Symposium Organizers
Joseph J. Berry National Renewable Energy Laboratory
Elvira Fortunato CENIMAT/13N
Yuzo Shigesato Aoyama Gakuin University
Julia Medvedeva Missouri University of Science and Technology
MM1:Transparent Conducting Oxides and Applications
Session Chairs
Monday PM, November 29, 2010
Constitution B (Sheraton)
9:30 AM - MM1.1
Transparent Conductive Nb12O29 Thin Films Deposited on Glass Substrates by Using Magnetron Sputtering.
Takeo Ohsawa 1 , Junpei Okubo 1 , Toru Suzuki 1 , Taro Hitosugi 1
1 , Tohoku University, Sendai Japan
Show AbstractIn the last decade, the industrial demands for transparent conductive oxides (TCOs) have been growing to provide a wide range of novel devices, flat panel displays, light-emitting devices, and solar cells. Further investigations of new class of TCOs on glass substrates have been more and more required in light of cost issues and improvement of various optoelectronic devices. Highly-conductive Nb12O29 thin films were fabricated on glass substrates using dc magnetron sputtering. Sputtering of Nb metal under a critical oxygen flow ratio of 6.5%, followed by annealing at 1000°C in vacuum, resulted in the transparent and conductive thin film. Structural properties characterized by grazing incidence x-ray diffraction (GIXRD) and high-resolution transmittance electron microscope (HRTEM) indicate that these films can be assigned to be Nb12O29. The HRTEM images strongly support that this film is composed of a Nb12O29 based on their rectangle block structures and the corresponding diffraction patterns. We employed synchrotron-radiation photoemission spectroscopy (SR-PES) to gain direct evidence for the metallic nature of Nb12O29, consequently revealing the metallic Fermi edge. In addition, this film showed relatively high transparency with the transmittance of ~70 % in the visible. Thus, we succeeded in the synthesis of transparent and metallic Nb12O29 films on glass substrates.
9:45 AM - MM1.2
Optimization of Metal-insulator-metal Diodes for High Frequency Rectification Using Different Insulator Candidate Materials.
Prakash Periasamy 1 , Philip Parilla 2 , Arrelaine Dameron 2 , Joseph Berry 2 , David Ginley 2 , Ryan O'Hayre 1
1 Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, Colorado, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractMetal-Insulator-Metal (MIM) diodes are actively considered for high frequency (THz) rectification when operated in the fast tunneling transport regime. Tunneling is an ultra-fast process and hence allows ∼THz frequency operation. As tunneling probability decays exponentially with insulator thickness, an ultra-thin (1-5 nm) high quality (pin-hole free) insulator layer is required to achieve tunneling in MIM diodes. In this work, MIM diodes with different insulator layers were fabricated and characterized with the focus on rectification characteristics at low frequencty. TiO2, Nb2O5, Al2O3, Y2O3 and HfO2 are the insulating oxides investigated in this study to fabricate MIM structures in point-contact configuration. Nb2O5 and TiO2 were deposited via anodization and pulsed layer deposition, respectively. Al2O3, Y2O3 and HfO2 were deposited via atomic layer deposition. The ultra-thin film oxides were characterized using x-ray reflectivity, atomic force microscopy and x-ray photoelectron spectroscopy. I-V response of the devices were measured using a custom built I-V test stand assembled for point-contact configuration. Among the devices tested, MIM systems based on Nb-Nb2O5-Pt and Nb-TiO2-Pt gave excellent diode characteristics such as low turn-on voltage (0.02 V), high asymmetry value (defined as |Iforward/Ireverse|) of 4 to 10 at a bias voltage of 0.5 V and high non-linearity. The other MIM structures based on Al2O3 or Y2O3 or HfO2 exhibited linear or symmetric I-V curves and hence are not suitable for rectification. Using a trapezoidal barrier model and the experimental results, an empirical theory relating the work function values of the metal and the electron affinity of the insulator layer is proposed. As per the theory, if one of the work function of the metals and the electron affinity of the insulator are close (i.e., the barrier height at the interface is close to zero), it results in a device with high asymmetry and if a suitable insulator thickness is chosen yields a very low turn-on voltage. Theoretical analysis using the SimmonsSimmons, JAP, 1963 models were carried out to identify the best barrier parameters to achieve desired I-V response of a MIM structure. The analysis clearly showed that best I-V responses were obtained when one of the barrier height is 0 eV, which is consistent with the observed experimental results.
10:00 AM - **MM1.3
Oxygen Exchange at SnO2 Surfaces.
Andreas Klein 1 , Christoph Koerber 1 , Andre Wachau 1 , Karsten Rachut 1 , Steven Harvey 2 , Thomas Mason 2 , Peter Agoston 1 , Karsten Albe 1
1 Materials Science, Darmstadt University of Technology, Darmstadt Germany, 2 , Northwestern University, Evanston, Illinois, United States
Show AbstractSnO2 is a prototype material for chemical gas sensing. Changes in electrical conductance in dependence on gas composition are typically explained by a variation of surface potential induced by adsorption/desorption of oxidizing or reducing gases (ionosorption model). Typical sensor operation temperatures are below 400°C. At higher temperature, the conductivity of SnO2 single crystals exhibits reversible changes of conductivity in dependence on oxygen partial pressure, which is related to changes in defect equilibria, typically attributed to oxygen vacancies. Establishing defect equilibrium requires the exchange of oxygen with the gas phase. We will address the dependence of oxygen exchange at SnO2 surfaces using a combination of electrical conductivity and photoemission experiments with density functional theory calculations of surface terminations and oxygen adsorption. It is shown that stoichiometric surfaces block the exchange of oxygen because charge transfer to adsorbed oxygen is not possible. This observation justifies the validity of the ionosorption model for SnO2 gas sensors.
10:30 AM - MM1.4
Impurity Distribution and Microstructural Characterization of GZO Films Grown on ZnO/Sapphire and on GaN/Sapphire Substrates by MBE.
Alexander Kvit 1 , Andrew Yankovich 1 , Vitaliy Avrutin 2 , Huiyong Liu 2 , Natalia Izyumskaya 2 , Hadis Morkoç 2 , Paul Voyles 1
1 Materils Science and Engineering, UW-Madison, Madison, Wisconsin, United States, 2 Electrical and Computer Engineering, Virginia Commonwealth University, Richmond, Virginia, United States
Show AbstractWe report aberration-corrected STEM microstructural and microanalytical characterization of heavily Ga-doped ZnO (GZO) thin films. Ga and Al doped ZnO films are important materials for high-performance transparent conductive n-type oxide, with applications as window materials in displays, light-emitting diodes, solar cells and other optoelectronic devices. We studied GZO films grown on ZnO/sapphire and GaN/sapphire layers by plasma-enhanced molecular beam epitaxy. All the structures demonstrate single-phase epitaxy of all layers, but metal-rich and oxygen-rich growth conditions lead to the formation of different extended defects. Metal-rich conditions for GZO on ZnO/sapphire results in an unusual GZO layer, which is porous but stills maintains excellent epitaxy with the underlying ZnO seed layer. GZO grown on GaN/sapphire is not porous and is otherwise highly structural perfect. These results correlate with photoluminescence and Hall effect measurements.We have also measured the gallium, zinc, oxygen and aluminum distribution of GZO on ZnO/sapphire thin films grown under metal- and oxygen-rich conditions using energy-filtered TEM, and aberration-corrected STEM EELS and EDS spectrum imaging. The Ga distribution is flat within the GZO layers, in good agreement with first-principles calculations for Ga diffusion in ZnO. Ga and O accumulate at the GZO surface, where Zn has minimum concentration, but not sufficiently to form a distinct gallium oxide phase.
11:15 AM - **MM1.5
High Mobility Transparent Conductive Oxides.
Rodrigo Martins 1 2 , Elamuguru Elangovan 2 1 , Shanmugam Parthiban 2 1 , Gonçalo Gonçalves 2 1 , Elvira Fortunato 2 1
1 CEMOP, Uninova, Caparica, Caparica, Portugal, 2 CENIMAT, FCTUNL, Caparica, Caparica, Portugal
Show AbstractIn this paper we report the role of process temperature (ranging from room temperature to 600 C) composition and process technology route selected (physical or chemical based) on the structure, electrical and optical performances exhibited by binary and ternary oxides. Here emphasis will be put on amorphous and polycrystalline films processed by rf magnetron sputtering at room temperatures exhibiting mobilities ranging from 20 to 75 cm2V-1s-1and in some cases with transparency exceeding 80% in the wavelength range from 400 nm to 2500 nm, while for films processed by spray pyrolysis mobilities exceeding 200 cm2V-1s-1 will be reported targeting their use in a wide range of optoelectronic devices with relevancy for solar cells.
11:45 AM - MM1.6
Characterization of the Surface Composition and Resultant Electrical and Electrochemical Properties of Ga-doped ZnO Electrodes: Effect of Surface Pretreatment Conditions.
Erin Ratcliff 1 , Ajaya Sigdel 2 , Ken Nebesny 1 , Paul Lee 1 , Dana Olson 2 , David Ginley 2 , Neal Armstrong 1 , Joe Berry 2
1 Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona, United States, 2 National Center for Photovoltaics, National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractGallium-doped zinc oxide (GZO) electrodes show promise as replacement transparent conductive oxides (TCOs) for indium tin oxide (ITO) electrodes because of their enhanced earth abundance, increased electrical conductivity, good thermal stability, and the possibility that MZO electrodes will be more electrically homogeneous than we have observed for ITO. In order to fully understand the role of these electrodes in organic photovoltaic applications, it is necessary to fully understand near-surface composition correlated with the electrical and electrochemical properties of GZO electrodes after various pretreatment conditions. GZO films with Zn:Ga atomic ratios of 95:5 were prepared by magnetron RF and DC sputtering. Surface characterization of the GZO films was evaluated using monochromatic X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) following surface pretreatments: RF air- or oxygen-plasma etching, high-vacuum argon-ion sputtering, and solution etching using concentrated HI or KOH. The O 1s spectra of the oxides have contributions from both the stoichiometric oxides and higher binding energy hydroxylated forms of the oxide, predominantly associated with the near-surface zinc sites. Solution etching procedures increase the work function and ionization potential of the GZO electrode by ~0.2-0.3 eV, but result in quite different near surface composition. The acid etch treatment changes the Zn:Ga ratio when bulk versus surface atomic ratios are compared, but does not appear to impact the net carbonaceous impurity surface coverage. The KOH etch shows a significant decrease in carbonaceous impurities with little change to the surface Ga:Zn ratio. Plasma treatments increase the work function and ionization potential of the GZO ~0.8 eV, accompanied by a decrease in binding energy and more pronounced structure of the valence Zn 3d and O 2p peaks, as confirmed by both He(I) and He(II) UPS. Elemental peak fitting was used to evaluate the ratio of stoichiometric oxides to hydroxides on the surface, which was found to vary depending on the pretreatment conditions, with monolayer coverages of hydroxides for the ZnO control under high-vacuum Ar-ion sputtered conditions. XPS results are compared with standard ZnO and Ga2O3 films and demonstrate that the solution and vacuum pretreatments change both the relative surface coverage of hydroxides and carbonaceous impurities. The near surface chemical composition has been shown to influence the rate of electron transfer to probe molecules in solution and rates of charge transfer across the oxide/organic interface in OPVs. The XPS and UPS results for the GZO electrodes are correlated with electron transfer rates of a solution probe molecule ferrocene in an effort to develop a surface model of the chemical composition and electronically active surface sites on the electrode surface for OPV applications.
12:00 PM - MM1.7
Transparent Schottky Barrier Metal/n-ZnO.
Eliana Kaminska 1 , Anna Piotrowska 1 , Iwona Pasternak 1 , Michal Borysiewicz 1 , Marcin Juchniewicz 1 , Marek Ekielski 1 , Piotr Kazmierczak 1 , Adam Barcz 1 2 , Elzbieta Dynowska 1 2
1 Department of Micro- and Nanotechnology of Wide Bandgap Semiconductors, Institute of Electron Technology, Warsaw Poland, 2 , Institute of Physics, Polish Academy of Sciences, Warsaw Poland
Show AbstractPrevious works on Schottky contacts to n-ZnO reported potential barriers heights in the range 0.6 to 0.8 eV, regardless of the metal used, what suggested that the interface properties are determined by the semiconductor surface rather than the work function of metal. Recent studies performed on ‘nearly ideal’ nominally undoped bulk ZnO substrates have shown that the height of the Schottky barrier depends on the density of oxygen vacancies at the metal/ZnO interface [1]. Consequently, various ZnO surface treatments including wet chemical etching, oxygen plasma treatment and UV ozone cleaning prior to metal deposition [2] as well as the use of oxidised Ag, aimed at restoring the stoichiometry of the superficial region, were reported [3].
In this work an approach was undertaken to solve this problem by using as a Schottky contact thin film Ru-Si-O [4]. Ru-Si-O is a conducting amorphous material of high thermal stability and chemical inertness [5].
ZnO films for this study were grown on (0001)-oriented sapphire by reactive sputtering in Ar+O2 atmosphere at RT, and subsequently annealed at 600oC. The films were (0001) preferentially oriented with surface roughness of ~2 nm. The carrier concentration and the mobility were ~1.0x1016 cm-3 and ~4 cm2/Vs, respectively.
Schottky diodes were fabricated in three steps. 1. Ti/Al ohmic contact rings were formed by photolithography, sputter-deposition, and annealing. 2. Ru-Si-O Schottky contacts were deposited and patterned into circular dots coaxial with ohmic contacts. 3. a thin insulating layer was deposited between the two metallisations to reduce surface conductivity.
The contacts were characterized by I-V and I-V-T measurements. They exhibit good rectifying ratios with Schottky barrier heights exceeding 0.8 eV. The ideality factors were in the range 2-4 indicating transport mechanisms such as thermionic-field emission and recombination. The use of the passivating layer allowed to substantially limit the leakage currents of the diodes. It is worthwhile to note that these results were obtained on a thin-film ZnO which definitely is of poorer quality than the bulk material.
Furthermore, transmission measurements showed that the Ru-Si-O contacts are optically transparent in the spectral range 400-2200 nm. The transmittance of a 50 nm thick layer is 80%.
The research was partially supported by the European Union within European Regional Development Fund, through grant Innovative Economy POIG 01.01.02-00-008/08 NANOBIOM
References:
1. M. W. Allen et al., Mater. Res. Soc. Symp. Proc. 957, K09-03 (2007)
2 K. Ip et al., Appl. Phys. Lett. 84, 5133 (2004)
3. M. W. Allen et al., IEEE Trans. El. Dev. 56, 2160 (2009)
4. S. M. Gasser, et al., J. Appl. Phys. 86, 1974 (1999)
5. E. Kaminska, A. Piotrowska et al., Mater. Res. Soc. Symp. Proc. 831, E3.41.1 (2005)
12:15 PM - MM1.8
Thermophysical and Electrical Properties of ITO, IZO, and AZO Films.
Nobuto Oka 1 , Takashi Yagi 2 , Naoyuki Taketoshi 2 , Tetsuya Baba 2 , Yuzo Shigesato 1
1 , Graduate School of Science and Engineering, Aoyama Gakuin University, Sagamihara, Kanagawa Japan, 2 , National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki Japan
Show AbstractThermal diffusivity of various transparent conductive oxide (TCO) films, such as polycrystalline Sn-doped In2O3 (ITO), amorphous indium zinc oxide (IZO), and Al-doped ZnO (AZO) films with a thickness of 200 nm has been measured using a “rear heating/front detection type” nanosecond thermoreflectance system [1, 2]. The wavelengths of pulse laser used in the thermoreflectance system were 782 nm and 1064 nm. TCO films are transparent at these wavelengths, hence reflective layers, i.e. Mo layers in this paper, were necessary. TCO films sandwiched between Mo films, Mo/(TCO films)/Mo three-layered films, were deposited on fused silica substrate by dc magnetron sputtering using oxide ceramics and Mo metal targets. The layered structure was fabricated without exposure to the atmosphere between each deposition. The interface between the Mo films and the fused silica substrate was irradiated by nanosecond laser pulse. Heat generated by the pump laser pulse diffuses toward the top Mo surface across the three-layered films. A probe laser pulse is used to detect a change in reflectivity at the front side corresponding to the temperature change. The normalized temperature rise, i.e. the thermoreflectance signal, is recorded as a function of the delay time relative to the pump laser pulse. To derive the thermal diffusivity of the TCO films, the thermoreflectance signals were analyzed based upon an analytical solution of the one dimensional heat flow across the three-layered film/substrate system [3]. The resistivity, carrier density and hall mobility of the IZO films ranged from 4.2×10-4 to 22.7 Ω cm, from 2.6×1016 to 4.2×1020 cm-3, from 10 to 51 cm2/Vs, respectively [4]. The heat diffusion time changed significantly with changing the resistivity. The thermal diffusivity of the IZO films was (0.6–1.3) ×10−6 m2/s, depending on the electrical resistivity, which was approximately equivalent as that of amorphous ITO films and smaller than the polycrystalline ITO films [5,6]. Thermal properties of all the TCO films will be discussed based on Wiedemann-Franz law and thermal transportation by phonon. [1] T. Baba, Proceedings of the Tenth International Workshop on Thermal Investigations of ICs and Systems, Sophia Antipolis, France (2004) 241.[2] T. Yagi, K. Tamano, Y. Sato, N. Taketoshi, T. Baba, and Y. Shigesato, J.Vac. Sci. Technol. A 23 (2005) 1180.[3] T. Baba, Jpn. J. Appl. Phys. 48 (2009) 05EB04.[4] T. Ashida, Y. Shigesato, et. al. J. Vac. Sci. Technol. A, Vol. 25, No. 4 (2007) 1178.[5] T. Yagi, Y. Shigesato, et. al. J. Vac. Sci. Technol. A, Vol. 23, No.4 (2005) 1186.[6] T. Ashida, N. Oka, Y. Shigesato, et.al. J. Appl. Phys. 105, (2009) 073709.ACKNOWLEDGMENTSThis work was supported by the New Energy and Industrial Technology Development Organization (NEDO) as a project of “Development of High-efficiency Lighting Based on the Organic Light-emitting Mechanism.”
12:30 PM - MM1.9
Fundamental Studies of UV Laser Modification of Single Crystal ZnO.
Tom Dickinson 1 , Enam Kahn 1 , Steve Langford 1
1 Physics, Washington State University, Pullman, Washington, United States
Show AbstractUV-Laser interactions with wide bandgap insulators and semiconductors has generated a number of examples of point defect production, surface and bulk modification, etching and re-deposition processes, as well as numerous PLD related applications involving the emitted particles. Here we examine the fundamental processes involving UV laser radiation interacting with a high potential transparent semiconductor: ZnO. We examine the fundamental mechanisms involving atomic level modifications in oriented single crystals. We discuss results on interactions of strongly absorbing 248 nm (5 eV), 193 nm (6.3 eV), and 157 nm (7.8 eV) excimer laser light with high purity ZnO surfaces in UHV. Using time resolved quadrupole mass spectroscopy, we show examine atomic and molecular emissions (Zn, O, and O2) generated at fluences below plasma formation threshold. Although the atomic Zn emission is robust, more total oxygen is observed to leave the surface. We have found a photochemical route via a single photon excitation that can lead to bond breaking and the formation of interstitial-vacancy pairs. Position annihilation experiments confirm the rates of Zn vacancy formation. Accompanying exposure of these single crystals to 193 nm light is coloration: i.e. gray to black spots (some preliminary evidence is showing detectable but less coloration at 248 nm); we show conclusively that this coloration is due to surface metallic zinc in the form of nanoparticles, typically 10-20 nm in diameter. We also examine the role of interstitial-vacancy pair formation on the electrical properties of ZnO—dramatic changes in conductivity and free carrier density as well as carrier mobility will be presented.
MM2: Transparent Conducting Oxides and Applications
Session Chairs
Monday PM, November 29, 2010
Constitution B (Sheraton)
2:30 PM - MM2.1
Wafer-scale High-throughput Ordered Growth of Vertically Aligned ZnO Nanowire Arrays.
Yaguang Wei 1 , Wenzhuo Wu 1 , Rui Guo 2 , Dajun Yuan 2 , Suman Das 2 , Zhong Lin Wang 1
1 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractAssembly and integration of highly-ordered nanowire (NW) arrays at large scales is essential for multi-functional devices and systems. Significant efforts have been made to assemble large quantity NWs through parallel processes. ZnO nanowire (NW) is one of the most important one-dimensional nanostructured building blocks for energy harvesting, sensing, optoelectronic and electronic applications. To achieve controllable growth of highly-ordered and aligned ZnO NW arrays with high-throughput and low-cost at large scale, it is necessary to adopt feasible combinations of patterning techniques and ZnO NWs synthesis methods. We present an effective approach for patterned growth of vertically aligned ZnO nanowire arrays by combining laser interference lithography (LIL), which is a large-scale, fast, maskless, and noncontact nanopatterning technique, and hydrothermal (HT) growth of ZnO NWs either homoepitaxially on textured ZnO layer, which is also synthesized wet-chemically, or heteroepitaxially on GaN film. The substrates can be patterned with periodic ordered, variable spacing and symmetry over large areas of up to a 2-inch wafer by LIL. Perfectly aligned vertical ZnO NW arrays were then grown at predefined positions via a low temperature HT method without using catalyst but with control over orientation, dimension and location. As-synthesized NWs are highly uniform in length and diameter with perfect alignment, and are single crystals with growth direction along [0001]. The development of textured ZnO seed layer for replacing single crystalline GaN and ZnO substrates extends the large-scale fabrication of vertically aligned ZnO NW arrays on substrates of other materials such as polymers, Si and glass. This combined approach demonstrates a novel method of manufacturing large-scale patterned one-dimensional nanostructures on various substrates for applications in energy harvesting, sensing, optoelectronics and electronic devices.[1] Yaguang Wei, Wenzhuo Wu, Rui Guo, Dajun Yuan, Suman Das and Zhong Lin Wang, submitted, 2010[2] For more information: http://www.nanoscience.gatech.edu/zlwang/
2:45 PM - MM2.2
Synthesis, Defect Chemistry and FET Properties of Crystalline and Amorphous Oxides.
Joerg Schneider 1
1 Fachbereich Chemie, Technische Universität Darmstadt, Darmstadt Germany
Show AbstractThe electronic properties of metal oxides can be tuned from an insulating ceramic over semiconducting to complete metallic behavior. In this respect, it is of utmost importance to understand the defect chemistry of solution processed size-selective nanoscale ZnO nanoparticles in more detail, because thin films of this material have unique performance properties in materials printing technology and a high application potential as active semiconductor in thin-film transistors (TFTs). In this contribution we would like to report on our studies towards formation of zinc oxide in crystalline and amorphous morphology synthesized either by microwave heating, by conventional thermal and a combination of thermal and UV processing. Based on electron paramagnetic resonance (EPR) studies, a particle model will be derived allowing to dwetermine a finite surface shell thickness. Field-effect transistor (FET) device properties of different crystalline and amorphous ZnO based oxide materials will be reported.
3:00 PM - MM2.3
ZnO/p-Si Heterojunctions with Arsenic Doped ZnO Films.
Oscar Garcia Serrano 1 , Marco Vazquez-Agustin 1 , Ramon Pena-Sierra 1 , Gabriel Paredes-Rubio 1 , Jose Andraca-Adame 1 , A. Esparza-Garcia 2
1 SEES, CINVESTAV-IPN, México D.F. Mexico, 2 , UNAM, México D.F. Mexico
Show AbstractZnO(n)/p-Si(100) hetero-junctions were growing by DC-sputtering. ZnO films as-grown presented high resistivity and showed n-type behavior. ZnO films were doped by arsenic diffusion in a quartz reactor at one atmosphere of pressure where Arsenic gas was obtained by heating solid arsenic at 550 C. The conductivity of the ZnO films was increased by diffusing arsenic at 700 C. After the arsenic diffusion the resistivity of the ZnO was reduced to 0.17 Ω-cm, with 3.81×1017 cm−3 and 95 cm2/V-s carrier concentrations and electron mobility, respectively. Current-voltage (I-V) characteristics of the p-n structures were measured using a Keithley System Model 90. The structural characteristics of the ZnO films and structures were measured by X-ray diffraction (XRD). The zinc arsenite phase was detected on the diffused samples.
3:15 PM - **MM2.4
High-Performance Oxide Semiconductors.
Thomas Mason 1
1 , Northwestern University, Evanston, Illinois, United States
Show AbstractHighly conductive ceramics (e.g., superconductors, semi-metallic oxides, ionic conductors) are well known, as are highly resistive ceramics (e.g., dielectrics, insulators, ferroelectrics). Since the advent of oxide-based chemical sensors (e.g., SnO2-based) and voltage-dependent resistors or “varistors” (e.g., ZnO-based) circa 1970, there has been a steady rise of interest in oxide semiconductors. The renaissance of oxide semiconductors over the past two decades has been particularly dramatic. For example, publications dealing with ZnO have doubled each half-decade since 1990 to more than 26,000 papers (2006-2010). This talk will focus on “medium band gap” (~3 eV) post-transition metal oxides, the basis set of which include CdO, ZnO, In2O3, and SnO2. (Ga2O3 is also of interest, although its band gap is significantly larger.) These compounds and their numerous binary, ternary and multinary compounds and solid solutions are known for their rare combination of high electronic conductivity (when degenerately doped) and optical transparency, and are collectively referred to as transparent conducting oxides or TCOs. TCOs find application as transparent electrodes in display technologies and photovoltaics. When non-degenerately doped, many of the same compounds/solid solutions can serve as thermoelectric oxides or TEOs for direct conversion of heat (solar, commercial, vehicular) to electricity. When very lightly doped, these same materials are excellent “transparent oxide semiconductor” (TOS) candidates for channel materials in oxide-based transparent thin film transistors (TTFTs), especially in the amorphous state (so-called “amorphous oxide semiconductors”). These can be deposited at low temperatures on flexible (polymer) substrates, thereby enabling oxide-based “transparent” and “flexible electronics.” This talk “dusts off” two long-standing (but under-utilized) semiconductor analysis procedures—so-called “Jonker” and “Ioffe” analyses—and applies them to the characterization/optimization of high-performance oxides for advanced applications in display, information technology, and energy conversion technologies.
3:45 PM - MM2.5
Thermoelectric Materials Discovery Using Combinatorial Chemistry.
