kuo-Chuang Chiu 1
1 Materials Research Laboratories, Industrial Technology Research Institute, Hsinchu Taiwan
Show AbstractLithium doped zinc oxide polycrystalline bulks (LiZO) were prepared by soft-chemical route process. The doping of lithium in the solution was varied from 0.05 to 0.3 mol%. The structural characteristics studied by X-ray diffractometry were complemented resistivity measurement by Hall Effect. Prepared under tartaric acid as chelating agents and sintered at different temperature were employed to elucidate the LiZO formulation for the bulk process. The lowest resistivity were obtained for the LiZO bulk containing 0.05 mol% of lithium. The Li-doped ZnO bulks prepared at 1400 oC possessed the lowest resistivity of 1.734 Ω-cm with a Hall mobility of 32.7 cm2 V-1 s-1 and hole concentration of 1.1 × 1017 cm-3.
9:00 PM - H10.7
Room Temperature Ferromagnetism in Spin-coated Anatase- and Rutile- Ti1-x MxO2 ( M= Fe, Mn, Co) Films.
Danilo Barrionuevo 1 , Surinder Singh 2 , Maharaj Tomar 3
1 Physics, University of Puerto Rico Mayaguez, Mayaguez, Puerto Rico, United States, 2 Engineering Science and Materials, University of Puerto Rico Mayaguez, Mayaguez, Puerto Rico, United States, 3 Physics, University of Puerto Rico Mayaguez, Mayaguez, Puerto Rico, United States
Show AbstractIn recent years, diluted magnetic semiconductors (DMS) have been studied extensively, because of their potential application in spintronic devices. TiO2 is a wide band gap semiconductor. We studied the structural, electronic, and magnetic properties of Ti1-x MxO2 ( M= Fe, Mn, Co) for (x = 0.00, 0.05, 0.08, 0.10, 0.15, 0.20, 0.25, and 0.30). Thin films of Ti1-x MxO2 (M= Fe, Mn, Co) were synthesized by sol gel technique and were deposited by spin coating on Pt/SiO2/Si and quartz substrates. X-ray diffraction studies and Raman spectroscopy reveal anatase and rutile phases of the synthesized films when annealed at 550 and 1000 oC, respectively. Optical transmission measurements show high degree of transparency that decreases with increase in transition metal ion concentration. Cobalt doped TiO2 has shown magnetoresistance with the possibility of magnetic coupling. The films show room temperature ferromagnetism, showing their potential in spin based heterojunction devices.
9:00 PM - H10.9
Vanadium Dioxide Nanowires: from Orientated Assembly to Electroactivity.
Liqiang Mai 1 2 , Yanhui Gu 1 , Lin Xu 1 , Pengchao Zhang 1
1 State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, School of Materials Science and Engineering, Wuhan University of Technology, Wuhan, HUbei, China, 2 Department of Chemistry and Chemical Biology, Harvard University, Boston, Massachusetts, United States
Show AbstractOne-dimensional (1D) nanostructures, such as nanowires and nanotubes, have attracted growing attention due to their unique structures, interesting physical properties, and potential for novel applications. A formidable challenge, however, is the hierarchical organization of these nanoscale building blocks into organized assemblies and, ultimately, useful systems. The Langmuir-Blodgett (LB) technique is an attractive methodology because it can readily help to assemble 1-D nanostructures into large-area ordered monolayer arrays at the air-water interface. Vanadium oxide nanowires have been the focus of intensive investigation due to their interesting electrochemical, electric and magnetic properties. Until now, however, there is poor understanding of orientated assembly and properties of vanadium dioxide (VO2) nanowires, which restricts the practical application of this kind of novel nanomaterials. Herein we report Langmuir-Blodgett based assembly, electronic structure adjustment and properties of VO2 nanowires. VO2 nanowires were functionalized with stearic acid (SA) and cetyltrimethylammonium bromide (CTAB) and then spread on the surface in an aqueous phase in a LB trough. The LB film of the VO2 nanowires was transferred at the certain surface pressure. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), fourier transform infrared (FTIR), and cyclic voltammetry (CV), current-voltage (I-V) investigation. Interestingly, XRD studies show that VO2 nanowire LB films exhibits (00l) crystal plane orientation, which results from preferential coordination of SA and CTAB-SA complex to (001) surface of VO2 nanowires thereby driving this orientation. I-V and CV investigation confirm the specific capacity, current density and conductivity of VO2 nanowires are improved through LB assembly by approximately two orders of magnitude, resulting from preferred orientation arrangement, locally ordered nanowire monolayer structure and resulting decrease of boundary resistance.ACKNOWLEDGEMENTThis work was supported by the National Nature Science Foundation of China (50702039), the Research Fund for the Doctoral Program of Higher Education (20070497012) and Innovation Special Foundation of Excellent Returned Scholars of Wuhan (2008-84). The authors are pleased to thank the strong support and helpful discussion of Prof W Chen, B Hu, YN Jiang and HR Xu of Wuhan University of Technology.