Matin Amani 1 , Ian Tougas 1 , Otto Gregory 1
1 Chemical Engineering, University of Rhode Island, Kingston, Rhode Island, United States
Show AbstractCommercially available transparent conducting oxides have been previously investigated for both bulk and thin film thermoelectric applications, and have shown promising results due to their thermal stability and electrical conductivity. Alloys of two or more transparent conducting oxides have been deposited using PLD and combinatorial sputtering, and the resulting films were optimized for optical applications. Using co-sputtering techniques, a chemical gradient in the systems indium tin oxide (ITO), indium zinc oxide (IZO), and zinc tin oxide (ZTO) were formed on alumina substrates that were patterned with hundreds of micro-thermocouples. Combinatorial libraries were rapidly screened to establish room temperature resistivity, Seebeck coefficient, and power factor as functions of both composition and heat treatment in nitrogen and air ambients. Because of their chemical stability, oxidation resistance, and large Seebeck coefficients relative to metal thermocouples these materials are ideal for temperature measurement in harsh environments such as gas turbine engines.
4:30 PM - MM2.6
Engineering of the Electronic and Electrical Properties of Amorphous Indium Zinc Oxide Sputtered Thin Films.
Mohammad Ebdah 1 , Martin Kordesch 1 , Wayne Jennings 2
1 Department of Physics and Astronomy, Ohio University, Athens, Ohio, United States, 2 Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, Ohio, United States
Show AbstractAmorphous transparent conductive oxides (a-TCOs) have drawn much attention and great consideration due to their flexibility, electronic and transport properties. The incorporation of a-TCOs in nano-scale devices and industrial applications such as thin-film transistors (TFTs), optoelectronic devices and other multidisciplinary applications represents a scientific nano-technological revolution in the 21st century. Out of a variety of a-TCOs, we engineer the electronic and electrical properties of amorphous semiconductor of indium zinc oxide (IZO) sputtered thin films. IZO films of 50 nm thickness have been reactively sputtered onto fused silica substrates in an atmosphere of oxygen and argon with manipulating the composition at.% of the films and the growth conditions ”semiconductors by design”. X-ray diffraction (XRD) and transmission electron microscope (TEM) imaging reveal that the grown films have an amorphous structure. The composition and electronic properties were studied by depth profile X-ray photoelectron spectroscopy (XPS) and ultra-violet photoelectron spectroscopy (UPS, He I radiation, 21.22 eV). We probe the valence band density of states (VBDOS), characterize the work function of the alloy, and set up the band diagram of the valence band maximum (VBM) and the conduction band minimum (CBM) for each composition. Our systematic approach is to study the influence of the composition at.% on the electronic structure. All measurements in this study are done over the In% range of (0 - 100)%. In addition, a complete investigation of the electrical properties is made using Hall Effect (HE) measurements at room temperature for characterizing the electrical conductivity, mobility, and both the type and size of free carrier concentrations. The electrical results are related to the electronic findings, and to the optical bandgap (Eg) measured by spectrophotometry (SP). Finally, the correlation between Eg and carrier concentration is analyzed in the frame work of Burstein-Moss effect.
4:45 PM - MM2.7
Data Mining-aided Crystal Engineering for the Design of Transparent Conducting Oxides.
Changwon Suh 1 , Kwiseon Kim 1 , Jinsuk Lee 1 , Joseph Berry 1 , Wesley Jones 1
1 , NREL, Golden, Colorado, United States
Show AbstractWith the advancement of crystal chemistry and theoretical calculations, the ability to understand structural factors governing transparent conductivity has critical responsibilities in better designing transparent conducting oxides (TCO). An example of the efforts for designing new TCO includes identifying the role of the cations, especially for basic continuous edge sharing octahedra suggested by Shannon et al in the 1970’s, to the phase stability, chemical bonding, and transport properties. Nevertheless, it is a hard task to elucidate interrelationships between structural factors and electrical/optical properties due to the lack of data analysis-oriented tools to systematically explore their inherent inter-complexities. In our approach, we revisit and enhance classical bivariate structure field maps from the work of Marquardt et al. for the delafossite ABO2 (ex. A=Cu, Ag, Pd, Pt; B=Co, Cr, Fe, Ga, In) structure. We utilize data mining tools as efficient exploring tools for the TCO data generated from theoretical calculations as well as experiments. We demonstrate how a dimension reduction technique defines and visualizes the design space by making various multivariate structure field maps of structure factors in terms of A-and B-site cations, transport properties, or structure-property to correlate multiple parameters of crystal structure levels with properties of TCOs and to identify key features (i.e. evaluation of the relative impact of structural parameters on the transport properties) that are favorable for transparent conductivity.
5:00 PM - MM2.8
Carrier Scattering Mechanisms and Mobility Limit in Epitaxial ZnO Films Heavily Doped with Ga.
Vitaliy Avrutin 1 , Huiyong Liu 1 , Natalia Izyumskaya 1 , Michael Reshchikov 2 , Stephen Wolgast 3 , Cagliyan Kurdak 3 , Andrew Yankovich 4 , Alexander Kvit 4 , Paul Voyles 4 , Umit Ozgur 1 , Hadis Morkoc 1 2
1 Electrical and Computer Engineering, Virginia Commonwealth University, Richmond, Virginia, United States, 2 Physics, Virginia Commonwealth University, Richmond, Virginia, United States, 3 Physics, University of Michigan, Ann Arbor, Michigan, United States, 4 Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States
Show AbstractZnO films heavily doped with Ga (GZO) and Al (AZO) have attracted considerable attention as the transparent conducting oxide (TCO) material for electrode applications in solar cells, flat-panel LCD displays, and GaN-based light emitting diodes (LEDs and vertical-cavity surface-emitting-lasers (VCSELs). To have highly transparent and at the same time highly conductive material, achieving high mobility of charge carriers is imperative. Moreover, solar-cell applications require transparency window as wide as possible. Therefore, light absorption on free charge carriers in the near-infrared range is unwanted, and it is more beneficial to achieve as high as possible mobility rather than carrier concentration. In this vein, understanding of carrier scattering mechanisms governing carrier transport and mobility limit in the TCO materials as well as their relationship with their structural and morphological properties is crucial for the proper choice of fabrication techniques and growth conditions.In this work, epitaxial ZnO layers heavily doped with Ga were grown by plasma-enhanced molecular-beam epitaxy (MBE) on a-plane sapphire and GaN(0001)/sapphire(0001) templates. Advanced control over the growth conditions and in situ monitoring capabilities of MBE made it possible to prepare a variety of samples with doping levels varying in a wide range as well as different surface morphologies and structural quality. Crystal structure of the samples was studied by transmission electron microscopy and high-resolution x-ray diffraction. Temperature- and magnetic-field-dependent Hall effect measurements were used to study carrier transport and establish correlations between sample structures and dominating carrier scattering mechanisms. Optical properties were investigated by photoluminescence and transmittance measurements. Generally, for the both types of substrates, growth under metal-rich conditions yielded two-dimensional growth mode and better structural and electronic properties. However, the observed temperature dependences of carrier concentration and electron mobility were found to be different, and layers grown on the GaN(0001)/sapphire(0001) templates showed much higher mobilities than the GZO films on a-plane sapphire. The room-temperature Hall mobility as high as 90 cm^2/V-s was achieved for a GZO layer with resistivity of about 1.4×10^-4 ohm-cm grown on GaN(0001)/sapphire(0001). To the best of our knowledge, this value exceeds mobilities reported for ZnO-based TCOs.
5:15 PM - MM2.9
Solid Phase Crystallization of ZnO Films via Nitrogen-atom Mediation.
Naho Itagaki 1 , Kazunari Kuwahara 1
1 Graduate School of Information Science and Electrical Engineering, Kyushu University, Fukuoka Japan
Show AbstractOxide semiconductors have attracted a great deal of attention not only for transparency but also for wide-ranging conductivity, which enable us to make high-performance electronic devices [1,2]. ZnO is one of the most fascinating oxide semiconductors with unique features and a wide application range. Especially, the large exciton binding energy and the material abundance make ZnO a very promising candidate such as light-emitting diode (LED) and laser diode (LD). For realization of ZnO based optical devices, high-quality epitaxial films are crucial to reduce the density of defect and dislocation. However, because the high energy of impingement of sputtered species may lead to random nucleation, it is difficult to fabricate highly orientated ZnO films by sputtering method, which is widely used for commercial production of transparent conductive oxide. Recently, we have developed a novel method of fabricating ZnO films via solid-phase crystallization (SPC) from amorphous phase films. The resultant ZnO films are well aligned out-of-plane even on quartz glass substrates and have higher crystallinity than the films prepared by the conventional sputtering method. In this report, we apply the SPC method mentioned above to the epitaxial growth of ZnO films. The effects of SPC on the crystallinity of ZnO are described with comparison to those of ZnO fabricated by a conventional method.As the amorphous phase films, we employ a-ZnON because we have difficulty in synthesizing amorphous ZnO films by sputtering. ZnON films were deposited on GaN/Al2O3 templates without intentional heating of the substrates. Ar and N2 mixing gasses with N2/(Ar+N2) flow ratio of 0.78 were used and the total deposition pressures were 0.47 – 1.33 Pa. After deposition, ZnON films were annealed at temperatures between 500 and 800 degrees C for 60 minutes in air to be converted to ZnO films. The structures of the films were examined by X-ray diffraction.We have succeeded in SPC of ZnO prepared on GaN/Al2O3 templates by annealing a-ZnON at the temperature above 600 degrees C. The resultant ZnO films are highly orientated to (001) and the rocking curves width is smaller than the films prepared by the conventional sputtering method. Furthermore, we observed that ZnO films are in-plane aligned with the substrate and the solid phase epitaxy is achieved. All the results show that SPC proposed here is a promising method for synthesizing epitaxial ZnO films by sputtering deposition.[1] N.Itagaki et al, phys. stat. sol. (a) 205 (2008) p. 1915.[2] N.Itagaki et al, International session in 19th Symposium of The Materials Research Society of Japan, I-06-G (2009).
5:30 PM - MM2.10
Deposition of Zinc Oxide Thin Films Using a Catalytic Reaction on Platinum Nanoparticles.
Hitoshi Miura 1 , Hiroshi Nishiyama 2 , Kanji Yasui 1
1 Electrical, Electronic and Information, Nagaoka University of Technology, Nagaoka Japan, 2 Material Science and Technology, Nagaoka University of Technology, Nagaoka Japan
Show AbstractZinc oxide (ZnO) is useful for many applications, and a wide range of growth techniques, including molecular beam epitaxy (MBE), pulsed laser deposition (PLD), and metal-organic chemical vapor deposition (MOCVD), have been used to prepare ZnO films. Despite the advantages of MOCVD in industry, ZnO deposition by conventional MOCVD consumes a lot of electric power to react the source gases and raise the substrate temperature. To overcome this, a more efficient means of reacting oxygen and metalorganic source gases is needed. In addition to the low reaction efficiency, conventional CVD methods yield low-quality ZnO films compared to those prepared by MBE and PLD, due to incomplete reaction of metalorganic source gases with oxygen source gases in the gas phase. However, if thermally excited water is used to hydrolyze the metalorganic source gases, reactive ZnO precursors are produced in the gas phase, allowing growth of ZnO epilayers in a manner similar to PLD and MBE. Here, we present a new epitaxial growth method for ZnO using the reaction between dimethylzinc (DMZn) and high-energy H2O produced by a Pt-catalyzed H2−O2 reaction. H2 and O2 gases were admitted by a pulsed valve into a catalyst cell containing a Pt-dispersed ZrO2 catalyst, whose temperature increased rapidly to over 1300 K due to the exothermic reaction of H2 and O2 on the catalyst. The resulting high-energy H2O molecules were ejected from a fine nozzle into the reaction zone and allowed to collide with DMZn ejected from another fine nozzle. ZnO epitaxial films were grown directly on a-plane (11-20) sapphire substrates at substrate temperatures of 773-873 K with no buffer layer. Growth rates were 0.02-0.08 μm min-1, and film thicknesses were 2-3 μm. X-ray diffraction patterns exhibited intense (0002) and (0004) peaks from the ZnO(0001) index plane. The smallest full width at half maximum (FWHM) value of the ω-rocking curve of ZnO(0002) was less than 0.1deg (203 arcsec). The Hall mobility and residual carrier concentration of the epilayers were in the ranges 140-170 cm2V−1s−1 and 1.7-6×1017 cm−3, respectively. This Hall mobility is very large compared with ZnO films grown directly on sapphire by other deposition methods. PL spectra at room temperature exhibited a sharp peak at 3.29 eV, with a FWHM of 109 meV, assigned to band edge emission. Green luminescence from deeper levels was as low as 0.2% of the band edge emission intensity. PL spectra at 16 K showed a strong emission peak at 3.357 eV, attributed to the neutral donor-bound exciton Dox. The FWHM was as low as 3.6 meV, which is smaller than that previously reported for ZnO obtained by MOCVD (reaction of Zn(C2H5)2 with N2O). It is also smaller than the values for ZnO grown by MBE (5.5 meV), and by PLD on a sapphire(0001) substrate (5.1 meV) without a buffer layer. Free exciton emission also appeared at 3.373 eV. Longitudinal optical phonon replicas of the Dox emission were clearly observed from the first to the fourth emissions
MM3: Poster Session: Transparent Conducting Oxides and Applications
Session Chairs
Tuesday AM, November 30, 2010
Exhibition Hall D (Hynes)
9:00 PM - MM3.1
Visible ITO Pattern in Capacitive Touch Lens: Cause and Countermeasure.
Duk Su Kim 1 , Gyeong-Geun Lee 2 , Jacob Yang 1 , Dong Hyuk Shin 1
1 , ELK Corporation, Daejeon Korea (the Republic of), 2 Nuclear Materials Research Division, Korea Atomic Energy Research Institute, Daejeon Korea (the Republic of)
Show AbstractRecent development around touch panel/lens industry has been overwhelming for the last 2 years. Now, most of major mobile phone companies launched touch panel-installed handset to the market, and this phenomenon was started from Apple’s iPhone. There were many ways to achieve touch functionality. Among these, the most promising and dominant one is projected capacitive type. There are many people who do not agree on this comment, but Apple’s iPhone and upcoming feature products are based on this technology. Projected capacitive touch panels/lenses can be made of film-based or glass-based. At ELK Corporation, the former solution was adopted for sensor making. Capacitive touch lens based on ITO film was made of more than 2 layers of ITO films in most of cases. To improve pattern invisibility, which is obviously not preferred by customer, many ITO film vendors implemented index matched ITO film. This specially developed ITO film is layered with optical coating to compensate the difference in refractive indices of ITO and PET (base film). However, certain numbers of ITO film-based capacitive touch lenses were failed due to visible ITO pattern. We approached various aspects around the fabrication processes of film-based touch lens. Methodology based on cause-and-effect diagram was adopted to locate the most influential factor in this type of failure. In this paper, the potential root causes of visible ITO pattern will be presented, and the way to ruling out the least influential aspect will be provided.
9:00 PM - MM3.10
Fabrication of P-Type Li-doped ZnO Films by Pulsed DC Sputtering.
Yi-Wen Kao 1 , Kuo-Chuang Chiu 1
1 Materials Research Laboratories, Industrial Technology Research Institute, Hsinchu Taiwan
Show AbstractThe p-type Li-doped ZnO films are successfully deposited on a glass substrate by pulsed DC sputtering. The pure, crystalline sputtering target of LixZn1-xO1-x/2 with X in the range of 0.001~0.005 is fabricated using the powder synthesized by the thermal decomposition of a metal-nitrate-tartrate gel complex at 600 oC. The complex is prepared by dissolving nitrates of zinc and lithium in an aqueous solution with nitric and tartaric acids. The resulting compacts, which can be densified at 1400 oC, exhibit a decreasing trend of electrical resistivity with increasing Li content. In our experiment, the high mobility and low resistivity film were obtained for the LiZO target containing 0.2 mol% of lithium. The Li-doped ZnO film prepared at 50 oC possessed the resistivity of 2.04×101Ω-cm with a Hall mobility of 2.68 cm2/V-s and hole concentration of 1.14*1017 cm-3.
9:00 PM - MM3.11
Improving Electrochromic Properties of WO3 Thin Solid Films Deposited by Chemical Spray by Mo Doping.
Dwight Acosta 1
1 Instituto de Fisica, Universidad Nacional Autonoma de Mexico, Mexico City, Mexico D.F., Mexico
Show AbstractDwight R. Acosta and Jesús M. Ortega Universidad Nacional Autónoma de México, Instituto de FisicaA.P. 20-3064, 01000 México
[email protected] effects of molybdenum (Mo) doping on the electrochromic behavior of chemical sprayed tungsten trioxide (WO3) thin films have been studied. It has been observed that the color bleaching kinetics, coloration efficiency, and stability of electrochromic WO3 films are closely related to molybdenum doping concentration, apart from variations on microstructure and crystallinity for each specific deposition conditions.To enhance the electrochromic efficiency through transitions between two kinds of metal sites, efforts have also been made to fabricated thin films of selective mixed oxides semiconductors, based on their EC performance. The electrochromic efficiency of WO3 thin films can also be modified by doping suitable metal ions with higher electro negativity or lower oxidizing capability than W ions. Doping of such material in electrochromic host lattice is expected to benefit the coloration efficiency and durability of the host, extending the switching potential range, and enhancing the reaction kinetics.The chemical spray technique technique was utilized to fabricate the doped and undoped WO3 thin films. The WO3 films were deposited on FTO/glass at different substrates temperatures. The final pH of the precursor solutions remained in between 4.0 and 5.0.The samples were characterized using X-rays diffraction and morphological variations in doped WO3 films as the Mo content was rised in the starting solution was followed using scanning electron and atomic forcé microscopy respectively. High resolution electron microscopy micrographs were used to study at nanometer scale the structure of our films together with expected structural modifications as substrate temperatura and Mo concentration were rised. The electrochomic behavior in our films, was induced and studied using cyclic voltammetry techniques. On increasing the Mo concentration in the films, while the cathodic/insertion peak moves to higher potential, the anodic/extraction peak moves to a lower potential, which indicates that the changes from colored to bleach state and viceversa are faster for the WO3 films doped with Mo. The bleaching reversibility of all the films was good up to 1000 cycles of the cyclic voltammograms.However, the electrochemical stability of the films increased substantially (up to 2000 cycles) on incorporating 2 atom % of Mo in them. Incorporation of Mo in WO3 thin films modifies their crystallinity, crystallite size, and optical band gap. Electrochomic performance of the spray pyrolised WO3 thin films improves significantly on Mo doping. While the best electrochromic response of WO3 thin films could be obtained through Mo doping of about 6.0 atom %, stability of the films is best when the nominal dopant concentration is about 2.0 atom %.
9:00 PM - MM3.12
Pressure Induced Structural Transition of ZnRh2O4.
Huiyang Gou 1 2 , Fuxiang Zhang 3 , Gongkai Wang 1 4 , Xiang Sun 1 , Fengyuan Lu 1 , Hui Niu 1 , Rodney Ewing 3 , Jie Lian 1
1 Department of Mechanical, Aerospace, and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Key Laboratory of Metastable Materials Science and Technology, , College of Material Science and Engineering, Yanshan University, Qinhuangdao, Hebei, China, 3 Departments of Geological Sciences and Materials Science & Engineering, University of Michigan, Ann Arbor, Michigan, United States, 4 Key Laboratory for Anisotropy and Texture of Materials, Ministry of Education, Northeastern University, Shenyang, Liaoning, China
Show AbstractZnRh2O4 with cubic structure were studied using both in situ high pressure X-ray diffraction (XRD) and first principle total energy calculations. Pressure-induced phase transition occurs at 28.1 GPa as observed by in situ high pressure XRD. The high-pressure phases of ZnRh2O4 are not quenchable upon pressure release. Based on first principles calculations using CASTEP, the cubic phase is found to transform to an orthorhombic phase at 25.5 GPa. The pressure dependence of total energy, structural parameters and band gap of these phases is calculated and the potential transition mechanism is also proposed. The results of density of states have been analyzed in order to understand the bonding characteristics of ZnRh2O4 phase.
9:00 PM - MM3.13
The Sputtering Deposition of Transparent Conducting ZnO: RuO2 Films with H2 Containing Atmosphere.
Shih-Min Lin 1 , Wen-Chih Chang 1 , Tai-Bor Wu 1
1 Department of Material Science and Engineering, National Tsing-Hua University, , Hsinchu, Taiwan, R. O. C, Hsinchu Taiwan
Show AbstractTCO films are extensively used as transparent electrode in OLED and solar cell. Zinc oxide has potential applications to optoelectronics. Pure ZnO has high resistivity about 102~104 Ω cm, however, ZnO films doped with impurities exhibit low resistivity. RuO2 is a minority of oxide that is conductive. The ZnO: RuO2 films nearly 200 nm thick were deposited on glass substrate in different work temperature by RF magnetron sputtering. A ceramic disc of ZnO mixed with different content of RuO2 was used as target (diameter of two inches). During ZnO: RuO2 deposition, 20% H2 and Ar mixed gas was transported to chamber. Moreover, a series of RZO films were deposition with changing the H2/ (Ar+H2) ratio that were controlled by mass flow controllers. The film thickness was evaluated by morphology profilometer (Alpha step). Using four- point probe and Hall Effect, the electrical conductivity properties were measured. The UV visible spectrometer was employed to measure the optical transmittance in the range from 300 nm to 800 nm. The crystallization of ZnO thin films was determined by x-ray diffraction (SHIMADZU XRD-6000). The composition of ZnO: RuO2 was examined by ESCA PHI-1600 photoemission spectrometer.The results indicate that target doping with 6 wt.% RuO2 powder and H2/ (Ar+H2) ratio of 8% and substrate temperature of 350 °C have the largest influence on reducing resistivity of RZO films. The resistivity is in the order of 10-4 Ω cm and the average transmittance about 90% in the visible region. In addition, the influences of RuO2 doped on bonding conditions have also been studied.
9:00 PM - MM3.16
Fabrication of Indium Tin Oxide Thin Films by Environmentally Friendly Aqueous Sol-gel Route.
Tor Olav Sunde 1 , Helle Ervik Fossheim 1 , Benjamin Otter 1 , Mari-Ann Einarsrud 1 , Tor Grande 1
1 Department of Materials Science and Engineering, Norwegian University of Science and Technology, Trondheim Norway
Show AbstractIndium oxide doped with tin oxide (indium tin oxide, ITO) is one of the most widely used transparent conducting materials. Here we report on a novel aqueous sol-gel route for synthesising ITO nano-crystalline powder and thin films using indium nitrate and tin acetate as precursors. Three different carboxyl acids, acetic, malic and citric, were used as complexing agents, and ethylene glycol was used as polymerization agent. Evaporation of the solvent resulted in an amorphous gel, which decomposed to an amorphous oxide and further crystallized to nano-crystalline ITO at temperatures below 400 °C. X-ray diffraction, IR spectroscopy and electron microscopy were used to characterize the nano-crystalline ITO. The aqueous sol-gel route was further used to prepare thin films of ITO on glass substrates. Optimization of the spin coating and thermal annealing to obtain continuous and crack free films are reported. Finally we report on the optical and electrical properties of the ITO thin films.
9:00 PM - MM3.17
Rapid Deposition of Adherent Oxide Thin Films on Semiconducting and Dielectric Substrates via Microwave Irradiation-assisted Chemical Synthesis.
Sanjaya Brahma 1 , S. Shivashankar 1 2
1 Materials Research Centre, Indian Institue of Science, Bangalore, Karnataka, India, 2 Center of Excellence in Nanoelectronics, 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. 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. Highly oriented layers of ZnO have also been obtained by this method on conducting surfaces, such as ITO-coated glass.
9:00 PM - MM3.18
Ion-beam-sputtering of ZnO by a Modified Radio-frequency Ion Thruster (RIT).
Bruno Meyer 1 , Martin Becker 1 , Martin Fischer 1 , Angelika Polity 1 , Peter Klar 1 , Markus Piechotka 1 , Davar Feili 1
1 1. Physics Institute, Justus-Liebig University, Giessen Germany
Show AbstractRadio-Frequency-Ion-Thrusters (RIT) have proven their exceptional quality as propulsion systems in satellite missions (ARTEMIS). Moreover, due to their design (ion extraction and grid system) and their ability to run with very different inert and reactive gases (Xe, O2, SF6) they are ideally suited for thin film deposition but also for surface cleaning and etching. We will report on the properties of ZnO thin films deposited on glass substrates with a modified RIT-10 (10cm diameter) source using a 3 inch ceramic ZnO target. With a heated substrate holder the deposition temperature can be controlled between RT and 500 0C. With an additional oxygen flow the stoichiometry of the films can be controlled to achieve films with high resistive up to highly n-type conduction. We compare the films properties in terms of grain size, density and morphology with film deposited under similar experimental conditions by RF-Magnetron Sputtering.
9:00 PM - MM3.19
Towards a Better Understanding of the Electrical and Optical Properties of ZnO:Al Films for a Complete Optically Determined Electrical Properties.
Patricia Prod'homme 1 , Enric Garcia-Caurel 2 , Coralie Charpentier 1 2 , Pere Roca i Cabarrocas 2 , Jean-Francois Besnier 1
1 NanoPV, Total, Palaiseau France, 2 LPICM, CNRS-Ecole Polytechnique, Palaiseau France
Show AbstractOne of the main approaches to increase the efficiency of thin film solar cells is to improve the optical confinement and the electrical properties of the Transparent Conductive Oxide layers. We believe that a detailed understanding of the growth process of these films is needed to engineer complex multi-layer structures with this material for light trapping. Consequently, we are currently studying which is the key process parameter for the nucleation and which one for the growth process of ZnO:Al films. In particular we focus on the effect of the working pressure, the partial oxygen pressure and the temperature during RF sputtering process. The morphological, optical and crystallographic properties have been determined by Atomic Force Microscopy, spectrophotometry (in situ and ex situ) and Raman for different thickness of film.Moreover we have also combined optical measurements by spectroscopic ellipsometry in a broad spectral range, from UV-visible to IR, with Hall Effect measurements to determine the electron’s effective mass, the carrier concentration and their mobility. Indeed, several previous works [1, 2] have shown the possibility of correlate the optical and Hall mobilities for lower doping concentrations. By performing spectroscopic ellipsometry studies in a broad IR region, we can achieve a better understanding of the dependence between the optical and electrical properties even for the upper doping concentrations. Another well known aspect is the sensitivity of the intragrain and intergrain of Hall measurement by contrast of the sensitivity of intragrain for ellipsometry measurements. Because the grain boundary scattering depends on the doping concentrations and the defect density at the grain boundaries [3], a better definition of what should be a grain size from a morphological, electronical and optical point of view in correlation with the growth process is necessary.Our studies demonstrate a difference between the electronic grain size, derived from the Seto’s model (~ 300 nm), and the morphological size determined by AFM (between 30 nm and 100 nm). More importantly we find a correlation between the electronic grain size and a characteristic distance in the AFM images which is different from the AFM grain size. [1] I. Volintiru and al., J. Appl. Phys. 103, 033704 (2008)[2] F. Ruske and al., Thin Solid Films 518 (2009) 1289-1293[3] K. Ellmer and al., Thin Solid Films 516 (2008) 4620-4627
9:00 PM - MM3.20
Hydrogen Incorporation Effects into ZnO Thin Films Synthesized by Sol-gel Method.
Takashi Yasuda 1 , Musashi Utsumi 1 , Yuma Suzuki 1
1 , Isinomaki Senshu Univ., Ishinomaki Japan
Show AbstractZnO is a wide band gap semiconductor with potential applications including transparent electronics as well as optoelectronics. This material also has practical benefits that the environmental impact and toxicity are lower than other competitive semiconductors, such as ITO (indium tin oxide) and GaN. While ZnO has many inherent advantages, the lack of control over dopants and defects, in particular, acceptors, presents an obstacle to the realization of practical devices. An enormous amount of experimental effort has been put forward to control the conductivity by doping different elements e.g. Al, Ga, N, As, Li, etc., into ZnO. As-grown ZnO usually exhibits n-type conductivity, which was historically ascribed to native defects. However, recent experiments and theory have shown that O vacancies are deep donors, while Zn interstitials are too mobile to be stable at room temperature [1]. Instead of these native defects, hydrogen has attracted much attention as an n-type impurity in ZnO, because first principles density functional calculations showed that hydrogen is easily incorporated into the ZnO lattice and forms a shallow donor state [2]. In this study, we have investigated hydrogen doping effect into ZnO thin films synthesized by sol-gel method, which is suitable for fabricating large area films at low cost. We have used the sol-gel source synthesized from the mixture of Zn-acetate, 2-methoxyethanol and 2-aminoethanol, which was spread on the quartz substrates by a spin-on coat method. The samples were then dried up in the oven at 600 °C for 10 minutes. This procedure was repeated several times to get appropriate film thickness. Finally the samples were heated at the temperature of 600 °C for 1 hour in air, and then divided into several pieces, which were annealed at temperatures between 100 °C and 450 °C in hydrogen atmosphere for 1 through 5 hours.Resistivity of the samples annealed for 4 hours decreases by increasing annealing temperatures from 100 °C to 400 °C. Resistivity of the as-grown ZnO exhibits approximately 10 Ωcm, while the sample annealed at 400 °C in hydrogen atmosphere showed two orders of magnitude decrease of resistivity. These results suggest that hydrogen can be easily incorporated into ZnO thin films at relatively low temperature, and the incorporated hydrogen acts as a shallow donor. As ZnO films annealed in hydrogen gas over 420 °C were chemically etched and disappeared completely, hydrogen may form structural defects during the 400 °C annealing. Therefore, we must take into account hydrogen complexes as well as interstitial and substitutional hydrogen donors in ZnO. Photoluminescence spectra and SEM photographs will be presented to discuss the hydrogen incorporation effects in ZnO. [1] M. D. McCluskey and S. J. Jokela, J. Appl. Phys.106(2009)071101.[2] C. G. Van de Walle, Phys. Rev. Lett. 85(2000)1012.
9:00 PM - MM3.21
Microstructure Analysis of Co-doped ZnO Films at Different Oxygen Pressure by PLD.
Xueqiong Su 1 , Li Wang 1 , Jiangbo Chen 1 , Xiaojing Wan 1
1 College of Applied Sciences, Beijing University of Technology, Beijing China
Show AbstractCo doped ZnO nanostructured thin films are grown with different oxygen pressure on sapphire at substrate temperature 500 degree by pulsed laser deposition(PLD). The surface topography and inner structural properties of film were characterized with X-ray diffraction (XRD), atomic force microscopy (AFM) and UV-visible absorption spectrum, respectively. The XRD patterns suggest a hexagonal wurtzite structure for the films. The crystallite size, lattice constants and lattice strain in the films are calculated. Co were substituted Zn in the film and retaining original structure lattice. Specially, AFM picture suggests that 5Pa pressure is evener than that of no-doped ZnO films. The optical properties of the sample were tested by UV-visible absorption spectrum. The UV-visible spectra show transparency of thin films has altered greatly when deposited at oxygen pressure or not, and the cause of the band-gap of ZnO crystal lattice in these films are narrowed.
9:00 PM - MM3.22
Photoelectrochemical Properties of Porous Metal Oxide Electrodes Deposited by Laser Ablation.
Rudresh Ghosh 1 , Matthew Brennaman 1 , Thomas Meyer 1 , Rene Lopez 1
1 Energy Frontier Research Center, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States
Show AbstractThe enhancement of performance of metal oxides as photoanodes in dye sensitized solar cells(DSSC) is a constant challenge in the field of renewable energy sources. Obtaining a film-like structure which simultaneously increases the surface area as well improvesthe charge transfer properties being the main goal. In this work we report the fabrication of such a structure.We have used a pulsed laser deposition (PLD) system to ablate different metal oxide targets to obtain porous and vertically aligned structures for enhanced photoelectrochemical performances of these materials. We have investigated the effect of background pressure of oxygen during deposition , film thickness, annealingconditions to optimize light absorption and incident photon to current efficiency (IPCE) of photoanodes.These chemical states, crystallinity, film structure, optical constants of these films have also been characterized using x-ray photon spectroscopy(XPS), x-ray diffraction(XRD), scanning electron microscopy(SEM) and ellipsometric measurements. Absorption studies show that in optimized structures for these metal oxides, obtained using PLD, more than 99% of the light can be absorbed. For titanium oxide films IPCE values comparable to nanoparticle films have already been obtained.
9:00 PM - MM3.23
Colloidal Synthesis of ITO Nanoparticles by Microwave Heating.
Cleocir Dalmaschio 1 , Edney Firminiano 1 , Adenilson Chiquito 2 , Edson Leite 1
1 Chemistry, Federal University of Sao Carlos, São Carlos, São Paulo, Brazil, 2 Physics, Federal University of Sao Carlos, São Carlos, São Paulo, Brazil
Show AbstractIndiums tin oxide (ITO) used as transparent conducting oxide (TCO) have drawn great interest due to their prominent electro-optical behavior. Moreover, microwave irradiation as a heating tool to ITO synthesis opens up new perspectives because microwave-mediated synthesis routes are characterized by short reaction times, often with excellent yields. Therefore a synthesis route carried out through microwave heating was used to obtain ITO nanoparticles. In this synthesis 3 mmol of indium acetylacetonate In(acac)3 and 0.3 mmol of tin(IV) tert-butoxide Sn(OC(CH3)3)4 were dissolved in 30 grams of Polietilenglycol Mw = 1000 and processed in a microwave oven. The resulting material was then separated by centrifugation with the use of organic solvents to destabilize the colloidal dispersion formed, and subsequently dried in a vacuum oven at 70 0C for 6 hours and then characterized. From X-ray diffraction (XRD) the pattern obtained are related to the cubic, single phased and crystalline system of the indium oxide doped with tin. Using Scherrer's model to determine the extent of crystallographic domain from XRD data a value of 9.9 nm was obtained, which is associated to the particle size of the synthesized material. Thermogravimetry (TG) measurements allowed to observe a mass loss of 10.3% at 3500C, which is directly associated with the presence of a high amount of organic material on the surface of ITO nanoparticles. Infrared spectra were obtained in order to confirm the presence of organic material on the surface of the nanoparticles, and the spectra allowed the observation of a shift in a series of characteristic absorption bands for the absorption bands characteristic of pure polyethyleneglycol. This displacement effect of the bands is a function of the electron donor effect from the oxide to the organic molecules attached to the surface of nanoparticles. The oxide was shaped in pellets to electrical characterizations. Measurements of current x voltage indicated ohmic behavior at different temperatures when contacts were investigated. Measurements of resistivity as a function of temperature were performed in the range from 10 to 250 K for a more accurate characterization of electronic transport, indicating a typical metal electronic transport behavior with resistivity of about 10-2 Ω.m. The results of structural and electronic characterization showed that the proposed synthetic route enables the achievement of ITO nanoparticles with surface functionalized with organic molecules. Besides obtaining the desired properties of the oxide, microwave solvothermal synthesis reduces the processing time compared to conventional heating methods.
9:00 PM - MM3.25
High Rate Deposition of ZnO:Al by Filtered Cathodic Arc.
Rueben Mendelsberg 1 , Sunnie H. Lim 1 , Delia Milliron 1 , Andre Anders 1
1 , Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractIndium tin oxide (ITO) has dominated the transparent conducting oxide (TCO) market for decades, providing highly transparent layers with good electrical properties. The strong increase in demand for TCOs in recent years is indicative that ITO needs to be replaced by lower cost materials wherever possible. Al-doped ZnO (AZO) is an abundant, non-toxic, lower cost alternative to ITO and has therefore gained much attention in recent years.No matter what the TCO is made from, it must be deposited at a high rate in order to be commercially viable. Filtered cathodic arc deposition (FCAD) produces high quality AZO, with the best material typically produced in pulsed mode at substrate temperatures of 200-300 C. However, running the arc continuously gives a much higher growth rate with an acceptable reduction in the quality of the deposited film. In pulsed mode, resistivity as low as 1-2 x 10^-4 Ohm cm can be achieved with carrier mobility of 30-45 cm^2/Vs at growth temperatures near 200 C. For continuous arc deposition at the same substrate temperature, resistivity of 6 x 10^-4 Ohm cm with carrier mobilities of 30 cm^2/Vs have been recently obtained without full optimization. In both cases the average visible transmission was greater than 85%. Growth rate in the continuous mode was 150 nm/min, which is 8 times faster than pulsed arc growth. This rate is higher than rates typically obtained by magnetron sputtering, and additionally, film damage by energetic negative ions is avoided.Continuous arc growth also shows a significant advantage over pulsed arc for deposition onto room temperature substrates. Resistivities of 1-2 x 10^-3 Ohm cm can be obtained for continuous arc deposition with growth rates exceeding 250 nm/min. The highly ionized plasma created by the arc can heat the substrate by over 150 C in two minutes, which does not occur for pulsed operation. This process heat is important for producing high quality material, but it must be carefully managed when depositing onto temperature-sensitive substrates.Work supported by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
9:00 PM - MM3.3
Structural Variation in ZnO-LiGaO2 System and Novel Wide Band Gap Oxide Semiconductor, Zn2LiGaO4.
Masao Kita 2 , Takahisa Omata 1 , Kosuke Tachibana 1 , Shinya Otsuka-Yao-Matsuo 1
2 Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, Suita Japan, 1 Department of Mechanical Engineering, Toyama National College of Technology, Toyama Japan
Show AbstractZinc oxide (ZnO) with a wurtzite structure is a well-known wide band gap oxide semiconductor, whose energy band gap is 3.37 eV. Its energy band gap is generally controlled and widened by alloying with MgO. However, the alloying region is limited, because the crystal structure of MgO is rock salt-type and it is quite different from that of ZnO. Recently, we have proposed an alternative system of ZnO-LiGaO2; the terminal β-LiGaO2 possesses wurtzite-derived structure, and the lattice mismatch between β-LiGaO2 and ZnO is smaller than ~4% for both the a- and c-axis in the wurtzite structure. The x(LiGaO2)1/2-(1-x)ZnO alloys fabricated by the simple ceramic process showed significant electrical conductivity and a wider band gap than ZnO, for example, 8.2 Ω-1cm-1 of conductivity and 3.7 eV of energy band gap for the 0.38(LiGaO2)1/2-0.62ZnO. In the present study, we investigated the structural variation in this system in the range of 00.2; it was sharpened and split into three diffractions and many other superlattice diffractions appeared at x=0.5. In the SAED for x=0.5, in addition to the fundamental reflections indexed assuming a hexagonal cell, clear satellite reflections that could not be indexed by the hexagonal cell were observed. This observation indicates that the cations in the alloyed material were ordered in the wurtzite framework; and the phase is a novel compound with a chemical formula of Zn2LiGaO4. The energy band gap of the Zn2LiGaO4 was evaluated to be ~4.0 eV from its optical absorption spectrum.
9:00 PM - MM3.4
Pulsed Laser Deposition of Single-crystalline Nanobricks of SnO2 on Supported Gold Nanoislands.
Samad Bazargan 1 , Nina Heinig 1 , Kam Leung 1
1 WATLab and Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada
Show AbstractControlled growth of one dimensional nanostructures of tin (IV) oxide is of great scientific and technological interest for gas sensing, nano(opto)-electronics, and solar cell applications. Different chemical methods, including spray pyrolysis, chemical vapor deposition, and template-assisted growth, have been used to produce a variety of nanostructures (e.g. nanorods, nanoribbons, and nanobelts). These nanostructures are, however, usually not well oriented on the substrate, with no preferred growth direction. To date, there have been a rather limited number of studies on the use of physical deposition methods to produce one-dimensional oriented nanostructures of SnO2. In the present work, we explore the use of Pulsed Laser Deposition (PLD) as a new method for synthesizing one-dimensional SnO2 nanostructures on different substrates.In order to deposit SnO2 nanostructures using PLD, we first fabricate a seed layer of gold nanoislands, which are used to introduce inhomogeneity in the substrate and, once activated, to serve as a catalyst through a Vapor-Liquid-Solid or similar growth mechanism. The gold islands are produced by sputter-coating thin films of gold on a Si substrate, followed by annealing in oxygen for 1 h. The average size of the gold nanoislands can be easily controlled between 50 and 100 nm by varying the thickness of the gold film and the annealing temperature. Depending on the size of the gold islands, a substrate temperature of 600°C or higher is required to activate the gold. In addition to the substrate temperature, oxygen pressure, and fluence and pulse repetition rate of the excimer laser (248 nm) are also used to control the morphology of the resulting nanostructures. The resulting films are found to consist of single-crystalline rectangular nanostructures, nanobricks, with average dimensions of 80×80×400nm. Electron diffraction results obtained by orientation imaging microscopy and transmission electron microscopy both indicate the highly ordered crystalline structure of these nanobricks. X-ray diffraction, UV-Visible and Raman spectroscopy have also been conducted on these nanostructures to further characterize these nanostructures. Our recent experiments further show that “oriented” SnO2 nanobricks can be grown by more exquisite control of the deposition conditions, including the substrate temperature and gold nanoisland size. These novel one-dimensional SnO2 nanostructures promise potential applications in chemical sensors, and transparent opto and nanoelectronics.
9:00 PM - MM3.5
Characterization of Mesoporous ZnO:SiO2 Films Obtained by the Sol-gel Method.
Rui M. Martins 1 2 3 , Viorica Musat 4 , Arndt Muecklich 5 , Elvira Fortunato 3
1 Unit of Physics and Accelerators, Instituto Tecnológico e Nuclear (ITN), Sacavém Portugal, 2 , Centro de Física Nuclear da Universidade de Lisboa (CFNUL), Lisboa Portugal, 3 CENIMAT/I3N, New University of Lisbon, Monte de Caparica Portugal, 4 Center of Nanostructures and Functional Materials (CNMF), “Dunarea de Jos” University of Galati, Galati Romania, 5 Institute of Ion Beam Physics and Materials Research, Forschungszentrum Dresden-Rossendorf, Dresden Germany
Show AbstractZnO:SiO2 films are intensively investigated for optical and electronic applications. Additionally, porous ZnO:SiO2 films are of great interest as catalyst and gas sensing materials. The sol-gel method is an efficient and low-cost process for the deposition of meso- and microporous silica-based films. The present paper studies the effect of the withdrawal speed on the microstructure and optical properties of mesoporous ZnO:SiO2 films obtained by the sol-gel method. The morphology of the films was investigated by atomic force microscopy and the overall structure was studied by x-ray diffraction. The structure and size of the zinc oxide nanoparticles embedded in the silica matrix was investigated in more detail by transmission electron microscopy. These techniques showed ZnO:SiO2 films with crack-free mesoporous morphology and highly efficient embedding of ZnO nanoparticles with (100) preferred orientation. Furthermore, the optical transmittance (in the visible and near infrared regions) and the optical band gap value were observed to vary with withdrawal speed. It is shown that ZnO:SiO2 nanocomposites films which possess ZnO particles exhibiting a (100) orientation, with possible special applications in non-linear optics, could be prepared by the low-temperature crystallization sol-gel method.
9:00 PM - MM3.6
Transparent Conducting Al-doped ZnO Thin Films Prepared by Laser Induced High Current Pulsed Arc at Low Deposition Temperature.
Jin-Bao Wu 1 , Chao-Ying Chen 1 , Ming-Sheng Leu 1 , Hong-Yih Tseng 2 , Ying-Cherng Lu 2
1 , Materials Research Laboratories, Industrial Technology Research Institute, Hsinchu Taiwan, 2 , BeyondPV Co., Ltd, Tainan Taiwan
Show AbstractHighly transparent conductive Al-doped ZnO (AZO) thin film was deposited at 100 oC by laser induced high current pulsed arc (LIHCPA) from an Al-Zn alloy target (2 and 3 wt.% of Al doping content). A pulsed current more than 2 kA was generated on the Al-Zn target in order to make high ion energy and fully ionized plasma. The film's properties were highly correlated to the growth conditions, including O2 flow rate and Al doping content. Electrical properties and transmittance of the AZO films were investigated by the Hall measurement and UV/VIS spectrometer. The microstructure properties of the films were studied using scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). It was found that the O2 flow rate and Al doping content affected not only the electrical properties of the films but also the band gap. The results clearly showed that when the O2 flow rate increased from 20 sccm to 100 sccm, the resistivity gradually increased from 4.2×10-4 to 1.9 ×10-3 Ω-cm and 5.2×10-4 to 2.3 ×10-3 Ω-cm for the 3 and 2 wt.% of Al-Zn target. Likewise, the band gap of the AZO films calculated by UV/VIS spectrometer measurement decreased from 3.77 eV to 3.58 eV and 3.56 to 3.44 eV as well. The experimental XRD results showed that the AZO films preferred c-axis orientation along the (002) plane and the grain size calculation was in the range of 12.2-15.1 nm. XPS analysis revealed that the Zn and O chemical state can be assigned to the Zn exits in the oxidized state and O occurs in two chemical state (I) O2- ions on wurtzite structure of hexagonal Zn2+ ion array, surrounded by Zn and the (II) chemisorbed oxygen species like O2-, O- and O2- at the grain boundaries, respectively.
9:00 PM - MM3.7
Growth of CuO and CuGaO2 Thin Films by Spin-coating Method.
Masato Sakamoto 1 , Afishah Alias 1 , Katsuhiro Uesugi 1
1 , Muroran Institute of Technology, Muroran Japan
Show AbstractCopper-based oxide materials have been widely studied for p-type semiconductors to realize first response gas sensors and transparent devices. In this paper, we report the preparation and growth of CuO and CuGaO2 thin films on Si(001) substrates using a sol-gel spin-coating method. 2-propanol was used as solvent and copper (II) acetate monohydrate and tris-acetylacetonato gallium (III) were used as solute for CuO and GaO solutions, respectively. As a stabilizing agent, monoethanolamine was added in the source solution. Each solution was separately stirred for 1 h at 50oC. A mixture of them with various Ga/Cu atomic ratios was used as the CuGaO solution. The sol layers were spin-coated on Si substrates at 3000 rpm in 15 s and were pre-baked at 100-300oC under N2 atmosphere for gelation process. The spin-coating and pre-baking processes were repeated to obtain a required thickness, followed with post-baked at temperature ranging 500~1000oC for crystallization. For the CuO growth, flat CuO films were successfully fabricated on the Si substrates at annealing temperatures between 500 and 800oC. X-ray diffraction (XRD) peak corresponding to CuO (111) and (002) planes was observed. However, at higher temperature (>800oC), pyramidal islands and their arranged structures as straight line parallel to the <110> and <010> directions were clearly observed on the surfaces. The width and height of the pyramidal island were 1~2μm and 0.4~0.8μm, respectively. The XRD peak of the CuO films shifted to (220) plane, which suggests that the self-organized CuO island with the growth orientation along (110) plane were fabricated on Si(001) surface.On the other hand, CuGaO2 films using the mixed solution with different Ga/Cu atomic ratios were annealed at 700oC for 1h. With increasing of Ga/Cu atomic ratio in the solution, CuO (111) and (002) peaks disappeared and a strong peak corresponding to CuGaO2 (012) orientation was clearly observed for Ga/Cu atomic ratio of 0.5. As the increase of Ga/Cu atomic ratio further, XRD peak corresponding to Ga2O3 (400) plane was observed. These results indicate that delafossite CuGaO2 were successfully fabricated by the sol-gel spin-coating method using the mixed solution. The growth process and optical property of the films will be discussed at the conference.
9:00 PM - MM3.8
Atomic Layer Deposition of Gallium-doped Zinc Oxide Films for Low Emissivity Glazing Applications.
Paul Chalker 1 , Paul Marshall 1 , Simon Romani 1 , Matthew Rosseinsky 2 , Simon Rushworth 3 , Paul Williams 3 , John Ridealgh 4 , Neil McSporran 4 , John Buckett 4
1 Engineering, University of Liverpool, Liverpool United Kingdom, 2 Chemistry, University of Liverpool, Liverpool United Kingdom, 3 , SAFC Hitech, Bromborough, Wirral, United Kingdom, 4 , Pilkington Technology Management Limited, Lathom, Lancashire, United Kingdom
Show AbstractTransparent conducting oxide (TCO) thin films are being increasingly exploited in a range of technological applications including architectural glazing, photovoltaic solar cells, flat panel displays, light emitting diodes and lasers. Doped zinc oxide based TCO’s are of particular interest and various substituents for zinc, including group-III aluminium and gallium have been investigated. The overwhelming majority of these studies have been made using physical vapour deposition techniques such as sputtering, pulsed laser deposition, molecular beam epitaxy or ion plating. Chemical vapour deposition processes have been demonstrated for TCO’s via metalorganic CVD and more recently atomic layer deposition. In this study, transparent conducting oxide films of gallium-doped zinc oxide have been deposited on zinc stannate coated glass substrates by atomic layer deposition using dimethyl zinc, triethyl gallium and water vapour as precursors. The gallium-doped zinc oxide films were deposited over the temperature range 373 – 623K. The influence of substrate temperature and gallium concentration on the film microstructure have been analysed by X-ray diffraction. The incorporation of gallium is strongly influenced by a ‘site-blocking’ mechanism which can be overcome at lower gallium incorporation levels. Strongly c-axis textured films are obtained at temperatures above 300°C and gallium contents in excess of 5 atomic percent. Transmission electron microscopy reveals that the as-deposited films are polycrystalline in character. The electrical resistivity of the gallium-doped zinc oxide films was evaluated using four-point probe and contactless measurement methods as a function of film thickness. The lowest resistivity of 400 micro-ohms centimetres was measured from a film thickness of 170nm and a gallium content of 5 atomic percent. The electron Hall mobility of this film was 16.2 cm2/Vs. The transparency of the films across the visible spectrum has been investigated. These results will be discussed in the context of exploiting transparent conducting oxide films in low emissivity, energy saving glass applications.The authors gratefully acknowledge the financial support of the UK Technology Strategy Board under contract DTI Project No: TP11/LIB/6/I/AM092J.
9:00 PM - MM3.9
Deposition of Al Doped ZnO Layers with Various Electrical Types by Atomic Layer Deposition.
Cheol Hyoun Ahn 1 , Dong Kyu Seo 1 , Ho Seong Lee 2 , Hyung Koun Cho 1
1 School of Advanced Materials Science & Engineering, Sungkyunkwan University, Suwon, Gyeonggi-do, Korea (the Republic of), 2 School of Materials Science and Engineering, Kyungpook National University, Daegu Korea (the Republic of)
Show AbstractAlZnO ternary oxide films consisting of ZnO and Al2O3 have wide applications in electronic and optoelectronic devices on account of their wide range of electrical conductivity [1]. Conventional Al2O3 is used as an alternative gate dielectric material to SiO2 in Si based electronic devices, such as complementary metal oxide semiconductors. High-k oxide thin films, such as Al2O3, HfO2, and ZrO2, can reduce the leakage current without reducing the oxide layer capacitance and it allows the shrinkage of transistor dimensions [2]. In addition, ZnO thin films are attractive materials in opto-electronic device due to their superior physical and chemical properties. ZnO has a wide bandgap in the ultraviolet range, high resistance to oxidation and a high exciton binding energy [3].Recently, among various transparent conducting semiconducting materials, metal(In, Ga, Al)-doped ZnO films have drawn a great deal of attention due to their low manufacturing cost and non-toxicity, and high stability under hydrogen plasma compared to the Sn-doped indium oxide. In additional, InZnO and GaZnO conducting oxide materials show the metallic and semiconducting properties according to the content of III group elements. These peculiar properties have prompted research into the development of transparent thin film transistors [4]. However, there are few studies on AlZnO films with the main focus on the deposition of transparent electrodes with low Al contents by sputtering. In this study, we investigated the electrical, structural, and optical properties of AlZnO thin films were deposited by ALD at 200 oC with various Al contents.AlZnO thin films were deposited on sapphire and glass substrates using an ALD system with horizontal transverse flow. The AlZnO films of ~ 200 nm thickness with different Al contents (0 ~ 80 at %) were grown at 200 oC. DEZn, TMAl, and high purity H2O vapor were used as the zinc, aluminum, and oxygen precursors, respectively. The Al contents incorporated in the oxide thin films were determined by the number of DEZn and TMAl cycles. The electrical characteristics of the AlZnO thin films were analyzed by Hall effect configuration. The microstructures, and optical properties of the thin films was investigated by field-emission scanning electron microscopy and x-ray diffraction, photoluminescence, and UV/visible/near-IR spectroscopy.References[1] B. Bayraktaroglu, K. Leedy, R. Bedford, Appl. Phys. Lett. 93 (2008) 022104.[2] D.G. Schlom, S. Guha, S. Datta, MRS Bull. 33 (2008) 1017.[3] G. C. Yi, C. Wang, W II Park, Semicond. Sci. Tecjnol. 20 (2005) S22-S34[4] M.P. Taylor, D.W. Readey, M.F.A.M. Van Hest, C.W. Teplin, J.L. Alleman, M.S. Dabney, L.M. Gedvilas, B.M. Keyes, B. To, J.D. Perkins, D.S. Ginley, Adv. Funct. Mater. 18 (2008) 3169.
Symposium Organizers
Joseph J. Berry National Renewable Energy Laboratory
Elvira Fortunato CENIMAT/13N
Yuzo Shigesato Aoyama Gakuin University
Julia Medvedeva Missouri University of Science and Technology
MM4/F4: Joint Session: Vacuum Deposited Metaloxide TFT
Session Chairs
Tuesday AM, November 30, 2010
Constitution B (Sheraton)
9:30 AM - **MM4.1/F4.1
Fully Transparent n and p-type Oxide TFTs.
Elvira Fortunato 1
1 Materials Science, FCT-UNL, Caparica Portugal
Show AbstractTransparent electronics is growing so fast and is today one of the most advanced topics for a wide range of device applications, where the key component are wide band gap semiconductors, and oxides of different origin play an important role, not only as passive component but also as an active component similar to what we observe in conventional semiconductors. In this paper we will review the main achievements related to fully transparent n-type oxide based TFTs as well as to p-type oxide based TFTs produced at CENIMAT/I3N.
10:00 AM - MM4.2/F4.2
P-Type Tin Monoxide Semiconductor Fabricated by Sputtering.
Po-Ching Hsu 1 , Wei-Chung Chen 1 , Tzu-Ming Wang 1 , Chung-Chih Wu 1 , Hsing-Hung Hsieh 2 , Ching-Sang Chuang 2 , Yusin Lin 2
1 Graduate Institute of Electronics Engineering, National Taiwan University, Taipei Taiwan, 2 , AU Optronics Corporation, Hsinchu Taiwan
Show Abstractp-type tin monoxide (SnO) is one of the most promising p-type oxide TFT material [1]. Here, we report the fabrication of SnO p-type oxide semiconductor on glass substrates by the TFT-industry compatible sputtering technique, which is different from the previous reports (e.g. epitaxy on crystalline substrates by pulsed laser deposition etc. [1, 2]). By using targets of appropriate compositions, SnO films on glass substrates were successfully obtained by deposition at room temperature, followed by post annealing. The as-deposited film showed an amorphous phase, while polycrystalline tin monoxide phase was obtained after thermal annealing. By judiciously controlling the working gas during the sputtering, we can not only modulate the Sn/O ratio in SnO films, but also improve the orientations/crystallinity of the SnO phase. We also observed that appropriate combinations of working gases can significantly decrease the post-annealing temperature required for the formation of the SnO phase, making the whole process temperature compatible with the glass substrates. With a <450°C process, the p-type Hall mobility of 0.1-1 cm2V-1s-1 and p-type carrier concentration down to the order of 1017 cm-3 were successfully achieved in the SnO films, which meet the requirements for active layers of thin-film transistors. The realization of p-type SnO films by room-temperature sputtering and low-temperature post annealing demonstrates the possibility of large-area and low-cost p-type oxide TFT fabrication. [1] Y. Ogo, H. Hiramatsu, K. Nomura, H. Yanagi, T. Kamiya, M. Hirano, and H. Hosono, Appl. Phys. Lett. 93, 032113 (2008). [2] Ho-Nyeon Lee, Hyung-Jung Kim, and Chang-Kyo Kim, Jpn. J. Appl. Phys. 49, 020202 (2010).
10:15 AM - **MM4.3/F4.3
The Improvement of Photo-induced Bias Stability of the Oxide TFT.
Chang Jung Kim 1 , Sun Il Kim 1 , Jae Chul Park 1 , Sang Wook Kim 1 , Ihun Song 1 , U-In Chung 1
1 , Samsung Advanced Institute of Technology (SAIT), Yongin-si Gyeonggi-do Korea (the Republic of)
Show AbstractWe successfully fabricated the high stable amorphous hafnium-indium-zinc-oxide (HIZO) thin film transistors (TFTs) with SiOF gate insulator layer and SiOx/SiON passivation layer by systematically investigating the role of various gate insulator layers and passivation layers under negative-bias-temperature illumination-stress (NBTIS) condition. For example, the instability of the TFTs with SiOx passivation layer (threshold voltage shift (ΔVth)~ -6.5V) is less than that of the TFTs with SiONx passivation layer (ΔVth ~ -8.5V). Also, we could get better photo-induced bias stability after back channel treatments. Finally, using the SiOF gate insulator, the back channel treatments, and the SiOx (inner)/SiONx (outer) passivation layers, the instability of the amorphous HIZO TFTs were drastically improved by the suppression of the positive charge trapping sites under the NBTIS conditions. These are very promising results for applications in large area AMLCD and AMOLED display with high intensity light source.
11:15 AM - **MM4.4/F4.4
Improvement of Performances and Stability of a-In-Ga-Zn-O TFT by Low-temperature Annealing.
Kenji Nomura 2 , Hideo Hosono 1 2 , Toshio Kamiya 1
2 Frontier Research Center, Tokyo Institute of Technology, Yokohama Japan, 1 Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama Japan
Show AbstractTransparent amorphous oxide semiconductor represented by a-In-Ga-Zn-O (a-IGZO) is widely accepted as a channel material of driving /switching thin-film transistors (TFTs) for next-generation flat-panel displays such as high-resolution active-matrix (AM) organic light-emitting displays and large-size / high fram rate AM –liquid-crystal displays (LCD) because they have better performances such as large mobilities of ~10 cm2(Vs)-1 and substhreshold slope swing (S) values than a-Si:H TFTs even when they are fabricated at room temperature. To date, many efforts have been devoted to improve TFT characteristics including stability against electrical stress for practical-use. It is known well that post thermal annealing over 300 oC is effective to improve TFT characteristics due to the reduction of subgap defect states in a-IGZO channels. However, it is indispensible to reduce annealing temperature for developing stable and high-performance flexible electronic devices. In this paper, we report improvement of performances and stability of a-IGZO TFTs by low-temperature annealing. Bottom-gate and top-contact a-IGZO TFTs were fabricated on thermally-oxidized SiO2/ c-Si. The a-IGZO channel layers were deposited at room temperature (RT) and subjected subsequently to annealing at 100-400oC in dry O2, wet O2, or ozone atmospheres. Electron-beam evaporated Ti / Au electrodes were used as source / drain contacts. Constant current stress and light illuminated negative bias stress tests were examined to evaluate TFT stability. We found that strong power oxidation atmospheres such as wet O2 are effective to reduce subgap defects state for high-temperature annealing over 300oC. The 400oC-wet annealed TFTs exhibited best performances such as saturated mobility (μsat) ~ 12.4 cm2(Vs)-1 and S ~ 112 mV/dec. However, both the dry and wetO2 annealing caused large negative threshold voltage shifts originating from increase of donor density in the channel for low-temperature annealing at < 200oC. It was found that ozone thermal annealing can improve TFT characteristics even at 150oC, and the 150oC-ozone annealed TFTs exhibited reasonably good performances such as μsat ~ 11.9 cm2(Vs)-1 and S ~ 260 mV/dec. The TFT stability under constant current bias and light illuminated negative bias for TFTs with a wider range of channel quality will also be presented.
11:45 AM - MM4.5/F4.5
High Mobility Amorphous Metal Oxynitride Thin Film Semiconductors.
Yan Ye 1 , Rodney Lim 1 , Anshu Gaur 1 , John White 1
1 AKT, Applied Materials, Inc, Santa Clara, California, United States
Show AbstractAmorphous thin film semiconductors are preferred over polycrystalline films for large-area electronics manufacturing because of better local uniformity and consistency of structure through the thickness of the deposited film. Linked to their bond ionicities, single-metal oxides, such as In2O3, Ga2O3, ZnO and SnO2, tend to form films that are highly crystalline in structure. Combining several of these metal oxides by simultaneous deposition to form a single film such as InGaZnO, creates competition amongst the various oxides and frustrates the formation of any long-range crystalline structures; thereby producing a substantially amorphous structure.A different approach to produce amorphous semiconductor materials has been explored, using a single-metal system. The amorphous phase is achieved by promoting competition between reactions responsible for the growth of compounds from the same metal but different crystalline structures. We used zinc as the metal and both oxygen and nitrogen as reactants in a reactive sputtering process. By adjusting the ratio of the reactants, we are able to make the reaction dominated by either oxygen or nitrogen, or balanced with equivalent reaction rates. As the result, we are able to produce films of crystalline zinc oxide ZnO, nitrogen doped crystalline zinc oxide ZnO:N, amorphous or highly disordered nanocrystalline zinc oxynitride ZnON, or crystalline zinc nitride Zn3N2. Each film has its own chemical and physical properties. We observed that the highest mobility is attained from amorphous zinc oxynitrides. Even though the films produced from the multi-reactant process have a highly disordered structure, they exhibit a higher Hall mobility than the crystalline films produced from preferential growth under the dominance of a single reaction. Moreover, the mobility of the ZnON films deposited at low temperature is higher than that of InGaZnO. Furthermore, the mobility increases as electron carrier concentration decreases over a wide range of the carrier concentration. This characteristic is opposite from the behavior of InGaZnO and indicates that carrier transport in the new semiconductor is dominated by a different mechanism, likely due to different constitutions of conduction band bottom, as will be discussed. Stability of the films and devices made with the new semicoductor has been tested under various stress conditions. Shelf life of an as-deposited film deposited at 50C was about 30 days when it was exposed to room air without any passivation or other protection. Failure is due to the adsorption of moisture and pollutants in the air. An annealed film has been exposed to room air without any protection for more than 600 days without degradation. Thin film transistors (TFTs), active matrix-TFT arrays, and e-ink displays have all been made successfully with the new semiconductor material. Tests of the TFTs under high-temperature and bias-temperature stress have been performed and results will be presented.
12:00 PM - MM4.6/F4.6
High Performance Amorphous-oxide-semiconductor with Indium Tin Zinc Oxide (ITZO) for Thin Film Transistor.
Masashi Kasami 1 , Mami Nishimura 1 , Masayuki Itose 1 , Masahide Matsuura 1 , Shigeo Matsuzaki 1 , Hirokazu Kawashima 1 , Futoshi Utsuno 1 , Koki Yano 1
1 Advanced Research Laboratories, Idemitsu Kosan Co., Ltd., Chiba Japan
Show AbstractTransparent amorphous oxide semiconductors (TAOS) are of growing interest in the context of thin film transistor (TFT) channel layers for transparent electronics and various electronics applications because of their high mobility and large area uniformity. In particular, the most popular TAOS is indium gallium zinc oxide (IGZO), and there have been many studies on practical applications as the backplane of AM-OLED and 2k×4k display panels. Although IGZO was mobility of the order of ~10 cm2/Vs, higher mobility has been needed to fabricate next generation applications such as a super high-vision panel and to integrate driver circuit. Moreover, an etching stopper layer was necessary for using in the conventional pattering process because IGZO was acid soluble. Therefore, the TAOS with insolubility in various acids was needed to be adopted the conventional TFT process. We developed new oxide semiconductor of indium tin zinc oxide (ITZO) with high electrical performances and processability. Since our developed ITZO target has low resistivity, it was possible to be deposited by direct current (DC) sputtering. The obtained film was amorphous and PAN (phosphoric, acetic and nitric acid) insoluble. In this work, we studied electrical and practical properties of ITZO films deposited under various deposition conditions at room temperature.In the as-deposited ITZO films, the carrier density and Hall mobility decreased with increasing oxygen partial pressure. In the post annealed films, however, their carrier density and Hall mobility maintained almost constant values independent of oxygen partial pressure. Hall mobility and carrier density of their post annealed films, which were annealed at 300 °C for 1 hour, were 30 cm2/Vs and ~1018 cm-3, respectively. ITZO film was soluble in oxalic acid, as same as transparent electrodes such as amorphous ITO and IZO films. However it was insoluble in PAN which was used in conventional pattering process of source and drain electrode. Also the etching rate of ITZO film for oxalic acid was about 2~5 nm/sec at 40 °C as same as that of IZO. We fabricated the ITZO channel TFT using photolithography technique. The channel length and width were 20 μm and 10 μm, respectively. The field-effect mobility was 20 cm2/Vs, it was higher than that of IGZO TFT. It was noted that its threshold voltage, S-factor, and On/Off ratio were -5 V, 0.4 V/dec., and 108, respectively.
12:15 PM - MM4.7/F4.7
Transparent MgZnO-based Metal-semiconductor Field Effect Transistors and Devices.
Alexander Lajn 1 , Heiko Frenzel 1 , Tobias Diez 1 , Fabian Kluepfel 1 , Friedrich Schein 1 , Holger von Wenckstern 1 , Marius Grundmann 1
1 Semiconductor physics group, University of Leipzig, Leipzig , Saxony, Germany
Show AbstractTransparent electronics combined with transparent light emitters permit the fabrication of fully transparent displays. New designs, which involve higher information content, better ergonomics, augmented reality applications, lower power consumption and new aesthetic aspects, are feasible; e.g. in car wind shields, windows, sun glasses, monitors or cell phones.
The authors report on the fabrication of transparent rectifying contacts (TRC) on MgZnO thin films and their application in field-effect transistors and inverters. The TRC are fabricated by reactive sputtering of an about 5 nm thick silver oxide or platinum oxide layer and a subsequently deposited metallic conducting capping layer of about 5 nm thickness. An average transmission of 70 % (60%) in the visible spectral range was achieved for the AgxO (PtOy)-based TRC. Using standard photo-lithographic techniques, transparent metal-semiconductor field-effect transistors (TMESFET) utilizing TRC as gate electrodes and ZnO:Al for the source and drain contacts were processed.
These devices reach a channel mobility of 12 cm2/Vs and on/off-ratios of 106 1. These performance values are only slightly lower compared with those of opaque MESFETs2. With that, the devices meet the requirements for the use in transparent displays formulated by Wager3. Furthermore the TMESFETs operate at low voltages; a voltage sweep of only about ΔV = 2.5 V is required to switch between on- and off-state. In addition, with 120 mV/dec, the sub-threshold slope of the TMESFETs is only a factor of two higher than the thermodynamic limit of 60 mV/dec.
The advantages of MESFETs (compared to state of the art transparent MISFETs) were successfully transferred to inverter circuits, yielding a maximum gain of 200 at a supply voltage of only 4 V and a low uncertainty range of 0.3 V. The effect of irradiating the inverter circuits with visible light was studied and no significant influence of red and green light was observed. When irradiated with blue light, a 15% decrease of the gain was observed. Furthermore, the stability of the inverter circuits at temperatures between room temperature and 150°C was investigated. Thermal degradation of the AgxO-gate electrodes started at temperatures of about 90°C4, nevertheless the inverters remained fully operational up to 150°C.
In summary, the authors present a promising approach to transparent electronics based on TRC including fully transparent MESFETs and inverter circuits.
1. Frenzel
et al. , J. Appl. Phys.,
107, 114515 (2010)
2. Frenzel
et al. , Appl. Phys. Lett.
92, 192108 (2008)
3) J. F. Wager, Science,
300, 1245 (2003)
4) Frenzel
et al. , Thin Solid Films,
518, 1119 (2009).
12:30 PM - MM4.8/F4.8
Low-temperature Processing of Metal-semiconductor Field-effect Transistors Based on Amorphous Gallium-indium-zinc-oxide and Indium-zinc-oxide Thin Films.
Michael Lorenz 1 , Alexander Lajn 1 , Heiko Frenzel 1 , Holger von Wenckstern 1 , Marius Grundmann 1 , Pedro Barquinha 2 , Elvira Fortunato 2 , Rodrigo Martins 2
1 , Universität Leipzig - Institut für Experimentelle Physik II, Leipzig Germany, 2 CENIMAT/I3N, Departamento de Ciência dos Materiais, Faculdade de Ciências e Tecnologia, FCT, Universidade Nova de Lisboa and CEMOP-UNINOVA, Caparica Portugal
Show AbstractThe physics and application of amorphous oxide semiconductors (AOS) are an emerging field of interest. Compared to covalent semiconductors like a-Si:H the influence of the disorder on the mobility of the charged carriers is negligible and not limited to values below 1cm
2/Vs[1]. Besides, AOS can be deposited at room temperature and thus electronics on flexible substrates are possible.
The authors demonstrate metal-semiconductor field-effect transistors (MESFET), which exhibit excellent electrical characteristics, e.g. channel mobilities up to 15cm2/Vs and a subthreshold swing of S=69mV/decade. The IZO or GIZO channel material and the AgxO-Schottky gate electrodes were all deposited at room-temperature. The influence of a low temperature annealing step (T=150°C) after device production is furthermore investigated.
IZO and GIZO thin films were grown by radio frequency magnetron sputtering at room temperature on Corning 1737 glass substrates. The composition of In and Ga controls the net doping concentration, which determines the extension of the space charge region below the Schottky contact. Accordingly, the IZO and GIZO thin films were grown with thicknesses of 16nm and 160nm, respectivelly. Subsequent annealing at 150°C has strong effects on the electrical properties of the thin films (e.g. Hall-effect mobility) as well as on the device characteristics, e.g. higher field-effect mobilities and lower values for the minimum subthreshold swing S=∂VG/∂log(ISD). The MESFET were fabricated by standard photolithography using lift-off technique. The ohmic source-drain electrodes were sputtered using an Au target in an argon atmosphere. The Schottky gate contacts were dc-sputtered by means of an Ag target in a mixed argon/oxygen atmosphere, which yields a partial oxidization of Ag and hence to higher effective barrier heights for the Schottky contact.
The effective barrier heights of the Schottky contacts (SC) on IZO and GIZO channel material are 0.85eV and 0.96eV, respectively. The ideality factor n of contacts on GIZO were determined for the annealed thin films to be n=1.5 and for the as-grown thin film n=1.8 while the values for n on IZO thin films are approx. 1.7 .
For MESFET on annealed GIZO channel material the source-drain current can be controlled over 8 orders of magnitude with a gate sweep voltage of ΔVG=2.5V. Transistors on the sample chip have the highest field-effect mobility of 15 cm2/Vs and best reproducibility. For annealed IZO channel material the minimum substhreshold swing is S=69mV/decade, which is near the theoretical minimum of 60mV/decade at room temperature and one of the best values reported so far even if compared with the extensive literature on oxide TFTs[2].
[1] R. Martins, P. Barquinha, I. Fereira, Goncalves, and E. Fortunato, J. Appl. Phys. 101, 044505 (2007)
[2] M. Grundmann, H. Frenzel, A. Lajn, M. Lorenz, F. Schein, and H. v. Wenckstern, Phys. Status Solidi A, 207 (2010)
12:45 PM - MM4.9/F4.9
Thermal Annealing and Time Dependent Electric Field Gating Studies of Conductivity Stability in Amorphous Indium Zinc Oxide Thin-film Transistors.
Charles Sievers 1 2 , Thomas Gennett 2 , Joseph Berry 2 , David Ginley 2 , John Perkins 2 , Charles Rogers 1
1 Department of Physics, University of Colorado, Boulder, Boulder, Colorado, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractThe high electron mobility (10 – 50 cm2/V-s) of low carrier concentration semiconducting amorphous indium zinc oxide (a-IZO) makes a-IZO an attractive channel layer material for transparent thin film transistors (TTFTs). However, even modest thermal processing, such as is required for lithographic processing, can lead to large increases in carrier density in a-IZO. Here, we are studying the temperature dependence and the electric field gating of a-IZO conductivity in thin-film transistors in an effort to understand and control these carrier density changes. For as-grown films, we find that the electric field gated a-IZO conductivity has a two-stage time dependent response. First, the electric field produces a prompt change in conductivity, consistent with electron depletion physics. This quick response is then followed by a slow relaxation process with a time sale of order 10 minutes, strongly suggesting the presence of positive mobile ions in the as-grown films. Sequences of short thermal anneals at temperatures ranging from 100 to 300 °C yield an initial increase the conductivity from 1e-4 to 1e2 S/cm and then a subsequent decrease to a more stable conductivity of 1-10 S/cm. Furthermore, these anneals reduce the magnitude of the long time scale conductivity changes in the gating. Comparative studies of exposed and SiO encapsulated a-IZO channel layers over a range of environments from vacuum to pure oxygen suggest that the thermal and slow electric field response both trace to mobile positive ions, which can be driven from the films by suitable baking during device fabrication.
MM5/F5: Joint Session: Solution Processed Metaloxide Semiconductor
Session Chairs
Erin Ratcliff
Heiko Thiem
Tuesday PM, November 30, 2010
Constitution B (Sheraton)
2:30 PM - **MM5.1/F5.1
Low Temperature Processed Amorphous Oxide Semiconductor Thin-film Transistors.
John Wager 1 , Ken Hoshino 1 , Layannah Feller 1 , Rick Presley 1
1 School of EECS, Oregon State University, Corvallis, Oregon, United States
Show AbstractAmorphous oxide semiconductor (AOS) thin-film transistors (TFTs) appear to be well positioned for near-term commercial entry into the flat-panel display market as a replacement for hydrogenated amorphous silicon TFTs. This application mandates a maximum process temperature of approximately 300-350 °C. Process constraints for future flexible display and flexible electronics products promise to be even more challenging, requiring a maximum temperature of ~150 °C, or even lower. The purpose of this presentation is to review our work on reduced temperature processing of AOS TFTs.
3:00 PM - MM5.2/F5.2
Solution-processed Aluminum Indium Oxide Thin-film Transistor with Low Temperature Annealing.
Young Hwan Hwang 1 , Seok-Jun Seo 1 , Jun-Hyuck Jeon 1 , Byeong-Soo Bae 1
1 Lab. Optical Materials & Coating (LOMC), Dept. of Materials Science & Engineering, KAIST, Daejeon Korea (the Republic of)
Show AbstractOxide semiconductor has plenty of advantages such as transparency due to their large bandgap, high electron conduction property, high uniformity in large-scale fabrication applications, and environmental stability. Thus, it is expected that oxide semiconductor do a significant role in many applications such as, flat-panel displays, flexible displays, radiofrequency identification tags, and smart windows. Typically, oxide TFTs have been fabricated using vacuum-process which enables low temperature process. Recently, solution-processed oxide TFTs get intensive attraction since they can be prepared by simple and low cost methods. However, its high annealing temperature restricts the use of the solution-processed oxide TFTs as an active layer in TFT applications. Previously we have reported the aluminum indium oxide (AIO) TFTs which can be prepared with relatively low temperature process. (i.e. 350 °C) [1] In this study, we have applied additional annealing for the purpose of decreasing the annealing temperature. The vacuum annealing, which is understood as an effective tool to remove the remaining organics inside the film and increase the carrier concentration, was adopted to increase the conductivity of the film. After the vacuum annealing, the film was annealed under oxygen surrounding to optimize the carrier concentration in the film and decrease the surface oxygen deficiency which is related to off current of the TFT for better TFT performance. All the annealing process was performed at the 200 °C and 250 °C, individually. The resultant AIO TFT annealed at 250 °C exhibits a channel mobility of 3.5 cm^2/Vs, a subthreshold slope of 0.6 V/dec, and an on-to-off current ratio of ~10^6. In addition, based on the results of low temperature annealing, solution-processed AIO TFT fabricated on the flexible substrate will be demonstrated.[1] Y.H. Hwang, J.H. Jeon, S. Seo, and B. Bae, “Solution-processed, high performance aluminum indium oxide thin-film transistors fabricated at low temperature”, Electrochem. Solid-State Lett. 12, H336 (2009).
3:15 PM - MM5.3/F5.3
ZnBeMgO Nanostructured Based UV Detectors by Spin Coating.
Neeraj Panwar 1 , Jose Liriano 1 , Ram Katiyar 1
1 Department of Physics, University of Puerto Rico, San Juan United States
Show AbstractThe detection of ultraviolet radiation is of prime importance in the field of space, environment and biological applications etc. ZnO is a direct band semiconductor having wurtzite hexagonal structure with a band gap Eg ~ 3.37 eV which can be easily tuned by alloying it with MgO (Eg= 7.5 eV) and BeO (Eg= 10.8 eV). By this approach, one can cover the deep UV range avoiding interference from the visible spectrum and these detectors can work in the harsh enviornment. Zn1-x-yBexMgyO films were prepared on sapphire (Al2O3) and Si substrates by novel spin coating method. Starting materials were dissolved in 2-methoxyethanol and monoethanolamine (MEA) used as solvent and stabilizer respectively. The molar concentration of the solutions was 0.5M. The solutions were coated on the substrates rotated with 3000rpm for 30s. The films were then dried at 300°C for 10 min for evaporating the solvents and removal of organic residuals. The effect of deposition steps, annealing temperature variation and variation in Be and Mg concentration at Zn-site have been carried out. It was identified that with 15 deposition steps and 15 and 20 at% of Be and Mg concentration and 450°C annealing temperature produce film with best properties. XRD patterns of the films revealed that the pristine film has a wurtzite type structure. However, the doped films exhibited the preferential c-axis orientation along (002) indicating that the c-axis of the grains becomes uniformly perpendicular to the substrate surface. No extra peaks corresponding to BeO or MgO could be noticed in the XRD patterns of the films. From the optical measurements it was observed that for the pristine ZnO film, the cutoff wavelength was 364nm which decreased to 320nm for Zn0.65Be0.15Mg0.20O film which lies in UV-B region. The dark current, photoresponsivity and other optical measurements have been carried out on the films with inter-digitated electrodes and the results will be presented in the meeting.
3:30 PM - MM5.4/F5.4
Synthesis of Morphologically Controlled ZnO Nanostructures and Spray Deposition of Hybrid ZnO/Ag Nanostructured Films for Transparent Conductor Applications.
Saahil Mehra 1 , Rodrigo Noriega 5 , Mark Christoforo 3 , Sujay Phadke 4 , Dietmar Knipp 2 , Alberto Salleo 1
1 Materials Science & Engineering, Stanford University, Stanford, California, United States, 5 Applied Physics, Stanford University, Stanford, California, United States, 3 Electrical Engineering, Stanford University, Stanford, California, United States, 4 Mechanical Engineering, Stanford University, Stanford, California, United States, 2 Electrical Engineering, Jacobs University, Bremen Germany
Show AbstractThe development of efficient back reflector light- trapping methods for long wavelength photons (λ > 850 nm) in thin film photovoltaic devices remains a major obstacle to making these technologies cost-competitive. Solution processed film deposition techniques are strong candidates for novel transparent conducting films, owing to their minimal fabrication costs and roll-to-roll processing compatibility. Here we explore the light-diffusing and electronic properties of solution processed nanostructured zinc oxide films fabricated on glass substrates using a scalable pneumatic spray coating process. Synthesis of hexagonal-base zinc oxide nanopyramids with controlled size distributions in the range of 100 and 500 nm diameters is achieved using low-temperature, solution-based decomposition of zinc acetate in organic solvents. The effects of Ga-dopant incorporation on ZnO nanopyramid morphology and electrical properties were characterized using Photothermal Deflection Spectroscopy (PDS). We further demonstrate a systematic optimization of spray-coating process variables for nanostructure film deposition, exhibiting control over film uniformity, thickness, and the resulting optical and electronic properties. Finally, the benefits of solution processed hybrid films using metallic (Ag) nanowires and highly scattering ZnO nanopyramids were explored as transparent conductive films. The effects of nanopyramid size, ZnO:Ag ratios, and post-processing steps on the light scattering and electrical properties of sprayed films were quantified using haze factor measurements, SEM, and electrical measurements. The fabrication requirements for thin film photovoltaics using spray-coating as a high-throughput deposition mechanism are estimated, and we show that solution-processed nanostructured films are promising materials for transparent top contacts or textured back electrodes for such thin film devices.
3:45 PM - MM5.5/F5.5
Sensor Devices Based on Ink-jet Printed ZnO Nanoparticle Thin Films.
Sonja Hartner 1 , Moazzam Ali 2 , Ahmed Khalil 2 , Markus Winterer 2 , Hartmut Wiggers 1
1 , Institute of Combustion and Gasdynamics and CeNIDE, Duisburg Germany, 2 , Nanoparticle Process Technology and CeNIDE, Duisburg Germany
Show AbstractFor device fabrication such as transparent conductive layer and sensor devices based on cost-efficient printing technologies, stable dispersions of smallsized (semi)-conducting particles in the nanometer scale are highly desirable. ZnO nanoparticles (NPs) are a very promising material due to the electrical and optical properties of zinc oxide, but due to its amphoteric behavior ZnO is not very stable in suitable dispersing solvents as the pH of the dispersion changes. By using chemical vapor synthesis, highly crystalline, less aggregated and narrow-sized ZnO nanoparticles can be obtained. Stable aqueous dispersions of these ZnO NPs have been successfully prepared after the addition of a polymeric stabilizer. The dispersions are stable for at least 2 months without any observable sedimentation. Interdigitating structures consisting of two opposite gold pads with gold fingers arranged in between were prepared by e-beam lithography and. Stable dispersions as mentioned above have been used to print ZnO NP films with a Dimatix 2800 ink-jet printer on the pre-structured substrates by ink-jet printing. The resulting films have a thickness between 100 and 250 nm, low porosity and could be fabricated on different substrates. The electrical and sensing properties of the as-prepared ZnO thin films are measured without any annealing steps in between from room temperature up to 473K in ambient conditions and in reducing atmosphere using impedance spectroscopy. Compared to the measurements in air, the resistance in hydrogen decreases by a factor of five even at room temperature. As a result we find that our nanosized ZnO ink enables for the formation of sensor devices without any annealing or post-processing. Substituting the interdigital structures prepared from e-beam lithograpy by printed structures (e.g. with silver ink) will lead to fully printed sensor devices even on temperature sensitive substrates.
4:30 PM - MM5.6/F5.6
Solvothermal Synthesis of Uniform ITO Nanoparticles and Their TCO Properties.
Kiyoshi Kanie 1 , Takafumi Sasaki 1 , Atsushi Muramatsu 1
1 IMRAM, Tohoku University, Sendai, Miyagi, Japan
Show AbstractHighly crystalline cubic indium tin oxide (ITO) nanoparticles with narrow size distribution were successfully prepared directly in one step from the mixed solution of indium and tin salts by the solvothermal method with lean ethylene glycol as a solvent. The addition of water must inhibit the formation of ITO crystals, but can strongly promote the formation of In(OH)3 and InOOH, including tin hydroxide. Since In(OH)3 and InOOH was not found in water-free EG system, the transformation of In(OH)3 and InOOH into In2O3 phase must be remarkably slow so that once formed indium hydroxides become final product in water-containing system. The as-prepared particles in BuOH as a solvent consist of irregular shaped nanoparticles of ITO and InOOH. In the solvothermal system with glycol as solvents, direct formation of ITO solid particles was observed starting from amorphous indium hydroxides, In(OH)3 and InOOH phases are not detected as intermediates. In addition, their size with the range from 15 to 40 nm was easily operated with changing conditions, such as aging period and sodium hydroxide concentration. X-ray diffraction measurement and high resolution transmission electron microscopic observation revealed that basically single-crystalline ITO nanoparticles were successfully obtained, and doped tin atoms were uniformly distributed in the nanoparticles.
4:45 PM - MM5.7/F5.7
Zinc Oxide Organic Composites for Thin Film Transistor Applications.
Simon Bubel 1 , Claudia Busch 1 , Andreas Ringk 2 , Ralf Theissmann 1 , Peter Strohriegl 2 , Roland Schmechel 1
1 Faculty of Engineering and CeNIDE, University of Duisburg-Essen, Duisburg Germany, 2 Macromolecular Chemistry I, University of Bayreuth, Bayreuth Germany
Show AbstractMetal oxide semiconductors are considered as candidates to substitute silicon in large area and low-cost electronics. Therefore, when compared to silicon, it is essential that similar or simpler ways to control their charge carrier transport properties are available. This is especially important for such applications as field effect transistor devices. Since many metal oxides exhibit a highly reactive surface, one approach to alter their electronic properties could be the use of organic adsorbates. For the case of ZnO, we present two examples in which surface engineering dominates the macroscopic electrical properties of thin film transistors. In one experiment we introduce a novel facile low temperature (T~125°C) ZnO precursor process which allows the formation of ZnO layers with a thickness of approximately 7 nm and a field effect mobility exceeding 1 cm^2/Vs. Because of the low thickness, these films facilitate the investigation of surface induced changes on their electronic properties such as thermal charge carrier concentration and atmospheric stability. The layers were modified using pyrrolidone and phosphonic acid linkers attached to different moieties. In another experiment we use ZnO nanoparticles functionalized with the same adsorbates as described for the first experiment. Here these moieties act as dispersing agents or electrically active spacers between the ZnO particles. It can be shown that this approach allows for a macroscopic doping of the nanoparticles, in addition to the low temperature processability of these nanoparticles from an ink like dispersion.
5:00 PM - MM5.8/F5.8
Spray Deposited Lithium-doped Zinc Oxide Thin-film Transistors with Electron Mobility Exceeding 50 cm2/Vs.
George Adamopoulos 1 , Donal D Bradley 1 , Thomas Anthopoulos 1
1 Physics, Imperial College, London United Kingdom
Show AbstractThe high optical transparency and excellent charge transport characteristics combined with their excellent chemical stability and mechanical tolerance make oxide semiconductors attractive for applications in large area opto-/electronics and particularly thin-film transistors (TFTs). However, the vast majority of high performance oxide-based transistors reported to date are fabricated using sophisticated deposition methods that are usually incompatible with large area processing and hence potentially expensive. Here, we show an alternative processing method based on spray pyrolysis and soluble precursor molecules for the deposition of high-performance doped oxide semiconductors onto large area substrates under atmospheric conditions. Using this simple technique we are able to realise lithium-doped zinc oxide n-channel TFTs that are characterised by field-effect mobilities of >50 cm2/Vs, channel current on/off modulation ratio >10^6 and almost hysteresis-free operation. The physical properties of Li-ZnO films were investigated using a range of characterisation techniques namely AFM, XRD, Raman and Photoluminescence spectroscopy, spectroscopic ellipsometry and FTIR. Structural studies show that Li doping can lead to either interstitial or substitutional doping depending on the doping level. Interstitial doping was observed for Li concentration <1% and found to yield TFTs of high mobility. The latter was attributed to an increase in the average crystal size of Li-ZnO films. For Li doping concentration >1% (stoichiometry of precursor solution) it was shown that substitutional doping of Zn by Li occurs resulting to a drastic reduction in the average crystal size and interplanar spacing accompanied by a significant reduction in the electron field-effect mobility. The present results demonstrate that spray pyrolysis is a versatile tool for the deposition of oxide semiconductors onto large area substrates and provides a new route for the rapid development of materials far beyond those accessible by traditional deposition methods.
5:15 PM - MM5.9/F5.9
Ink-jet Printing of In2O3-ZnO 2D-structures from Solution.
Jenny Tellier 1 , Marija Kosec 1 , Barbara Malic 1 , Danjela Kuscer 1
1 , Jozef Stefan Institute, Ljubljana Slovenia
Show AbstractOxide semiconductors in the In2O3-ZnO system are highly transparent in the visible range and their electrical conductivity can be tuned by varying the ratio of constituents, making them good candidates for transparent electronic devices. We have studied the processing of solutions that enable the in-situ shaping of the structures with the thickness of a few nanometers and the lateral resolution of about 40 μm for the design of thin-film-transistors (TFTs) by piezoelectric ink-jet printing.The printing ink was based on the precursor solution consisting of In-isopropoxide and Zn-acetate in 2-methoxyethanol which was originally designed for Chemical Solution Deposition (CSD) of thin films on glass and silicon substrates. The thin films were organics-free and amorphous upon heating at as low as 150°C and crystallised upon heating at 450°C as determined by FTIR and XRD analyses, respectively.The viscosity and the surface tension of the ink needed to be adapted to reach the target values of the Dimatix printer: 10-12 mPas and 28-32 mN/m, respectively. The viscosity of the original CSD-solution was 3.4 mPas and by admixing a highly viscous solvent 1,3-propanediol (41 mPas) in a 55/45 volume ratio it reached 9.6 mPas. The surface tension of the ink was 34 mN/m. Further, the printing parameters: the cartridge and substrate temperature, and the drop spacing were adjusted to allow patterning with a good resolution on selected substrates.Precise and reproducible structures were obtained by ink-jet printing According to microstructural, chemical and XRD analyses, the structures heated at 150°C in air were amorphous, contained no organics, and crystallized upon heating at 450°C. The thickness of one printed layer was about 35 nm. Functional properties of films will be reported as well.
5:30 PM - MM5.10/F5.10
A Low-temperature Solution Precursor for In2O3-based Transparent Conductors.
Robert Pasquarelli 1 , Maikel van Hest 2 , Alexander Miedaner 2 , Calvin Curtis 2 , John Perkins 2 , Ryan O'Hayre 1 , David Ginley 2
1 Metallurgical and Materials Engineering, Colorado School of Mines, Golden, Colorado, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractTransparent conducting oxide (TCO) thin-films play a critical role in many current and emerging opto-electronic devices due to their combination of high transparency in the visible region of the spectrum and tunable electronic conductivity. Atmospheric-pressure solution deposition is an attractive alternative to conventional vacuum-based TCO deposition techniques due to its ease and potential to lower device manufacturing costs. Solution precursors for In2O3 are of particular interest for material systems such as indium-tin-oxide (ITO) and more recently amorphous indium-zinc-oxide (a-IZO). We have reported on IZO thin-films prepared by ultrasonic spray deposition from an indium-zinc formate precursor at 300-400°C with good optical transmittance (>80%) and conductivities of ~50 S/cm [1]. However, the processing temperatures required for this and other traditional precursors are too high for many applications, such as thin-film photovoltaics and processing on flexible plastic substrates, which generally require temperatures < ~200°C. Here, we report on the exploration of the pentanuclear oxo-alkoxide cluster of In5O(isopropoxide)13 as a new low-temperature precursor for In2O3 thin-film deposition. The initial decomposition of this precursor occurs at temperatures as low as 150-170°C. The decomposition process and phases formed under various atmospheres were investigated as a function of temperature using thermogravimetry, differential scanning calorimetry, mass spectrometry, and X-ray diffraction. Characterization of the decomposition of the precursor, as well as initial results on the structural, optical, and electronic properties of the alkoxide deposited films, will be presented.[1] Pasquarelli et al., Inorg. Chem. 49, 5424 (2010).
MM6: Poster Session: Transparent Conducting Oxides and Applications I
Session Chairs
Wednesday AM, December 01, 2010
Exhibition Hall D (Hynes)
9:00 PM - MM6.1
Low Resistivity of ZnO:( Li,Ga ) Powder Prepared by Soft Chemical Route Process.
Yi-Wen Kao 1 , Kuo-Chuang Chiu 1
1 Materials Research Laboratories, Industrial Technology Research Institute, Hsinchu Taiwan
Show AbstractLi- and Ga-codoped zinc oxide polycrystalline powder were prepared by soft-chemical route process. The quantity of Lithium and Gallium in the sol was varied from 3.8 to 3.95 mol% and 0.05 to 0.2 mol%. The structural characteristics studied by X-ray diffractometry were complemented resistivity measurement by Four-Point Probe. Prepared under tartaric acid as chelating agents and sintered at 1400 oC was employed to elucidate the ZnO:( Li,Ga ) formulation for the powder process. The lowest room-temperature resistivity is found to be ~ 5 Ω in air annealing environment was optimized for production. The best conductors were obtained for the (Li0.0005Ga0.0395)Zn0.96O powder.
9:00 PM - MM6.10
The Properties of Un-doped SnO2 Buffer Layers and Their Effect on the Performances of CdTe Solar Cells.
Zeng Guanggen 1 , Feng Lianghuan 1 , Cai Yaping 1 , Zheng Jiagui 1 , Li Bing 1 , Zhang Jingquan 1 , Wu Lili 1 , Lei Zhi 1 , Li Wei 1
1 College of Material Sci. and Eng., Sichuan University, P.R.China, Chengdu China
Show AbstractDecreasing CdS thickness is one of the effective ways to improve the conversion efficiency of CdS/CdTe solar cells. However, the decrease in CdS thickness may lead to the adverse effects on devices. In order to eliminate these effects, it is necessary to introduce a buffer layer between CdS and front electrode. In this paper, the un-doped SnO2 thin films with different thickness, as buffer layers, have been deposited by magnetic reactive sputtering. Then the films have been annealed at 550 degree Celsius in N2/O2=4:1 ambience for 30 minutes. The properties of the films before and after annealing were studied by AFM, XRD, UV-vis and XPS. The results show that, after annealing, the oxygen content of the film increase and SnO has been oxidized into SnO2. After annealing, surface topography and electricity uniformity have been improved, higher than 80% transparency has been obtained and resistance increase to meet the requirements of the buffer layers. Finally, the cells with Glass/TCO/un-doped SnO2/CdS/CdTe/ZnTe/ZnTe:Cu/Ni structure have been prepared. The performances of cells with un- doped SnO2 show that the response of short-wave has been improved, lower series resistance and higher Jsc, FF have been achieved. As a result, after introducing a buffer layer, the device stability and duplication has been improved.
9:00 PM - MM6.11
Microstructural and Optical Properties of NiO Thin Films by Electrodeposition and Post-annealing.
Ho Lee 1 , Jong Do 1 , Hyunghoon Kim 1 , Jin Moon 1 , Hyung Cho 2
1 Materials Science and Metallurgical Engineering, Kyungpook National University, Daegu Korea (the Republic of), 2 Materials Science and Engineering, Sungkyunkwan University, Suwon Korea (the Republic of)
Show AbstractNickel oxide (NiO) has attracted global interest for potential applications, such as resistive random access memory, electrochemical capacitor, electrochromic devices, chemical sensors, and light-emitting diodes, because of its wide bandgap of about 3.54 eV and p-type conductivity. For the realization of these NiO-based devices, NiO has been prepared by many different techniques, such as electrodeposition, dc magnetron sputtering, thermal oxidation of Ni, chemical bath deposition, thermal evaporation, spray pyrolysis, sol-gel, and pulsed laser deposition. Among them, electrodeposition is suitable for cost-effective and low-temperature growth of thin film NiO.In this present, we investigated the microstructural and optical properties of NiO thin films prepared by a two-step process: electrodeposition and post-annealing. NiO films were electrodeposited on ZnO nanorods in electrolyte solutions composed of 5 mM NiCl2 and 10 mM C6H12N4 and then annealed at 300oC for 1 h in air ambient. The electrodeposition was performed in a two-electrode electrochemical cell at various temperatures from 60oC to 90oC. The ZnO nanorod substrates and Pt foil were used as a working and a counter electrode, respectively. The potential applied to the working electrode was -1.0 V. The deposition time was 30 min. During the electrodeposition, oxygen gas was continuously bubbled near the working electrode. The size and thickness of NiO films were investigated using scanning electron microscopy (SEM). Auger electron spectroscopy (AES) is used to investigate interfacial reaction between NiO film and ZnO nanorods. In order to investigate the crystal structure, x-ray diffraction (XRD) pattern and cross-sectional transmission electron microscopy (TEM) analysis was performed. The optical property was characterized by photoluminescence (PL) measurement at room temperature under excitation with a 30 mW He-Cd laser operating at 325 nm.
9:00 PM - MM6.12
Tolerance of Indium Zinc Oxide Films for the TCO Layer of Cu(In,Ga)Se2 Solar Cells.
Mutsumi Sugiyama 1 , Moe Warasawa 1 , Akira Kaijo 2 , Chizuru Yamazaki 1 , Yuiko Hirose 1 , Kenichiro Takakura 3 , Hideroni Ohyama 3
1 Department of Electrical Engineering, Tokyo University of Science, Chiba Japan, 2 , Idemitsu Kosan Co.,Ltd., Chiba Japan, 3 , Kumamoto National College of Technology, Kumamoto Japan
Show AbstractIndium zinc oxide (IZO) thin films are expected to use as a new candidate for transparent electrodes for optoelectronics devices, especially for high efficiency of solar cells such as Cu(In,Ga)Se2 (CIGS) or Si. In fact, IZO films have high mobility (30 - 40 cm^2/Vs) and low resistivity (2 - 3 x 10^-4 Ωcm) compared with those of ZnO:Al films that is a conventional TCO material for CIGS solar cell. These results indicate that using IZO as a window layer is expected to increase the efficiency of CIGS solar cells.For the commercial use, understanding the degradation mechanisms of IZO is necessary because long-term reliability is very important for solar cell. However, only a few degradation studies have been carried out on IZO thin film and entire optoelectronics devices such as solar cell, resulting in limited knowledge on the mechanisms responsible for the damage. In this presentation, degradation effects will be discussed under accelerated situation for CIGS solar cells such as electron irradiation, thermal annealing, and chemical resistance.IZO thin films were deposited on soda-lime glass substrates by conventional RF sputtering using unintentional heating. The substrate was located parallel to a 4-inch-diameter target. The chamber background pressure was 0.1 Pa, and the depositions of IZO were carried out at 0.3 Pa in a 6N-pure Ar atmosphere.For example, the IZO films did not change their transmittance properties under the electron irradiation up to 1.0 x 10^18 cm^-3. XRD patterns of IZO were the same before and after irradiation (amorphous structure). In addition, the resistivity of IZO did not change regardless of irradiation. Therefore, these results suggest that IZO thin films have a good tolerance of electron irradiation. This study clarifies a part of the degradation mechanism of CIGS-based solar cells.
9:00 PM - MM6.13
Effects of Air-annealing on Structural and Optical Properties of P-type Nickel Oxide Films Deposited by the RF Reactive Sputtering Method.
Yoshitsuna Murata 1 , Mutsumi Sugiyama 1 , Shigefusa Chichibu 2
1 Department of Electrical Engineering, Faculty of Science & Technology, Tokyo University of Science, Noda Japan, 2 CANTech, Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, Sendai Japan
Show AbstractTransparent conducting oxide (TCOs) films are widely used in flat panel displays, touch panels, and solar cells. Most of TCOs including indium-tin-oxide (ITO) and ZnO exhibit n-type conductivity. Recently, p-type TCOs such as CuAlO2 , CuInO2, and NiO have been attracting attention, because they are a possible candidate for the use in transparent transistors, UV-visible light emitting diodes, and solar cells consisting of a transparent p-n junction. Among these, NiO films have been used in resistance random access memory (ReRAM) devices, and Au-Ni-O alloy is frequently used as an transparent ohmic contact layer for p-type wide bandgap semiconductors. The resistivity of NiO can be controlled by doping column-I impurities such as Li, or by introducing intrinsic defects such as Ni vacancies and/or interstitial oxygen. Therefore, NiO has been used as an insulator with a resistivity of the order of 1E13Ωcm and a semiconductor with a resistivity of the order of 1E-1Ωcm.NiO films have been deposited by a variety of methods including sputtering, pulsed laser deposition, and spray pyrolysis. Sputtering is the most desirable method, because large-area films with well-controlled compositions can be deposited economically. It is known that the heterophase of NixOy films were deposited under various growth conditions. However, few experimental results have been reported for the correlation between the structural and optical properties of NixOy films and growth parameters. In this presentation, the effect of air-annealing will be shown for the structural and optical properties of NixOy and NiO films.Approximately 200–500-nm-thick NixOy films were grown by the RF reactive sputtering method on soda-lime glass (SLG) substrates. Ar and O2 were used as the sputtering and source gases, respectively. The O2 fraction in the sputtering gas was 0.2. With increasing the annealing temperature, the diffraction peak shifted toward that of NiO (200). The result indicates that annealing in air (oxygen or H2O) may change NixOy into NiO. The increase of the annealing temperature also gave rise to the increase in the bandgap energy of the films, reaching that of NiO. The result indicates that the bandgap of NixOy film may be lower than the NiO film.
9:00 PM - MM6.14
Advantages of Using Amorphous Indium Zinc Oxide Films for the Transparent Conducting Oxide Layer in Cu(In,Ga)Se2 Solar Cells.
Moe Warasawa 1 , Akira Kaijo 2 , Mutsumi Sugiyama 1
1 Electrical Engineering, Faculty of Science & Technology, Tokyo University of Science, Noda Japan, 2 , Idemitsu Kosan Co.,Ltd., Sodegaura Japan
Show AbstractVarious transparent conducting oxide (TCO) films such as Sn-doped indium oxide (ITO) and F-doped tin oxide (FTO) are used as window layers for electrical current collection in thin film photovoltaic devices. Al-doped zinc oxide (ZnO:Al) has been used in high-efficiency Cu(In,Ga)Se2 (CIGS) solar cells and is attracting attention as an alternate TCO. However, the Hall mobility of ZnO:Al decreases and its resistivity increases with decreasing film thickness, because target erosion during sputtering causes an inhomogeneous distribution of the resistivity. Furthermore, optical transmittance in the near-infrared region tends to decrease because of plasma oscillations and poor crystal quality.Indium zinc oxide (IZO) is a promising alternate candidate for TCOs, because it is possible to obtain IZO with an entirely amorphous structure with high reproducibility under a wide range of deposition conditions. In fact, several groups have reported that the transmittance decreasing of IZO due to plasma oscillation is smaller than that of ZnO:Al in case of the same carrier concentration. Therefore, IZO thin films are expected to increase the efficiency of CIGS-based solar cells and also to serve as a new candidate for transparent electrodes in optoelectronics devices. In this study, advantages of using IZO films for CIGS solar cells are describing with electrical properties and providing TEM images, and comparing them with conventional ZnO:Al thin films.IZO and ZnO:Al thin films were deposited on soda-lime glass (SLG) substrates by conventional RF sputtering using unintentional heating. An IZO (In2O3:ZnO = 89.3:10.7 wt%) or a ZnO:Al (ZnO:Al2O3 = 97:3 wt%) target was used for the sputtering. To fabricate conventional CIGS solar cells, an IZO or a ZnO:Al layer was deposited on an undoped ZnO/CdS/CIGS/Mo-coated soda-lime glass substrate structure. To adjust the sheet resistance of the TCO layer for the solar cell, the thickness of ZnO:Al layer was larger than that of IZO layer. The electrical properties of IZO films as a function of film thickness, with the properties of ZnO:Al serving as a reference are indicate that IZO films values of n was about 5-10 E20 cm-3, while μ (30 - 40 cm2/Vs) was higher and ρ (2 - 3 E-4 Ω cm) was lower by a factor of ten than the corresponding values for ZnO:Al. Representative cross-sectional TEM images of IZO/ZnO/CdS/CIGS/Mo/SLG and ZnO:Al/ZnO/CdS/CIGS/Mo/SLG are shown that an amorphous layer of IZO has been deposited on undoped ZnO, while ZnO:Al layer with an orientation along the c-axis has been deposited on the same substrate. The μ of ZnO:Al is dominated by grain-boundary scattering, while the μ of IZO is independent of grain-boundary scattering at approximately same carrier density. These results indicate that the efficiency of CIGS solar cells is expected to increase using IZO as a TCO layer.
9:00 PM - MM6.15
Transport in Thin Silver Films.
Martin Philipp 1 , Christian Hess 1 , Hartmut Vinzelberg 1 , Bernd Buechner 1 , Hadia Gerardin 2 , Jacques Jupille 3
1 Institute for Solid State Physics, Leibniz Institute for Solid State and Materials Research Dresden, Dresden Germany, 2 , Saint-Gobain Recherche, Aubervilliers Cedex France, 3 Institut des NanoSciences de Paris, Université Paris 6, Paris France
Show AbstractAl-doped ZnO / Ag / Al-doped ZnO layer stacks are widely used as low-emissivity coatings for building glazing due to their high reflectance in the infrared and high transmittance in the visible spectrum. The determining parameter for the IR reflectance is the electrical conductance of the layer stack - the better the conductance the higher the reflectance. To achieve a fundamental understanding of the prevailing electron scattering mechanisms, the layer stacks, which were produced by magnetron sputtering, were investigated by means of electrical transport measurements. The resistivity was measured in dependence of the temperature on layer stacks with different silver film thickness varying in the region between 4 nm and 200 nm. Afterwards, the data was analyzed using different thin film resistivity models, describing electron scattering at interfaces (Fuchs-Sondheimer model) and electron scattering at grain boundaries (Mayadas-Shatzkes model). The main objective was to differentiate between these two models and to identify the dominant scattering mechanism, which is an important issue for the improvement of the coating. Due to the fact that both models exhibit a similar film thickness dependence of the resistivity the differentiation is not trivial and has been rarely discussed on experimental data yet. In order to determine the average diameter of the grains inside the silver film transmission electron microscopy in combination with electron diffraction was done on thin cross-sectional slices of the layer stack for different silver film thicknesses. With this technique it is possible to examine several grains of the silver film and to estimate their diameter. From these results a simple model for the dependence of the average grain size on the silver film thickness was established and implemented into the Mayadas-Shatzkes model.From resistivity measurements at room temperature the increase of the resistivity with decreasing film thickness becomes obvious. However, the application of the models does not allow a differentiation between the two scattering mechanisms, because both models fit the data equally well.The two models predict a different temperature dependence of the resistivity for different film thicknesses. We could show that, by measuring the temperature dependent resistivity in the region from 4.5 K to 295 K in dependence of the film thickness, it is possible to differentiate between the different contributions to the total resistivity. It appears that the main contribution to the total film resistivity comes from the scattering at the grain boundaries.The results of the analysis of the basic Al-doped ZnO / Ag / Al-doped ZnO layer stack were compared to results of more sophisticated systems, where several inter- and underlayers were added in order to improve the conductivity of the silver film. Furthermore the effect of annealing was studied at all these systems.
9:00 PM - MM6.16
Transition of Transparent Conducting Oxide to Amorphous Transparent Oxide Semiconductor by Simple Composition Modification for Oxide TFT.
Junhyuck Jeon 1 , Young Hwan Hwang 1 , Seok-Jun Seo 1 , Byeong-Soo Bae 1
1 Lab. Optical Materials & Coating (LOMC), Dept. of Materials Science & Engineering, KAIST, Daejeon Korea (the Republic of)
Show AbstractRecently, oxide thin film transistor (TFT) including transparent oxide semiconductor (TOS) has been studied for transparent electronics because of its high mobility, good uniformity and large band gap. To improve the TFT performance, the defect control by varying compositions of TOS has been considered. a-IGZO, typical TOS reported by Hosono group, shows high carrier mobility even in amorphous phase because it has a different conduction band derived from isotropic and spatially expanded atomic state while covalent material such as has strongly-directed sp3 bonds. In addition, the carrier concentration could be effectively controlled through addition of Ga because Ga has higher bond energy with oxygen than In or Zn, resulting in suppressing of oxygen vacancy formation.From the concept, we observed the transition from transparent conductor to transparent oxide semiconductor by simply adding Al into indium tin oxide (ITO) thin film to optimized the composition for oxide TFT. Al has higher bond energy with oxygen than Ga, so it can formed AlOx in ITO thin film. AlOx is well-known as a good amorphous state stabilizer and it has large band-gap. We can control carrier concentration of ITO with suppressing oxygen vacancy formation and enlarging band-gap. Also, Al is light metal cation of small orbital so it contributes to the nonguaranteed electronic path due to insufficient overlapping. Each optimized composition of AITO channel layer which were annealed at 400°C and 500°C is x=0.35 at (Al2O3)X(In2O3)0.9-X(SnO2)0.1. They showed high carrier mobilities of 5.5 and 10.9 cm^2V^−1s^−1, threshold voltages of 7.7 V and 3.87 V, high on-to-off current ratios of 107 at the same, low sub-threshold swings about 1 V/dec. We could approach from this study how aluminum affects to the electronic structure of Al-added oxide thin film with annealed at two different temperatures and obtain the AITO channel property which is applicable to transparent electronics at each annealing temperature.
9:00 PM - MM6.17
ALD-grown Zinc Tin Oxide for TCO and TFT Applications.
Woon-Seop Choi 1
1 , Hoseo University, Asan-city Korea (the Republic of)
Show AbstractThere were some reports on zinc oxide/tin oxide by the sequential layer deposition of sputter and combinatorial approach of sputter. However, there was no report on the zinc tin oxide thin film by ALD technology yet. Zinc tin oxide thin film was prepared by plasma enhanced atomic layer deposition (PEALD) using diethyl zinc and tetrakis(ethylmethylamino) tin precursors for the first time. The ratios of zinc and tin were formulated from 1:1 to 10:1. The average growth rate of zinc tin oxide film is about 1.4 ~ 1.9 A/cycle with composition. The average growth rate increases gradually with zinc ratio at the substrate temperature of 150 oC. ALD-grown zinc tin oxide thin film was characterized with XRD, AFM and XPS. The morphology of ZnSnO film has a needle-like texture for all cases. Thermal annealing effect shows increased crystallinity, similar surface roughness, and increased zinc amount in the composition. From the atomic ratios by the XPS spectrum, the composition of ZnSnOx metal oxide is expected to be a ZnSnO. The potential application for TCO and TFT was examined.
9:00 PM - MM6.18
High Performance ZnO Thin Film Transistors Using Novel High-k Dielectrics.
D. Ngwashi 1 , R. Cross 1 , S. Paul 1 , A. Milanov 2 , A. Devi 2
1 Emerging Technologies Research Centre, Department of Engineering, De Montfort University, Leicester United Kingdom, 2 Inorganic Materials Chemistry, Ruhr University Bochum, Bochum Germany
Show AbstractTransparent oxide semiconductors have recently been subject to intense investigation for optoelectronic applications such as light-emitting diodes and display technologies, e.g., thin-film transistors (TFTs). Major benefits of these materials include the potential for simple low-cost deposition at temperatures compatible with polymer substrates and high electron mobilities. There is also the possibility of exploiting the wide bandgaps of these materials for transparent electronics. Of particular interest is zinc oxide (ZnO), a wide bandgap (~3.37 eV) n-type semiconductor that exhibits excellent optical, electrical, catalytic and gas sensing properties, and has many applications in numerous fields. There is also great potential in the ability to deposit high-quality ZnO films over large areas and at low temperatures, which has facilitated the demonstration of TFTs with impressive performance metrics [1]. However, for any TFT device, of great importance is the quality of the gate insulator material as it can have a significant affect on device performance. An issue arising from this is the relatively high operating voltages often reported in the literature for ZnO-TFTs. One way in which this could be ameliorated is to incorporate a high dielectric constant (high-k) insulator into the device [2], hence increasing the coupling of the gate voltage to the ZnO channel layer .In this paper, we describe the material properties and the performance of ZnO-TFTs incorporating high-k thin films deposited from novel precursor materials. The high-k material was deposited using metal-organic chemical vapour deposition operating under reduced pressure. The precursors was synthesised following a similar procedure published earlier [3]. For the active ZnO channel layer, a ZnO target (99.999 %) was used at a substrate sputtering distance of 6 cm. The film stoichiometry was controlled via the radio-frequency 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 to improve the film’s stability in air [4]. Thickness and refractive index measurements were carried out using ellipsometry. The capacitance-voltage behaviour of metal-insulator-semiconductor (MIS) structures and the current-voltage characteristics of the MIS devices and the TFTs were investigated using an HP4192A and an HP4140B picoammeter respectively. Typical performance metrics of the complete devices, including effective channel mobilities of up to 35 cm2/Vs and On/Off current ratios of 10^7, will also be presented.[1].R. Martins, Barquinha, P., Ferreira, I., Pereira, L., Gonalves, G., Fortunato, E. , Journal of Applied Physics 101, 044505 (2007). [2].Hideya Kumomi, Kenji Nomura, Toshio Kamiya and Hideo Hosono, Thin Solid Films 516, 1516 (2008).[3].A. Milanov, R. A. Fischer, A. Devi, Inorg. Chem. 47, 11405 (2008). [4].Divine Ngwashi, Richard B. M. Cross, Shashi Paul, Mater. Res. Soc. Symp. Proc. 1201, H05-39 (2009).
9:00 PM - MM6.19
Application of Indium Tin Oxide Nanowires, Nanopowders and Thin Films for Dye Sensitized Solar Cells.
Halil Ibrahim Yavuz 1 , Ahmet Ozenbas 1
1 Metallurgical and Materials Engineering, Middle East Technical University, Ankara Turkey
Show AbstractITO (Indium Tin Oxide) Semiconductor Metal Oxide nanowires (SCMONWs), favorable materials for various quantum devices which have 1 D interaction and huge surface-volume ratio, were synthesized by a novel technique. Growth of nanowires is commonly performed using expensive techniques requiring high vacuum and high temperature conditions, which also consume some hazardous raw materials. The sol-gel technique employed in this study is used primarily for the fabrication of materials including metal oxides beginning from a wet chemical solution, where low temperatures and non-vacuum conditions are utilized. Indium tin oxide (ITO) is primarily used as a transparent conducting oxide for electronics such as Solar Cells and LCD applications. In this work; indium tin oxide (ITO) nanoparticles, nanowires and thin films have been realized on glass substrates by sol-gel technique and these were used for the production of nano-crystalline dye-sensitized solar cells (nc-DSSC), which are a relatively new class of low-cost thin film solar cells. Structural, topographical and chemical analysis were made using XRD, SEM and EDS. 15 nm thick and 500 nm long ITO nanowires were observed in SEM analysis. The transparency of the ITO films was analyzed using UV spectrometer. Then 5% addition of ITO nanowires in nc-DSSCs caused an improvement of 33% in the efficiency with respect to pure TiO2 dye sensitized solar cells.
9:00 PM - MM6.2
Scanning Probe Spectroscopy on Transparent Conducting Al-doped ZnO Thin Films.
Rafael Jaramillo 1 , Edward Likovich 1 , Venkatesh Narayanamurti 1 , Shriram Ramanathan 1
1 , Harvard University, Cambridge, Massachusetts, United States
Show AbstractWe will present results obtained via scanning probe spectroscopic studies of ZnO:Al transparent conducting thin films grown by low-temperature reactive sputtering. Local density of states (LDOS) measurements made using scanning tunneling microscopy reveal a grain boundary density of states on the order of 1013 cm-2eV-1 that is peaked ~200 meV below the conduction band edge, thus identifying directly the trap states that limit electron mobility in low-Al-content ZnO:Al films. Complimentary Kelvin force microscopy measurements of the variation of work function across granular films demonstrate the importance of understanding local spectroscopy for the design and realization of advanced optoelectronic. We further show the effect of common post-processing steps on the “electronic granularity.” The results are of relevance towards improved understanding of electronic properties of oxide electron conductors for energy conversion.
9:00 PM - MM6.20
Indium-tin Oxide (ITO) Thin-film Transistors Fabricated by RF Magnetron Sputtering at Room Temperature and Thickness Dependence on the Transistor Performance.
Joo Hyon Noh 1 , Seyeoul Kwon 1 , Jiyong Noh 1 , Philip D. Rack 1 2
1 Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee, United States, 2 The Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractOxide TFTs have recently attracted a great deal of attention because their role as devices in realizing the development of transparent circuits, which are expected to serve as the basis for new optoelectronic devices. We have fabricated thin-film transistor (TFT) using a standard In2O3-10% SnO2 (ITO) sputter target by rf magnetron sputtering at room temperature. The ITO thickness dependence on the TFT performance was investigated. An ITO thin film active layer was deposited on a Si/SiO2 substrate with a thickness gradient by tilting the sputtering gun relative to the substrate axis. The deposition was performed at room temperature in a mixture of argon and oxygen (Ar:O2 = 9:1). The thickness was varied in the range of 12-31 nm. The 13 nm-thick ITO TFT operates in a normally off-mode with a field-effect mobility of 2.6 cm2 V−1 s−1, a threshold voltage of 6.2 V, an on-off current ratio of 9.0×104, and a subthreshold gate voltage swing of 2.5 V decade−1. Conversely the 26 nm-thick ITO TFT operates in a normally on-mode with a field-effect mobility of 0.2 cm2 V−1 s−1, a threshold voltage of -22.7 V. This thickness-dependent performance is ascribed to the total carrier number in the active layer. In this presentation we will review the synthesis process and will compare and contrast the TFT characteristics as a function of the active layer thickness and will show how the total carrier concentration affects the transport properties. Furthermore, we will show how the oxygen partial pressure affects the carrier concentration due to the oxygen vacancy concentration.
9:00 PM - MM6.21
Structural and Electrical Characterization of Oxide Spinels as Candidates for P-type Transparent Conductors.
Nicola Perry 1 , Arpun Nagaraja 1 , Thomas Mason 1 , Joanna Bettinger 2 , Michael Toney 2
1 Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States, 2 Stanford Synchotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California, United States
Show AbstractSpinel-structured oxides, such as ZnB2O4 (B = Rh, Co), show potential as p-type transparent conductors. Bulk, polycrystalline specimens of these Zn-based spinels were fabricated with a range of stoichiometries using low-temperature powder synthesis and solid-state sintering. In situ equilibrium conductivity and thermopower measurements, in combination with Rietveld analysis of X-ray diffraction patterns, were employed to investigate phase equilibria, dominant defect species, and conduction mechanisms. The corresponding distributions of cation species on octahedral and tetrahedral sites were investigated through thermodynamic-based calculations and anomalous X-ray diffraction (AXRD) measurements at the Zn, Co, and Rh K absorption edges to correlate the electrical properties with the crystallographic structure. Optical band gaps were measured by diffuse reflectance spectroscopy. Structure-property relationships will be discussed in the context of improving the performance of spinels as transparent conductors.
9:00 PM - MM6.22
Pluse Laser Deposited ZnBeMgO Thin Films for UV Detection.
Jose Liriano 1 , Neeraj Panwar 1 , Ram Katiyar 1
1 Physics, University of Puerto Rico, San Juan, Puerto Rico, United States
Show AbstractZnO is a energy band gap of 3.37eV and high excitation binding energy (60meV) at room temperature which makes its versatile for potential applications in many areas like flame detection, missile plume sensing, air quality monitoring, piezoelectric and ferroelectric usage, accurate measurement of radiation for the treatment of UV irradiated skin and ozone hole detection. The present day detectors based on photomultipliers tubes (PMT) and micro-channel plates (MCP) are heavier and not efficient in discarding the radiation in the visible range and hence put questions for their proper utilization at sophisticated places. The versatility of ZnO is that it is a direct band semiconductor with wurtzite hexagonal structure which can be easily tailored by allyoing it with MgO and BeO. We have prepared thin films of ZnBeMgO on sapphire (Al2O3) substrates at 650°C using pulsed laser deposition (PLD) technique and studied their structural and optical behaviour. From the X-ray diffraction patterns it was observed that single phase formed. Optical absorption measurements carried out on these films revealed that the optical band gap of the pristine ZnO film the cutoff wavelength was 364nm which decreased to 266nm for Zn0.7Be0.1Mg0.2O film which is quite well in the solar blind region. The ZnBeMgO are show that can be can be used for the manufacture of UV detectors. Other results like Raman, the deposition temperature and oxygen partial pressure were also studied and will be presented.
9:00 PM - MM6.23
Characterization of P-type ZnO Films Produced by Thermal Oxidation.
Oscar Garcia Serrano 1 , Marco Vazquez-Agustin 1 , Ramon Pena-Sierra 1 , Gabriel Paredes-Rubio 1 , Jose Andraca-Adame 1
1 SEES, CINVESTAV-IPN, México D.F. Mexico
Show AbstractZnO films were grown by thermal oxidation of metallic Zn films in air-laboratory at normal environment conditions in the temperature range of 450 C to 700 C. Metallic Zn films of 190 nm were deposited by DC-Sputtering at 1.3x10-4 Torr on n-type Si and glass substrates. The complete oxidation of metallic Zn films was determined by the ZnO films transparency. Hall-effect measurements were done at room temperature. Hole concentrations of ~ 1017 cm-3, carrier mobility of 3 to 8 cm2V-1s-1 and resistivities of ~ 12 Ω-cm were obtained. The measured carrier mobility is affected by the thin thickness of the characterized films. The used n-type silicon substrate assures that the measured electrical properties came from the ZnO films. The origin of intrinsic p-type conductivity in ZnO thin films might be related to oxygen interstitials (Oi), zinc vacancies (VZn) acceptor levels and the no-polar prismatic superficial planes with energy gap lower than 0.8 eV. The ZnO films were analyzed by x-ray diffraction (XRD), Raman scattering and photoluminescence (PL) measurements. The XRD spectra shows that the resulting ZnO films are slightly textured in the {100} prismatic planes. The PL spectra of the ZnO films are composed by the high energy band (3.28 eV) and the defect band (2.33 eV). On the oxidized samples at low temperatures, band-to band transitions for the high energy band dominates the PL spectra, however as the oxidation temperature increases, the defect band increases.
9:00 PM - MM6.24
Electrical Properties of Highly Conducting SnO2:Sb Nanocrystals Synthesized by a Nonaqueous Sol-gel Method.
Tiago de Goes Conti 1 , Adenilson Chiquito 2 , Elson Longo 1 , Edson Leite 1
1 LIEC-Química, Universidade Federal de São Carlos, São Carlos, São Paulo, Brazil, 2 NanOLab-Física, Universidade Federal de São Carlos, São Carlos, São Paulo, Brazil
Show AbstractThe growing demand for advanced electronic devices based on nanomaterials has motivated efforts to produce nanoscale building blocks with designed functions. Transparent conducting oxide nanocrystals (TCONs), processed in organic media, are particularly promising for the development of such devices. In recent decades, many researchers have concentrated on developing methods to process Antimony-doped Tin Oxide (ATO) thin films on plane substrates and have achieved significant results in terms of electrical properties and processing techniques. However, most of these methods are limited to thin film processing, which restricts the use of the material for other applications such as conductive molecular composites and gas sensors.This work describes the synthesis of highly conducting antimony-doped tin oxide (ATO) nanocrystals prepared via a nonaqueous sol-gel route in the size range of 4-6 nm, as well as insights into its electrical properties. The antimony composition was varied from 1 to 18 mol% and the lowest resistivity (4.0x10-4 Ωcm) was observed at room temperature in the SnO2:8.8%molSb composition. The samples were evaluated by XRD, HRTEM, EDX, and SEM, and resistivity measurements were taken in the four-probe mode in the temperature range of 13-300K. The results show highly crystalline nanoparticles in a monodisperse colloidal system, dependence on the shape of ATO nanoparticles as a function of Sb distribution, low resistivity and semiconductor-metal transition. Moreover, this methodology renders nanoparticles suitable for thin film processing, for application as building blocks for hybrid materials and for other advanced ceramic electronic devices. The preparation of crystalline ATO nanoparticles fully re-dispersible in organic solvents consists in one pot reaction under solvothermal treatment of tin (IV) chloride and antimony (III) chloride in benzyl alcohol at 150°C for 48h. The ATO nanocrystals were collected by centrifugation and washed twice with THF, and placed in a concentrated THF dispersion. The nanoparticles were kept into organic media (20g.L-1) and not dried before the re-dispersion. Electrical measurements were taken of 13mm diameter pellets sintered at 500°C for 3h as well as of 45nm thickness thin film. The pellets showed ohmic resistor behavior at room temperature, and the sample containing 8.8mol% of Sb presented the best conductivity performance (lowest resistivity) and an unexpected semiconductor-metal transition, which is currently under investigation. Furthermore, we successfully used this material as nanoscale building blocks for ultrathin film deposition. Finally, it is important to mention that the ATO nanoparticles reported in this work are promising materials for ultrathin and thin film processing, for build blocks for hybrid materials, and for other advanced electronic devices.
9:00 PM - MM6.25
Surface Characterization of Ga-doped ZnO Layers.
Joy McNamara 1 , Iwona Ruchala 1 , Josephus Ferguson 1 , Michael Reshchikov 1 , Alison Baski 1 , Huiyong Liu 2 , Vitaliy Avrutin 2 , Hadis Morkoc 2
1 Physics, Virginia Commonwealth University, Richmond, Virginia, United States, 2 Electrical and Computer Engineering, Virginia Commonwealth University, Richmond, Virginia, United States
Show AbstractZnO that has been heavily doped with Ga (GZO) is a promising material for use as a transparent conducting electrode in optoelectronic devices such as solar cells and LED's. Such applications require both a highly transparent and conductive material with a high mobility of charge carriers. In this work, epitaxial ZnO layers heavily doped with Ga were grown by plasma-enhanced molecular-beam epitaxy (MBE) on sapphire substrates. The GZO samples studied here were grown at 400 °C with a range of oxygen pressures (4×10-6 to 1.5×10-5 Torr) and film thicknesses (100 to 700 nm). We used atomic force microscopy (AFM) to characterize the surface topography, and then studied surface electrical characteristics by locally injecting charge and measuring the resulting change in surface potential. Topographical studies show that layers grown under oxygen-rich conditions (1.5×10-5 Torr) have a dense packing of uniform surface features that produce increased surface roughness with increased sample thickness (rms = 1 nm for 100-nm thickness; rms = 15 nm for 700-nm thickness). Samples grown under metal-rich conditions (4×10-6 Torr) have a lower density of surface features that result in a comparable surface roughness for corresponding sample thicknesses. To characterize the surface charging behavior, a positive 1 to 3 V bias was placed on the sample during contact-mode AFM imaging using a metallized tip. As we have previously shown for wide-bandgap materials such as GaN, a reverse-bias polarity at the tip-surface junction should result in excess negative charge being deposited on the surface [1]. It should be noted that bias voltages above ~3 V in the case of GZO result in sample damage due to enhanced current flow and electrochemical reactions in the region between the tip and sample. After charging, we measured the surface contact potential difference between charged and uncharged regions using scanning Kelvin probe microscopy (SKPM). These measurements were performed in dark to prevent charge redistribution due to surface photovoltage effects. The charged regions were well-defined with surface potentials measuring 0.2 to 0.4 V below the surrounding regions, where larger differences occurred for higher charging voltages. The surface charging of layers grown under oxygen-rich conditions was more sensitive to ambient conditions such as humidity, as compared to layers grown under metal-rich conditions. Our results indicate that we are able to successfully inject and store charge on the surface of GZO at moderate applied voltages, similar to our previous experiments with GaN. Additional measurements are in progress to obtain local current-voltage spectra using conductive AFM and to determine any correlation with charging behavior. [1] J.D. Ferguson, M.A. Foussekis, M.D. Ruchala, J.C. Moore, M. A. Reshchikov, and A. A. Baski, “Manipulation of Surface Charge on GaN,” Mat. Res. Soc. Symp. Proc. 1202, I04-01 (2010).
9:00 PM - MM6.26
Doped Zinc Oxides as Alternative Transparent Conductive Oxides for Organic Photovoltaics.
Andres Garcia 1 , Ajaya Siddel 4 1 , Nicodemus Edwin Widjonarko 3 1 , Ken Xerxes Steirer 2 1 , David Ginley 1 , Joseph Berry 1 , Dana Olson 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States, 4 Physics and Astronomy, University of Denver, Denver, Colorado, United States, 3 Physics, University of Colorado, Boulder, Colorado, United States, 2 Physics, Colorado School of Mines, Golden, Colorado, United States
Show AbstractThe wide use of indium tin oxide (ITO) in organic photovoltaics (OPV) has led us to explore the use of alternative transparent conductive oxides (TCO) based on less expensive zinc oxide. Sputtered zinc oxide films doped with a small percentage of gallium (GZO), aluminium (AZO), and indium (IZO) were observed to exhibit excellent optical and electronic properties for photovoltaics i.e. high light transparency in the visible and low sheet resistance, and were used as TCOs in solution processed polymer-fullerene bulk heterojunction photovoltaics. Incorporation of these TCOs in OPV devices containing a poly(3-hexylthiophene):[6,6]-phenyl C60 butyric acid methyl ester (P3HT:PCBM) active layer resulted in high performance devices comparable to those utilizing ITO electrodes. The importance of the appropriate choice of electron or hole transport layers to obtained high efficiency devices due to the sensitivity of these doped zinc oxide TCOs is also presented. This work indicates that doped zinc oxide TCOs can function as low cost alternatives to ITO in organic photovoltaics.
9:00 PM - MM6.27
Electric Property of Transparent Conducting Oxide Films Toward High Frequency Applications in GHz Range.
Tsuyoshi Ogino 1 2 3 , Naoki Ohashi 1 3 , Shunichi Hishita 3 , Isao Sakaguchi 3 , Yutaka Adachi 3 , K. Nakajima 2 , Hajime Haneda 1 3
1 Department of Applied Science for Electronics and Materials, Kyusyu University, Kasuga, Fukuoka, Japan, 2 Research and Development Center, Taiyo Yuden Co. Ltd., Haruna, Gunma, Japan, 3 , National Institute for Materials Science, Tsukuba Japan
Show AbstractDownsizing of electric components is critical for integration of many functions into mobile electronic devices. Acturally, many modules, such as Bluetooth, digital TV, and GPS etc, are installed in small cell phones in addition to CDMA/GMS modules. Considering such technological trend, size and location of antenna in mobile deives are serious problems on further integration of functions, because performance of antenna is improved with its size. A possible solution is fabrication of transparent antenna to be installed on display panel of mobile devices. Fron this viewpoint, we investigated electronic conductivity of transparent conductor oxide (TCO) films in GHz range and developed possible device structures utilizing TCO thin films for transparent antenna applications. TCO films were deposited on SiO2 glass substarte by sputtering technique. The structure and composition of the films were characterized by AFM, SIMS, and XPS, and their electronic transport was studied by DC resistivity and Hall effect measurements. Microstripline (MSL) structures composed of TCO films were made for high frequency measurements and impedance profile of MSLs in GHz range was measured with a vector network analyzer at room temperature. The obtained impedance profiles were compared with simulated ones to reveal their high frequency properties. The simulations were performed referring measured DC conductivity. Since experimental and simulated impedance profiles showed good agreement, we proceeded to develop possible device structures for utilization of ZnO films to transparent antenna applications.
9:00 PM - MM6.28
RF-Superimposed DC Sputter Deposition Process of Amorphous InZnO Transparent Conductors.
Thomas Gennett 1 , John Perkins 1 , Marie Galante 1 , Jennifer Leisch 1 , Joshua Martin 1 , Philip Parilla 1 , David Ginley 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractTransparent conductive oxides (TCOs) used for applications such as flat panel displays and solar cells, are usually deposited via a DC magnetron sputter process in order to deposit films of reasonable material quality at reasonably high deposition rates to maximize process efficiency and minimize overall cost. However, as has been shown with Indium Tin Oxide (ITO) [1, 2] and aluminum doped zinc oxide (AZO) [3], appropriate incorporation of both DC and RF processes (RF-superimposed DC) results in enhanced substrate bombardment by moderate energy plasma ions which can in turn yield denser, better films at viable industrial level sputter rates. In this work, we apply RF-superimposed DC sputtering to the deposition of amorphous InZnO (a-IZO) thin films and optimize the deposition process across the 4-dimensional multivariate parameter space of RF/(RF+DC) power ratio, total power (RF+DC), oxygen concentration and metals composition (In/(Zn+In)). Across this coupled parameter space, we obtain exhibit extensive control of the various electrical, optical and physical properties of the deposited films that allow for maximize performance for specific applications of the TCO. In particular, a-IZO films were deposited at ambient temperature using normally incident RF superimposed DC magnetron sputtering from various single composition 4” diameter ceramic targets spanning the composition range from 60:40 to 87:13 wt% In2O3:ZnO. The total sputtering power (DC + RF) was varied from at 40 to 200 W and the RF/(RF+DC) fractional power was varied from 0% to 100%. As the relative RF portion of the total power was increased, the discharge voltage and deposition rate decreased. Also, through the use of an RGA mass spectral monitoring system, the amount of oxygen necessary to maximize conductivity of the a-IZO films was determined to be a coupled function of the RF/(RF+DC) power ratio, the total power and the metals composition. Of particular interest, we found that a-IZO films with > 90% visible transparency and electrical conductivity >3250 S/cm could be achieved with a relative indium content as low as 70:30 wt % In2O3:ZnO at deposition rates comparable to an all DC sputter process. Further, for applications requiring specific conductivities, we find that the parameter space control, and hence process stability, can be increased dramatically with the correct coupled choice of relative RF/(RF+DC) power ratio and total power. A summary of these results will be presented. [1] Stowell et al., Thin Solid Films 515, 7654 (2007); [2] Kim et al., Current Appl. Phys., 9 (2009) S262-S265; [3] Minami et al., J. Vac. Sci. Technol. A 25, 1172 (2007).
9:00 PM - MM6.3
Bandgap Engineered High Mobility Indium Oxide Thin Films for Photovoltaic Applications.
R. Gupta 1 , K. Ghosh 1 , P. Kahol 1
1 Physics, Astronomy, and Materials Science, Missouri State University, Springfield, Missouri, United States
Show AbstractMagnesium (Mg) and titanium (Ti) doped indium oxide (IMTO) thin films were grown using pulsed laser deposition technique. Magnesium was added to enhance the bandgap, whereas titanium was added to improve carrier concentrations and mobility of indium oxide films. IMTO thin films were deposited on quartz substrate by ablating the sintered target of indium oxide containing 5 at% of magnesium and 2 at% of titanium with a KrF excimer laser (λ = 248 nm and pulsed duration of 20 ns). The effect of growth temperature on structural, optical, and electrical properties were studied. X-ray diffraction studies showed that films grown at low temperatures are amorphous in nature. It was observed that the optical transparency of the films strongly depends on growth temperature and increases with increase in growth temperature. The films grown at 600 °C showed optical transparency > 85%. We observed widening in bandgap of indium oxide by doping with magnesium and titanium. The bandgap of IMTO films increases with increase in growth temperature. The maximum bandgap of 3.84 eV was observed for film grown at 600 °C. It was observed that growth temperature strongly affects the electrical properties such as resistivity, carrier concentration, and mobility. The electrical resistivity of the films increases from 7.69×10-4 to 1.25×10-3 Ω.cm with increase in growth temperature from 50 °C to 600 °C respectively. On the other hand, carrier concentration decreases with increase in growth temperature. The electron mobility of the films increases from 15 to 71 cm2/Vs with increase in growth temperature from 50 °C to 600 °C respectively. Temperature dependence electrical resistivity measurements showed that films grown at low temperatures were semiconducting in nature, while films grown at high temperature showed transition from semiconducting to metallic behavior. These wide bandgap, highly transparent and high mobility films could be used for photovoltaic applications. Detailed study based on effect of growth temperature on structural, optical, and electrical properties will be presented.
9:00 PM - MM6.30
Magnetron Sputtering of Fluorine-doped Tin Oxide.
Dane Gillaspie 1 , John Perkins 2 , Thomas Gennett 1 , David Ginley 2
1 Chemical and Materials Science Center, National Renewable Energy Laboratory, Golden, Colorado, United States, 2 National Center for Photovoltaics, National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractTin oxide (SnO2) is a ubiquitous transparent conducting oxide, due to its high durability, low toxicity, and excellent price/performance ratio. For example, glass coated with fluorine-doped tin oxide is often used as a substrate for thin film solar cells, electrochromic windows, and touch panels. Even so, conventional pyrolytically grown fluorine-doped tin oxide has a number of drawbacks, including high haze, roughness, and incompatibility with some device fabrication processes due to the high substrate temperatures required (T > 400 °C). Here we present results on the growth of fluorine-doped tin oxide films (SnO2:F) using magnetron sputtering. Deposition parameters such as dopant source, temperature, pressure, and gas ratio were varied, which yielded transparent films with a maximum conductivity of 880 S/cm to date. For these highest conductivity films, CF4 mixed into the Ar sputtering gas was used as the fluorine source. For the as-deposited films, the highest conductivity, ~ 500 S/cm, was obtained with ambient temperature deposition and the conductivity decreased monontonically with increasing substrate temperature to ~ 600 °C, the highest temperature tested. A subsequent post-deposition anneal at ~ 450 °C in 1 Atm Argon increased the conductivity to 880 S/cm. The results compare very favorably with those in the literature for sputtered tin oxide.
9:00 PM - MM6.31
Pulsed Laser Deposition of P-type Transparent Conducting Cobalt Oxide Spinels.
Paul Ndione 1 , Andriy Zakutayev 1 , Nicodemus Widjonarko 1 , Ajaya Sigdel 1 , John Perkins 1 , Philip Parilla 1 , Dana Olson 1 , David Ginley 1 , Joseph Berry 1
1 , NREL, Golden, Colorado, United States
Show AbstractTransparent conducting oxides (TCOs) are generally based on a limited class of metal oxide semiconductors such In2O3, ZnO, SnO2, Ga2O3 and CdO which are both transparent and conductive. These basic TCOs are under development for a myriad of applications including transparent transistors, flat panel displays, solid state lighting modules, sensors, optical limiters, switches, and thermally reflective windows. However, all these traditional TCOs materials are n-type conductors and alone, they do not meet the TCOs performance needs of emerging photovoltaics. In this work, we use pulsed laser deposition to explore the p-type (Ni, Zn)Co2O4 spinels for functional TCOs. Despite recent studies of these compounds, many of their physical properties are still unknown. Using a combination of electrical, X-ray diffraction, X-ray fluorescence and spectroscopic measurements, we find the deposition and post-deposition processing resulting in marked changes to both optical and electrical properties. For example, depending on the deposition temperature, conductivity of 3-6 S/cm (10 times higher than what has been previously reported in the literature) is measured for p-type Co2ZnO4. Device applications using these p-type oxide materials are presented with the specific example of the use of Co2ZnO4 in organic photovoltaics.
9:00 PM - MM6.32
Solution-phase Synthesis of Infrared Reflective and Transparent Conductive Al-doped ZnO Nanocrystals.
Hyun-Jong Kim 1 , Chi-Ho Shin 1 , Hoyun Lee 1 , Hong-Ki Lee 1 , Tae-Wook Shin 2
1 Surface Technology Center, Korea Institute of Industrial Technology (KITECH), Incheon Korea (the Republic of), 2 , SB Chemical, Ansan Korea (the Republic of)
Show AbstractPure ZnO is a direct-band gap semiconductor with a wide band gap of 3.37 eV and a large exciton binding energy of 60 meV. Indeed, pure ZnO is highly resistive, but doping of ZnO with ions of higher valency such as Ga3+, Al3+ and In3+ can induce dramatic changes. A degenerate gas of current-carrying electrons is created which gives rise to infrared absorption and high electronic conductivity. However, at same time the fundamental host band gap is left intact, that is the electrically conductive material remains optically transparent in the visible region. Therefore, Al-doped ZnO (AZO) semiconductor is promising as alternatives to ITO for transparent electrode applications due to their low resistivity, abundance in natural resource, low cost, non-toxicity and good optical transmission in the visible wavelength region. Here, we describe a one-pot synthesis method to fabricate infrared reflective and electrically conductive Al-doped ZnO nanoparticles. This technique was designed on the basis of a polyol process without additional supplement of water. Also, further heat treatment was not required for the infrared reflection and electrical conduction.The TEM measurement revealed that the 100 nm AZO nanocluster was formed with uniform size of 100nm by the aggregation of nanocrystals. The actual percentage of aluminum in AZO was changed from 0.03 % to 0.5%. The size AZO nanoparticle was slightly reduced as increasing the concentration of aluminum, which might be due to the acidity of aluminum precursor. The electrical resistivity of AZO was measured by 4-point probe. The resistivity of 22 kΩ/sq was obtained, when 0.5% aluminum was doped in ZnO. This value is quite low as comparing with that of commercial AZO powder (39 kΩ/sq). The optical diffuse reflectance spectra of AZO were measured in UV, visible and NIR region. The typical UV-absorption bands of AZO nanoparticles, at about 300~380nm, can be clearly seen for all samples. In the NIR region ranging from 780nm to 2500nm, however, the absorbance was obviously affected by the doping level of aluminum. While the absorbance of ZnO was just 25%, that of AZO was increased up to 85% as increasing the concentration of aluminum. In spite of the effective absorption of NIR, the transmittance of visible light was still higher than 75%. We could concluded that the AZO synthesized in this study has better performance in electrical conduction and NIR absorption than commercial AZO powder.
9:00 PM - MM6.4
Zinc Doped Indium Oxide Pyramids for Optoelectronic and Field Emission Applications.
Javier Bartolome 1 , Ana Cremades 1 , Javier Piqueras 1
1 , Universidad Complutense de Madrid, Madrid Spain
Show AbstractSemiconducting oxides represent a rich family of functional materials that have a significant impact in areas such as energy conversion, solid state lighting, and electronics. In particular, Indium Zinc Oxide (IZO) is an interesting material system for instance in organic related technology, or due to superconductivity properties of IZO films.
In this work, micro-pyramids of zinc doped indium oxide has been grown by thermal treatment of compacted powders under argon flow, so that neither a catalyst nor a foreign substrate is used during this vapor-solid process (1). Micro-pyramids present a square (400) bottom face and four (222) triangular faces. Pyramids with controlled size and forming a continuous film present completed tips and exhibit field emission properties. Some of the pyramids with rough surfaces and truncated tips show pinholes with regular crystalline facets forming an inverted pyramid with a square base. The apexes of these pinholes present a hollow core or pipe, as those often reported for SiC and GaN films. The presence of a pipe could be responsible for the activation of a dislocation driven growth of rods growing at the pyramid bases that develop during longer treatments. Energy dispersive X-ray (EDS) mappings as well as local EDS spectra enable the identification of the rough surfaces of the pyramids with a secondary deposition of a Zn enriched material due to the different rates of incorporation of precursor species to the gas phase. The CL images reveal an enhancement of the luminescence in regions with a higher content of Zn. A new luminescence band at 2,38 eV could be related to the incorporation of Zn. XPS measurements are used to discuss the Zn incorporation as a dopant and the formation of ZnkIn2Ok+3 ternaries.
1. D. A. Magdas, A. Cremades, J. Piqueras, Appl. Phys. Lett. 88 (2006) 113107
9:00 PM - MM6.5
Optoelectronic Characterization of Morphology-controlled Zinc Oxide Nanowires.
Shou-Yi Kuo 1 4 , Fang-I Lai 2 , Woei-Tyng Lin 2 , Hsin-I Lin 3 , Hsueh-Chao Chang 1
1 Department of Electronic Engineering, Chang Gung University, Tao-Yuan Taiwan, 4 Green Technology Research Center, Chang Gung University, Tao-yuan Taiwan, 2 Department of Photonics Engineering,, Yuan-Ze University, Tao-yuan Taiwan, 3 Graduate Institute of Electro-Optical Engineering, Chang Gung University, Tao-yuan Taiwan
Show AbstractIn this paper, we report the characterization of vertically aligned ZnO nanowire (NW) arrays synthesized by metal-catalyzed chemical vapor deposition. The growth mechanism of ZnO NWs may be related to vapor-solid-nucleation. Morphological, structural, optical and field emission characteristics can be modified by varying the growth time. For growth time reaches 120 min, the length and the diameter of ZnO NWs are 1.5 μm and 350 nm, and they also show preferential growth orientation along the c-axis. Room-temperature photoluminescence spectra exhibit a sharp UV emission and broad green emission, and the enhanced UV-to-green emission ratio with increasing growth time might originate from the reduced concentration of surface defects. Moreover, strong alignment and uniform distribution of ZnO NWs can effectively enhance the antireflection to reach the average reflectance of 5.7% in the visible region as well. Field emission measurement indicated that the growth time play an important role in density- and morphology-controlled ZnO NWs, and thus ZnO NWs are expected to be used in versatile optoelectronic devices.
9:00 PM - MM6.6
Novel Electron Injection Layers (EIL-AZO) for Inverted Organic Solar Cells.
Hyunchul Oh 1 , Johannes Krantz 1 , Christoph J. Brabec 1
1 , i-MEET: institute Materials for Electronics and Energy Technology, Department of Materials Science (WW6), Friedrich-Alexander-Universität Erlangen-Nürnberg , Erlangen Germany
Show Abstract Inverted bulk-heterojunction solar cells have recently captured high interest due to their environmental stability as well as compatibility to mass production. This has been enabled by the development of solution processable n-type semiconductors, mainly TiO2 and ZnO. However, the performance of device is very dependent on the property of interfacial materials, and here specifically to their crystallinity, their surface states as well as to their stochiometry. In this study, we synthesized aluminum-doped zinc oxides (AZO) and investigated their properties as electron injection layers for inverted bulk-heterojunction solar cells. The performance of the AZO based cells is compared to the one with intrinsic ZnO as well as TiOx. The metal oxides are characterized with respect to their transmittance, absorbance, conductivity and their band-gap. Further, we build the correlation between the doping level of the AZO and the device performance and discuss Voc and FF losses in the picture of a 1-d numerical model.
9:00 PM - MM6.8
Radio-frequency Sputtered Amorphous Nickel Oxide Thin Film as an Efficient Hole Transport Layer for PCDTBT:PC70BM Solar Cells.
Nicodemus Edwin Widjonarko 1 2 , Ajaya Sigdel 3 2 , Ken Xerxes Steirer 4 2 , Andres Garcia 2 , Paul Ndione 2 , Matthew Lloyd 2 , David Ginley 2 , Dana Olson 2 , Joseph Berry 2
1 Physics, University of Colorado at Boulder, Boulder, Colorado, United States, 2 National Center for Photovoltaics, National Renewable Energy Laboratory, Golden, Colorado, United States, 3 Physics and Astronomy, University of Denver, Denver, Colorado, United States, 4 Physics, Colorado School of Mines, Golden, Colorado, United States
Show AbstractThe hole transport layer (HTL) in an organic photovoltaic (OPV) device is a critical component in maximising power conversion efficiency (PCE). Traditionally, the HTL function in OPV systems is performed by a polymer material such as PEDOT:PSS. Here we present our work to replace this HTL with a tuneable inorganic nickel oxide (NiOx) layer. Recent developments in photo-active materials for solar cells have revealed very promising substitutes for the widely-used poly(3-hexylthiophene) (P3HT). Among these, poly(N-9’-heptadecanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thi-enyl-2’,1’,3’-benzothiadiazole) (PCDTBT) can be paired with a PC70BM electron acceptor to yield a PCE of 5.7% with a PEDOT:PSS HTL. However, the energy level of PEDOT:PSS (~5.0 eV) is not very well-aligned with that of PCDTBT (5.45 eV). Moreover, PEDOT:PSS is deposited from an acidic suspension, resulting in incompatibility with the emerging zinc oxide based transparent conducting oxides. NiOx has deeper and more easily tuneable work-function (between 5.1 - 5.8 eV) than PEDOT:PSS, which better facilitates Ohmic contact to the deep donor level of PCDTBT, leading to improved PCE. Here we demonstrate the use of radio-frequency sputtered amorphous NiOx as an efficient HTL for PCDTBT:PC70BM solar cells, with less leakage current compared to its PEDOT:PSS counterpart. We discuss the physical mechanism that lead to such reductions in leakage current. Additionally, we present our work examining the effect of systematically changing NiOx deposition conditions on work-function of the film and device performance.
9:00 PM - MM6.9
Study on Electronic Structure of in Tin-doped Indium Oxide by First-principles Calculations.
Hiroshi Mizuseki 1 , Madhvendra Nath Tripathi 1 , Ryoji Sahara 1 , Takashi Nakamura 2 , Yoshiyuki Kawazoe 1
1 , Institute for Materials Research, Tohoku Univ., Sendai, Miyagi, Japan, 2 , IMRAM, Tohoku University, Sendai Japan
Show AbstractTin doped indium oxide (commonly known as ITO) are widely used because of high transparency in the visible region with high electrical conductivity. Its optical and electrical properties are exploited in expanding variety of transparent electrode applications, including flat-panel displays, electrochromic windows, organic light-emitting diodes. In the present study, first-principles calculations are used to estimate the energy of interstitial oxygen in tin-doped indium oxide (ITO). To evaluate the transmittance and absorption for the long wavelength between 300 to 800 nm, we have calculated the frequency dependent dielectric parameters after getting the electronic ground states of the structures. A part of this work has been supported by New Energy and Industrial Technology Development Organization (NEDO).
Symposium Organizers
Joseph J. Berry National Renewable Energy Laboratory
Elvira Fortunato CENIMAT/13N
Yuzo Shigesato Aoyama Gakuin University
Julia Medvedeva Missouri University of Science and Technology
MM7: Transparent Conducting Oxides and Applications II
Session Chairs
Wednesday AM, December 01, 2010
Constitution B (Sheraton)
9:15 AM - MM7.1
Electrochromic Behavior of Indium Tin Oxide/Niobium Oxide Nanocomposite Films.
Guillermo Garcia 1 2 , Ravisubhash Tangirala 2 , Raffaella Buonsanti 1 3 , Thomas Richardson 2 , Delia Milliron 2
1 Mechanical Engineering, UC Berkeley, Berkeley, California, United States, 2 , Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 , UCLA, Los Angeles, California, United States
Show AbstractA recent focus on the enhancement of energy efficiency for buildings has generated new interest on the development of fast switching and durable electrochromic windows. Investigation of antimony doped tin dioxide nanocrystalline films1 and sol gel processed niobium oxide2 has demonstrated electrochromic properties in the near infrared and visible spectra respectively.1 In order to explore the ability to tune spectroelectrochemical characteristics across both the visible and near infrared, we prepared nanocomposites of indium tin oxide (ITO) nanoparticles in a niobium oxide matrix. Using a combination of X-ray diffraction, cyclic voltammetry, spectroelectrochemical measurements, and impedance spectroscopy, we correlate the morphological and the electrochromic properties of our nanocomposite films. These are compared with properties of single-component films. For example, preliminary results on niobium oxide thin films show coloration at ~1.5V and bleaching at ~1.75V vs. lithium. A change in 7% transmission between colored and bleached states is seen for films of only ~20 nm thick. Scanning electron microscopy of single-component ITO particle films reveals the development of a porous network that can enhance lithium intercalation during switching. Herein, we compare the electrochromic properties of these materials to the rich behavior in electrochromic properties of ITO/niobium oxide nanocomposite films.[1] zum Felde, U., et al., J. Phys. Chem. B, 2000, 104 (40), 9388-9395.[2] Ozer, N., et. al., Solar Energy Materials and Solar Cells, 40 (1996) 285-296.
9:30 AM - MM7.2
Defect Chemistry of CuAlO2 and CuCrO2: A HSE06 Study.
David Scanlon 1 , Graeme Watson 1
1 School of Chemistry, Trinity College Dublin, Dublin, Leinster, Ireland
Show AbstractFabrication of high figure-of-merit p-type transparent conducting oxides (TCOs) is a much sought after goal for optoelectronic devices. Realization of a p-type TCO to rival the current industry standard n-type TCOs (e.g. In2O3:Sn, ZnO:Al ), would open up the possibility of “transparent electronics”. An explosion of interest in Cu(I) wide band gap materials was spearheaded by Hosono and co-workers with their 1997 report of simultaneous p-type conductivity and optical transparency in thin films of delafossite CuAlO2.[1] Subsequently many other p-type TCOs with the delafossite structure (CuMO2 M= Sc, Y, In, Ga, B) have been investigated, but to date the delafossite with the highest reported conductivity (220 Scm-1 [2]) is Mg-doped CuCrO2. The defect chemistry of the delafossites is not well understood, however, and to date the nature of the dominant p-type defect in CuAlO2 is still under debate.[3,4] Previous theoretical studies of the defect chemistry of the CuAlO2 have been hampered by the use of standard DFT functionals, which erroneously favour delocalization of defects states, with these methods having been shown to fail to accurately model p-type defects in Cu2O.[5,6] There has also been some controversy about the p-type conductivity mechanism of CuCrO2, with both Cu(I)/Cu(II)and Cr(III)/Cr(IV) hole mechanisms being proposed.[6,7]In this presentation we will examine the defect chemistry of CuAlO2 and CuCrO2 using the screened hybrid DFT functional, HSE06. We will: (i) outline the thermodynamic growth conditions that should be adopted to maximize device performance, (ii) identify the dominant p-type charge carrier for both materials, (iii) investigate a novel method to improve the p-type conductivity of CuAlO2, and (iv) rationalize the role of the Cr d states in the improved conductivity of CuCrO2.[8]
[1] H. Kawazoe et al. Nature, 389 (1997) 939
[2] R. Nagarajan et al., J. Appl. Phys., 89 (2001) 8022
[3] J. Tate et al., Phys. Rev. B, 80 (2009) 165206
[4] B. J. Ingram et al., Chem. Mater., 16 (2004) 5616
[5] D. O. Scanlon et al., Phys. Rev. Lett., 103 (2009) 096405
[6] D. O. Scanlon et al., J. Chem. Phys., 131 (2009) 124703
[7] A. Maignan et al., Solid State Commun., 149 (2009) 962
[8] D. O. Scanlon and G. W. Watson, In Submission (2010)
9:45 AM - **MM7.3
Non-oxide p-type Wide-gap Semiconductors.
Janet Tate 1
1 Physics, Oregon State University, Corvallis, Oregon, United States
Show AbstractWide-gap semiconductors find application as transparent conductors, active layers in transparent electronics, injectors in solar cells or light-emitters, and have many other uses. Ideally, a useful semiconductor materials system would be environmentally benign, allow n- and p-dopability, with a wide range of carrier concentrations and high mobility carriers. It is hard to achieve all of these in any one materials system, and in particular the lack of a high-mobility p-type transparent oxide has been a problem for some time. Non-oxide candidates are therefore of interest. I will review various several chalogenide wide-gap semiconductor systems including BaCuChF and related structures, Cu3MCh4, and SnMCh3.
10:15 AM - MM7.4
Solution-based Vapor-phase Epitaxy for Functional Oxide Semiconductors and Alloys.
Kentaro Kaneko 1 2 , Shizuo Fujita 2
1 Department of Electronic Science and Engineering, Kyoto Univ., Kyoto Japan, 2 Photonics and Electronics Science and Engineering Center, Kyoto Univ., Kyoto Japan
Show AbstractWe have been researched a series of corundum-structured oxide semiconductors such as α-Ga2O3, α-Fe2O3, α-Cr2O3, and α-V2O3, because they have many physical functions of magnetic, optical, and electronic properties. α-Ga2O3 is a wide band gap oxide semiconductor which optical band gap is 5.3 eV. α-Fe2O3 is a weak ferromagnetic oxide with a band gap of 2.2 eV. α-Cr2O3 is a anti-feromagnetic oxide with a band gap of about 3.0 eV. So we expect that these alloys of α-(GaFe)2O3, α-(GaCr)2O3 have physical functions of magnetic and electronic properties. From our previous studies, we found that they were epitaxially grown on c-plane sapphire substrates. Especially, α-(GaFe)2O3 thin films have very high crystallinity which the full-widths at half maximum of X-ray diffraction rocking curves were as small as 100 arcsec for the entire range of Fe concentration (K. Kaneko et al., APEX 2 (2009) 075501 K. Kaneko et al., pss(c) DOI 10.1002). In addition, from our recent reserches, TEM observation showed that the lattice relaxation of the α-Ga2O3 thin film occured at the interface immediately and then the crystal grew without inclusion of severe defects. The lattice relaxation mechanism is unclear, but it is certain that this abrupt relaxation is the reason for highly crystalline α-Ga2O3, α-Fe2O3 α-(GaFe)2O3 layers.These oxides and alloys were prepared by the mist Chemical Vapor Deposition (CVD) method which was originally developed by our laboratory. It is different from trivial CVD technique for using of atomized precursor sources and all equipments is performed under atmosphere. And in case of making alloy oxide thin films, there is no need to set up new gas or precursor source line, only mixing of precursor sources. This system is very suitable for making of oxides because oxygen is not impurities. And more we focus on the α-V2O3. It is an oxide semiconductor with the band gap of 0.3 eV at insulator phase and shows anti-ferromagnetic properties. It is well known as metal-to-insulator transition materials and its transition temperature is about 160 K. There are many reports about Ti2O3, Cr2O3 and Fe2O3 doped α-V2O3 and its physical properties. But there are slight reports about alloy or doping with other corundum structured oxides. In this symposium, we report our efforts to fabricate and characterize α-V2O3 and α-V2O3-Ga2O3 alloy thin films. X-ray diffraction results show that α-V2O3 thin films were poly crystalline but strongly grown to c-plane axis on c-cut sapphire substrates, but α-V2O3-Ga2O3 alloy thin films were not poly crystalline and these were grown only 0001 axis. The details of α-V2O3 and α-V2O3-Ga2O3 alloy thin films will be presented at the symposium.
10:30 AM - MM7.5
A Study of Increased Resistivity of FTO Back Contact for CZTS Based Absorber Material Grown by Electrodeposition-Annealing Route.
Prashant Sarswat 1 , Michael Free 1 , Ashutosh Tiwari 2
1 Metallurgical Engineering, University of Utah, Salt Lake City, Utah, United States, 2 Materials science and engineering, University of Utah, Salt Lake City, Utah, United States
Show AbstractCZTS (Copper zinc tin sulfide) thin films have been synthesized using transparent conducting oxide (TCO) back contacts, allowing sun light to pass through the entire solar cell. Aqueous solution based co-electrodeposition - high temperature sulfurization methodology was adopted to grow CZTS on fluorinated tin oxide (FTO) back contact. Enormous loss in conductivity of FTO back contact is observed after sulfurization, causing reduced device efficiency. Increased resistivity of the FTO is due to outdiffusion process. A systematic study of resistivity of back contact at various sulfurization temperature and time is discussed. We propose and conduct various remedial measures for improved conductivity of back contact, specifically for CZTS based bifacial solar cell grown by aqueous solution based technique.
11:15 AM - **MM7.6
Progress in Oxide-based Electrochromics: Towards Roll-to-roll Manufacturing.
Claes Granqvist 1
1 Department of Engineering Sciences, The Angstrom Laboratory, Uppsala Sweden
Show AbstractElectrochromic devices allow the transmittance of visible light and solar energy to be varied between widely separated extrema. Such devices have large potential applications for energy efficient and comfort enhancing fenestration. A typical device has a centrally positioned electrolyte connecting two electrochromic oxide layers (one with anodic coloration and another with cathodic coloration), and this three-layer stack is positioned between thin films serving as transparent electrical conductors. The optical properties are modified when charge is shuttled between the electrochromic layers. The first part of this paper reviews recent progress regarding electrochromic materials, transparent conductors, and electrolytes with a focus on work in the author's research division. The second part of the paper discusses roll-to-roll manufacturing of electrochromic foil-based devices and presents experimental data. The paper is ended with some comments on the combination of electrochromics and thermochromics in order to produce "super windows" with optimized properties.
11:45 AM - MM7.7
Thermochromism of VO2 Nanoparticles: Calculated Optical Properties and Applications to Energy Efficient Windows.
Shuyi Li 1 , Gunnar Niklasson 1 , Claes Granqvist 1
1 Engineering Sciences, Uppsala University, Uppsala Sweden
Show AbstractThermochromic VO2 based films go through a reversible insulator-metal transition at a “critical” temperature of ~68 °C and become infrared reflecting above this temperature. This behavior is well known and has been widely discussed for temperature-dependent modulation of solar energy throughput of windows. However practical applications of VO2 based films for energy efficient windows have been severely hindered by the undesirably high luminous absorptance and the relatively low solar modulation due to the small solar intensity for the wavelengths at which the optical modulation is large. This work presents calculations based on effective medium theory applied to dilute composites of VO2 and VO2:Mg nanoparticles embedded in hosts representing glass or polymer. Computations were made with 1 vol. % of nanoparticles in layers of different thicknesses and for spherical and spheroidal particles with different orientations. The results show that the plasma absorption in the metallic phase of VO2 is tuned to the near infrared region where the solar intensity is high, and hence the modulation of the solar energy throughput is significantly enhanced and is more than twice as large as for the equivalent thin films. Moreover, the luminous transmittance is very much higher than for equivalent thin films. Composite layers incorporating VO2:Mg show even higher luminous transmittance. These results indicate that VO2-based “nanothermochromics” can yield high luminous transmittance and large modulation of solar energy throughput, which opens up new vistas for energy efficient fenestration.
12:00 PM - MM7.8
Computational Design of Dew ZnO-based Transparent Conducting Oxides.
Maria Stoica 1 , Mark Pellerite 2 , Cynthia Lo 1
1 Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis, St. Louis, Missouri, United States, 2 Display and Graphics Business Laboratory, 3M Company, St. Paul, Minnesota, United States
Show AbstractIndium Tin Oxide (ITO) is a widely-used transparent conductor that offers excellent performance in applications ranging from displays to photovoltaics, but its high cost and manufacturing limitations have spurred the search for alternative transparent conductors. While a wide range of binary, ternary, and even quarternary oxides are potential candidates, the sampling of all available permutations using experimental approaches would be prohibitive, both in terms of time and cost. Thus, we present a computational approach for rapidly screening new transparent conducting oxides. We rigorously calculate the electrical conductivity of the transparent conductor from the electronic band structure and density of states of the bulk unit cell, as obtained using electronic structure calculations following the density functional theory approach. We demonstrate the effectiveness of this approach on a well-characterized transparent conducting oxide: F-doped ZnO, where we have calculated the electrical conductivity as a function of F-dopant concentration and temperature. As experimentally-measured conductivities vary greatly with processing conditions, including the deposition method and location of the incorporated dopant (e.g., interstitial vs. substitutional), the computationally-derived conductivities may not necessarily be a quantitative match due to limitations of the density functional theory approach. However, we show that the computational trends provide predictive guidelines for controlling structure, composition, and processing conditions to achieve new transparent conducting oxides with optimal electronic and optical transport properties.
12:15 PM - MM7.9
Expressing an Old Material as a New TCO that Exhibits Broadband Transparency and Intrinsic Metallic Conductivity.
Debra Rolison 1 , Christopher Chervin 1 , Jeffrey Long 1 , Jeffrey Owrutsky 2 , Joseph Melinger 3
1 Surface Chemistry, U.S. Naval Research Laboratory, Washington, District of Columbia, United States, 2 Chemical Dynamics & Diagnostics, U.S. Naval Research Laboratory, Washington, District of Columbia, United States, 3 Solid State Devices, U.S. Naval Research Laboratory, Washington, District of Columbia, United States
Show AbstractWe recently demonstrated that depositing ~2-nm-thick shells of RuO2 onto silica dielectric cores (initially submicron-to-micron silica fibers in filter paper) creates an RuO2(SiO2) extended structure with only 0.1 vol.% of RuO2 that expresses the impressive properties of ruthenium dioxide: high electronic conductivity, high gravimetric capacitive charge storage for pulse power, and fast electron transfer for catalysis [1,2]. With 90% of the ruthenia expressed at the surface, this physically stabilized, air- and water-stable, metallically conductive oxide adds a non-graphene chemical system with which to probe the physics of long-range electron transport under dimensional confinement, but with control of conductivity over three orders of magnitude for the same morphology of ruthenia in the nanoskin and for aspect ratios that range from 10 to >106. We have also demonstrated the versatility of the benchtop-based synthetic protocol—a scalable, atom-efficient, liquid-phase, self-limiting growth process—to coat planar optical and spectroscopic substrates such as quartz, Si, and CaF2 with 2-nm-thick skins of RuO2. The RuO2-coated windows are broadband transparent over an atypically large range for an oxide: from UV to THz. The nanoparticulate coating also expresses low sheet resistance (~300 μΩ cm). Spectroelectrochemical experiments in the UV, Vis, and IR are readily accomplished using RuO2(CaF2) and will be discussed for nanometric overlayers of inorganic and organic redox-active species.[1] C.N. Chervin, A.M. Lubers, K.A. Pettigrew, J.W. Long, M.A. Westgate, J.J. Fontanella, D.R. Rolison, Nano Lett. 2009, 9, 2316. [2] C.N. Chervin, A.M. Lubers, J.W. Long, D.R. Rolison, J. Electroanal. Chem. 2010, 644, 155.
12:30 PM - MM7.10
Transparent Conductivity in Fluorine-doped Anatase TiO2 Epitaxial Thin Films.
Satoru Mohri 1 2 , Yashushi Hirose 1 2 , Shoichiro Nakao 2 , Naoomi Yamada 2 , Toshihiro Shimada 1 2 , Tetsuya Hasegawa 1 2
1 , University of Tokyo, Tokyo Japan, 2 , Kanagawa Academy of Science and Technology, Kawasaki Japan
Show AbstractAnatase TiO2 shows high optical transparency and good conductivity when the tetravalent Ti4+ ions are substituted by pentavalent cations such as Nb5+ or Ta5+. Substituting F- for O2- in anatae TiO2 is also expected to provide carrier electrons. Indeed, F- is known to work as an electron donor in the host materials of conventional transparent conducting oxides, such as SnO2 and ZnO. F-doping into TiO2 has not been attempted so far, mainly due to the difficulty in selecting an appropriate F source. In this study, we report on F-doping into PLD-grown antatase TiO2 epitaxial films using both gas phase and solid phase F sources.F-doped TiO2 (F:TiO2) films were fabricated by pulsed laser deposition (PLD) method. We examined two F sources: Firstly, TiO2 was deposited under a mixed gas atmosphere of O2 and CF4 (gas phase source). Secondly, TiF3 target was ablated under O2 atmosphere (solid phase source). In each case, a thin (~5nm) film of non-doped anatase TiO2 was epitaxially grown on lattice matched substrate (LaSrAlO4 or LSAT) as a seed layer prior to the growth of F:TiO2. Substrate temperature and partial pressures of process gases were optimized as growth parameters. The crystal structures of the F:TiO2 films were determined by X-ray diffraction (XRD) measurements. The fluorine contents were measured by X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS). Transport properties were evaluated by standard 6-probe resistance method.In the process using O2/CF4, (001)-oriented epitaxial anatase TiO2 films were obtained over a wide growth parameter range. However, F was not detected by XPS and the resistivity of the films was relatively high. In contrast, when starting with TiF3 target, anatase F:TIO2 was found to grow epitaxially without any impurity phases under the condition of P(O2) = 1 x 10-5 Torr and TS = 400 - 500 °C. In addition, XPS and SIMS measurements confirmed that F is almost uniform distributed in the films. The F:TiO2 film obtained under the optimized condition of TS = 400 °C showed resistivity as low as 1.6 × 10-3 Ωcm with high optical transparency. The electron density of the film was 5 × 1020 cm-3, which is an order of magnitude higher than the value of undoped anatase TiO2 films. Therefore, we concluded that F works as an electron donor in anatase TiO2 matrix. The activation rate of F is approximately 30% in a wide F content range of 5 × 1019 - 2 × 1021 cm-3.
12:45 PM - MM7.11
Orientation Dependence of the Work Function of Indium Oxide.
Mareike Hohmann 1 , Andre Wachau 1 , Peter Agoston 1 , Karsten Albe 1 , Andreas Klein 1
1 Materials Science, Darmstadt University of Technology, Darmstadt Germany
Show AbstractSn-doped In2O3 (ITO) is the most widely used electrode material in organic light emitting diodes and organic solar cells. Its work function, which determines the band alignment in contact to the organic materials, has been the subject of numerous investigations. Despite the use of polycrystalline ITO films, only a single value is typically given for the work function. This neglects the orientation dependence of the work function, which is well established for a number of materials. In this contribution we will present photoemission measurements of the work function of epitaxial In2O3 thin films grown on ZrO2:Y single crystal substrates by magnetron sputtering. The values are compared to experimental results for polycrystalline In2O3 and ITO films and to ab-initio density functional theory calculations.
MM8: Transparent Conducting Oxides and Applications III
Session Chairs
Wednesday PM, December 01, 2010
Constitution B (Sheraton)
2:30 PM - **MM8.1
Low Temperature Solution-processed Carrier Transport Layers for Organic and Hybrid Photovoltaic Devices.
Yun-Ju Lee 1 , Summer Ferreira 2 , Robert Davis 2 , Robert Copeland 2 , Diana Moore 2 , James Voigt 2 , Julia Hsu 1
1 Materials Science and Engineering, University of Texas at Dallas, Richardson, Texas, United States, 2 , Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractOrganic and hybrid photovoltaic (PV) devices represent promising routes toward low cost, lightweight solar energy conversion. It has been shown that the addition of thin hole or electron transport layers can enhance device performance. Here we present several examples of electron and hole transport layers deposited on top of the active layer using low temperature solution-based methods. The deposition of carrier transport layers on top of the active layer is challenging due to the thermal and chemical sensitivity of the active layer, imposing severe restrictions on the materials and techniques. For example, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is commonly used as hole transport layer because it can be solution deposited and processed at low temperature (120 °C). However, instability of PEPOT:PSS and its acidic nature have been attributed to device degradation. In addition, neat PEDOT:PSS does not wet organic films uniformly and the acidic solution attacks ZnO in an active layer. To potentially resolve these issues, we describe room temperature deposition of WO3 and MoO3 hole transport layers (HTLs) on top of poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) blends by spin coating butanolic solutions of the corresponding polyoxometalate anions, and examine the OPV device performance using these metal oxide HTLs benchmarked to standard PEDOT:PSS HTL. We also demonstrate the deposition of PEDOT:PSS hole transport layer on acid-sensitive P3HT:ZnO blends by increasing pH of the PEDOT:PSS solution, and study the effect of pH on PEDOT:PSS electrical properties and device performance. In addition, we explore the deposition of ZnO electron transport layers on organic and hybrid blends from sol-gel and nanoparticle precursors, and quantifies the improvement in device performance and stability in air. Advantages and disadvantages of these solution processed transport layers and their applicability to current and future devices will be discussed.
3:00 PM - **MM8.2
Transparent Oxide Conductors and Semiconductors. New Science and Potential Technologies.
Tobin Marks 1
1 Chemistry and Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States
Show AbstractControl of metal oxide compositions and film growth parameters affords broad and sometimes surprising tunability of many useful electrical, optical, and mechanical properties. In this lecture, three topics are briefly surveyed: 1) Polycrystalline and amorphous oxide semiconductors for the fabrication of transparent, mechanically flexible transistor-based electronic circuitry, 2) Oxides as enablers of high-performance electro-optic phase modulators, 3) Oxides as electrodes and interfacial layers for high-efficiency photovoltaic cells.
3:30 PM - MM8.3
Solution Deposited NiO as an Inorganic Hole Transport Layer for High Performance Organic Solar Cells.
K. Xerxes Steirer 1 2 , N. Edwin Widjonarko 1 3 , Andres Garcia 1 , Paul Ndione 1 , David Ginley 1 , Joseph Berry 1 , Dana Olson 1
1 NCPV, National Renewable Energy Laboratory, Golden, Colorado, United States, 2 Department of Physics, Colorado School of Mines, Golden, Colorado, United States, 3 Department of Physics, University of Colorado, Boulder, Colorado, United States
Show AbstractThe power conversion performance of organic solar cells is dependent upon the creation and subsequent collection of charges at the electrodes. High performance devices employ bulk heterojunction active layers with tunable photovoltaic properties. However, the morphology of the bulk heterojunction necessitates electrodes that are able to distinguish between charge carrier types and thus maintain directional current flow and high fill factors. This approach typically relies on careful choice of the electrode materials and the use of charge selective contacts. When holes are collected by the transparent electrode, then this contact layer is referred to as a hole transport layer (HTL). While various organic layers can be employed, such as poly(3-ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS) we present results using an inorganic oxide layer. Our results indicate that NiO acts as a tunable, deep work function contact layer which is especially advantageous when the bulk-heterojunction consists of a deep energy level donor. We demonstrate a NiOx HTL that is deposited from a solution precursor optimized to reduced series resistance and improved fill factors in such organic photovoltaic (OPV) devices. We compare NiOx to PEDOT:PSS HTLs to modify indium tin oxide anodes in bulk heterojunction solar cells and correlate open circuit voltage, short circuit current, and series resistance with device efficiencies of 5.7% and 6.7%, respectively. In order to probe the physical mechanisms for enhanced performance we fit device data to the Shockley diode model. Fits for PEDOT:PSS and NiOx give diode factors of 2.3 and 1.6 respectively indicating markedly improved diode performance and charge selectivity for NiOx in this particular architecture. We present a detailed band structure diagram explaining the improved operation of the NiOx hole transport layer.
3:45 PM - MM8:TCOs
Duplicate Abstract Was Here (MM5.3/F5.3)
Show Abstract4:30 PM - MM8.5
Laser Induced P to N-type Conductivity Transformation in NiO.
Pranav Gupta 1 , Titas Dutta 1 , Jagdish Narayan 1
1 MSE, 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 transformation of conductivity type in NiO thin films grown by pulsed laser deposition technique from p to n-type through irradiation with a UV laser. The characteristics of the films have been investigated in detail using XRD, TEM, and electrical and optical property measurements. XRD results show NiO (111) grows along the c-axis on α-Al2O3. The epitaxial growth of [111] NiO on [0001] sapphire was confirmed by using high resolution X-ray Φ scan. Post irradiation, a small change in the lattice parameter was observed from the X-ray diffraction, though the crystalline orientation of the thin film remained the same. The electrical conductivity of the NiO film increased significantly by ~3 orders of magnitude. The change in the type of major carrier concentration was confirmed using the Hall measurement. This was accompanied by reduction in transmittance from 90% to 70%, as observed in the film thickness of ~ 250 nm. The n-type conductivity was observed to be stable after 30 days post-irradiation. Mechanism behind the modified electrical and optical properties when subjected to Laser irradiation will be presented.
4:45 PM - MM8.6
Self-organised Transparent and Flexible Metal Nanowire Electrodes.
Francesco Buatier de Mongeot 1 , Daniele Chiappe 1 , Andrea Toma 1 , Corrado Boragno 1 , Christian Martella 1
1 Dipartimento di Fisica, Universita di Genova, Genova Italy
Show AbstractHere we report on a self-organised approach for the synthesis of laterally ordered arrays of noble metal nanowires supported on transparent substrates (glass slides or flexible polymer films) which feature a dual functionality [1]: (i) they exhibit anisotropic conductivity along the nanowire axis, with sheet resistances comparable to the best transparent conductive oxides (TCOs), (ii) the noble metal nanowires support localised plasmon resonances which can be tuned by modifying the nanowire morphology. The nanowire network appear thus as alternatives to TCO’s endowed with the additional plasmonic functionality in view of Plasmon enhanced photon harvesting applications [2].The formation of periodic metal nanostructures is induced by self-organized Ion Beam Sputtering (IBS) of polycrystalline Ag and Au films supported on soda lime glass slides or on flexible polymer films. A defocused broad beam ion source destabilizes in a self-organised way the surface with the formation of a regular nanoscale pattern extended over a macroscopic scale (cm^2 range). Regular arrays of ripples with a wavelength in the 100 nm range are formed by the competition of a smoothing term induced by thermally activated diffusion and by the destabilising erosive action of the ion beam [3]. By increasing the ion dose, the rippled metal film decomposes into an array of disconnected nanowires when the troughs of the valleys reach the supporting glass substrate [4]. Alternatively the metal nanowires are agglomerated on pre-structured substrates [5]. The metal nanowire arrays exhibit anisotropic resistivity along the nanowire direction, with low sheet resistances in the 3-5 Ohm/square range [2]. Given their low sheet resistances, and high optical transparency in the 80 % range, the nanowire arrays appear as promising alternatives to the best TCOs employed in photovoltaic or OLED applications. Additionally, far-field optical characterisation demonstrates that the nanostructured surfaces exhibit a tunable plasmonic response which matches the solar spectrum [4,5], a crucial issue in view of photovoltaic applications.[1] “Plasmonics for improved photovoltaic devices” H.A. Atwater and A. Polman Nature Materials 9, 205 (2010)[2] F. Buatier de Mongeot, C. Boragno, U. Valbusa, D. Chiappe and A. Toma, International Patent PCT Pub. (2009) No.:WO/2009/109939. [3] A. Toma, D. Chiappe, B. Šetina Batič, M. Godec, M. Jenko, F. Buatier de Mongeot Physical Review B 78, 153406 (2008)[4] A. Toma, D. Chiappe, C. Boragno, and F. Buatier de Mongeot, Phys. Rev. 81, 165436 (2010)[5] A. Toma, D. Chiappe, D. Massabò, C. Boragno, and F. Buatier de Mongeot Applied Physics Letters 93, 163104 (2008)
5:00 PM - MM8.7
Fabrication of Amorphous and Crystalline In-Zn-Sn-O Thin Film Using Radio Frequency Magnetron Sputtering for OPV.
Ajaya Sigdel 2 1 , N. Edwin Widjonarko 3 1 , Paul Ndione 1 , Andres Garcia 1 , Dana Olson 1 , Thomas Gennett 1 , Sean Shaheen 1 , John Perkins 1 , David Ginley 1 , Joseph Berry 1
2 Department of Physics and Astronomy, University of Denver , Denver, Colorado, United States, 1 , National Renewable Energy Laboratory (NREL) , Golden, Colorado, United States, 3 Department of Physics, University of Colorado, Boulder, Colorado, United States
Show AbstractTransparent conducting oxides (TCOs) with high work function, greater stability, yet lower fabrication cost compared to traditional materials such as indium tin oxide (ITO) are of great interest for a variety of opto-electronic device applications. Here, we present studies of varying surface morphology of a ternary oxide Indium-Zinc-Tin-Oxide (IZTO) films and its correlation with the organic photovoltaic (OPV) device performance. We will present a comparative study of OPV devices incorporating high conductivity amorphous and crystalline IZTO films as stand alone contacts and in combination with other functional oxides such nickel oxide (NiO). Additionally, the potential for modulating the carrier concentration of IZTO for use as a selective contact on other TCOs such as ITO and gallium doped zinc oxide (GZO) will also be explored. IZTO films with varying morphology were deposited by radio frequency magnetron sputtering using target of Zn: In: Sn cation ratio of 1:2:1. Crystalline IZTO films have conductivity (σ) of 2260 S/cm with rms surface roughness less than 2 nm. Amorphous IZTO films have σ of 1470 S/cm with surface roughness less than 0.2 nm. The films have tunable work functions ranging from 4.9 eV to as high as 5.4 eV. Using a standard device architecture of IZTO/NiO/BHJ/Ca/Al where the BHJ is P3HT:PCBM or similar polymer/fullerene system, our preliminary results indicate that the surface morphology and crystallinity of the TCO interface have varying degree of interactions with the polymer layer. Investigation with conductive atomic force microscopy (C-AFM) showed amorphous IZTO to require larger applied tip to sample biases, similar to amorphous indium-zinc-oxide and zinc-tin-oxide films, which correlated with poor OPV device performance with these oxides. C-AFM study of crystalline IZTO shows surface conductivity superior to both ITO and amorphous IZTO but less uniform conductivity compare to GZO. Use of this deep work-function IZTO material in OPV devices will be presented and correlated with the scanning probe measurements.
5:15 PM - MM8.8
Synthesis of Colloidal Aluminum-doped Zinc Oxide Nanoparticles.
Raffaella Buonsanti 1 2 , Guillermo Garcia 3 2 , Jeff Zink 4 , Delia Milliron 2
1 , UC-CEIN, UCLA, Los Angeles, California, United States, 2 , The Molecular Foundry- Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 , UC Berkeley, Berkeley, California, United States, 4 Department of Chemistry and Biochemistry, University of California, Los Angeles, California, United States
Show AbstractAluminum-doped zinc oxide (AZO) is one of the most promising alternatives to replace the most commonly used indium tin oxide (ITO) since comparable electrical and optical performance has been demonstrated and the material cost is lower. Reducing production cost is becoming a critical issue considering the ever-increasing demand for flat-panel displays, solar cells and smart windows. An opportunity exists to both improve the performances and reduce the cost of these optoelectronic devices through the integration of nanocrystals (NCs), which can exhibit peculiar size-dependent chemical-physical properties as well as a larger surface area in comparison to their bulk and thin film counterparts.[1,2] Herein, we report a colloidal wet-chemical approach for preparing highly crystalline AZO NCs. Wet-chemistry represents a low cost alternative production technology compared to traditional techniques such as sputtering or evaporation, and NCs have the advantage over sol-gel methods that they are already crystalline before film deposition, reducing the thermal budget required for annealing.[2,3] By a careful manipulation of the reaction condition, we have reached a high level of control on both the size and the doping content of AZO NCs. A growth mechanism is proposed which is consistent with FT-IR measurements. The NC chemical and structural identity has been investigated by a combination of different techniques such as powder X-ray Diffraction (XRD), high resolution transmission electron microscopy (HRTEM) analysis, energy dispersive spectroscopy (EDS), inductively coupled plasma atomic emission spectroscopy (ICP-AES). Optical properties have been assessed by UV-VIS-NIR absorption spectroscopy. Finally, we also briefly outline the technological potential of this promising material by showing some preliminary results on the integration of these novel AZO NCs in an electrochromic device of interest for smart windows.1. Roduner, E. Chem. Soc. Rev. 2006, 35, 583-592.2. A. Rogach, D.V. Talapin, H. Weller “Applications of semiconductor nanocrystals” in Colloids and Colloidal Assembly, F.Caruso Editor, 2004, Wiley-VCH Verlag GmbH&Co3. Peng, X. Nano Res. 2009, 2, 425-447
MM9: Poster Session: Transparent Conducting Oxides and Applications IV
Session Chairs
Thursday AM, December 02, 2010
Exhibition Hall D (Hynes)
9:00 PM - MM9.1
Fabrication of Alumina and Scandia Doped Zirconia Thin Films by Pulsed Laser Deposition.
Muthukkumaran Karthikeyan 1
1 Science and humanities, MIT,Anna University, chennai India
Show AbstractAlumina and scandia doped zirconia is prepared through a soft chemistry synthesis route and sintered at 1873 K. X-ray diffraction patterns indicate a pure cubic phase, for the compositions 0.88 ZrO2- (0.12-x) Sc2O3 – xAl2O3 (0.006 ≤ x ≤ 0.008). Thin films are fabricated on Al2O3 <0001> substrates using pulsed laser deposition technique. Dense films of 1.0 micron thickness were obtained at 873 and 1023 K substrate temperatures at a pressure of 0.15 mbar oxygen. The ionic conductivity of both thin film and bulk was measured using AC impedance spectroscopy in air. The conductivity values are higher for thin films compared to that of bulk.
9:00 PM - MM9.10
Controlled Growth of Monodisperse Tin (IV) Oxide Nanocrystallites in an Amorphous Matrix on Glass or Si Substrates.
Samad Bazargan 1 , Nina Heinig 1 , Debabrata Pradhan 1 , Kam Leung 1
1 WATLab and Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada
Show AbstractUsing a simple spin-coating method of a SnCl2.5H2O isopropanol precursor solution followed by post-annealing in oxygen at different temperatures for 1 h, we demonstrate that high-quality nanocrystalline films of tin oxide can be grown on glass and Si substrates. X-ray photoelectron spectroscopy indicates the presence of tin (IV) oxidation state and confirms the formation of SnO2 over the entire film. Transmission electron microscopy and X-ray diffraction studies further show an onset at 350°C of the formation of nanocrystalline SnO2 in a largely amorphous SnO2 film. Of special interest is that the growth of nanocrystallites in a narrow size range of 7-10 nm can be achieved over the entire thickness of the films prepared at 350-500°C, with a larger density near the surface, as illustrated by our depth-profiling X-ray diffraction studies. The size and density of these nanocrystallites do not appear to depend on the amount of post-oxygen-anneal time (24-96 h at 400°C). The remarkable size stability of these nanocrystallites along with the observed inherent strain in these nanocrystallites favours the mechanism of strain-limited growth as a result of change in the density upon crystallization. Use of a higher annealing temperature (600-900°C) for preparing the SnO2 films on oxidized Si substrates leads to an increase in the volume of the nanocrystalline phase, concomitant with an exponential growth in the nanocrystallite size to 30 nm. This suggests a second onset at 500°C, at which a transition from a strain-limited growth regime to an Arrhenius (exponential) growth regime occurs. The facile fabrication and stability of the nanocrystallites in a desirable size regime (7-30 nm) promises a cost-effective, easily scalable fabrication method of SnO2 nanofilms for gas-sensing and nanoelectronic applications.
9:00 PM - MM9.11
First Principles Study of Surfaces in In2O3.
Peter Agoston 1 , Karsten Albe 1
1 Department for Material and Earth Sciences, University of Technology Darmstadt, Darmstadt, Hessen, Germany
Show AbstractWe present a comprehensive first-principles study on surfaces of In2O3, an important transparent conducting oxide (TCO). On the basis of density-functional theory total energy calculations we have determined the surface energies for (111), (100), (110) and (112) faces with various terminations and stoichiometries. Our findings show that the stoichiomety of the (100) surface strongly depends on variations of the external chemical potential, while other orientations, like the (111) remain structurally and chemically stable. Since the surfaces energies of all orientations are very similar, a large variation of grain orientations in sputtered samples is likely. The surface stoichiometry in turn shows a pronounced impact on the ionization potential of the material which we have calculated for all stable surface configurations. We discuss the implications of our results on the injection properties of In2O3 based anodes in conjunction with organic semi-conductors and on the sensor activity of this material.
9:00 PM - MM9.12
High mobility, High NIR Transparency and Wide Work Function Indium Molybdenum Oxide Thin Films RF Sputtered at Room Temperature.
Elangovan Elamurugu 1 , Parthiban Shanmugam 1 , Rodrigo Martins 1 , Elvira Fortunato 1
1 Materials Science, FCT-UNL, Caparica Portugal
Show AbstractIndium molybdenum oxide thin films were RF sputtered at room temperature (RT) and studied as a function of base pressure (BP) ranging 1.0 - 6.0×10-4 Pa. The crystalline structure obtained from a BP of 1.0×10-4 Pa become amorphous at high BP. The atomic force microscope analysis authenticated the evolution of surface microstructures. The obtained maximum mobility (~50 cm2V-1s-1 ) is the best reported so far for these films deposited at RT. The films show a wide work function of ~4.91 eV, with high visible transmittance over ~80 % and near infrared transmittance above ~70 %.
9:00 PM - MM9.13
Synchrotron X-ray Measurements of Local Structure in Amorphous In-Zn-O Thin Films.
Brandon Reese 1 2 , J. Leisch 1 , P. Parilla 1 , N. Widjonarko 1 , T. Gennett 1 , J. Perkins 1 , D. Ginley 1 , D. Miracle 3 , M. Toney 4
1 , National Renewable Energy Laboratory, Golden, Colorado, United States, 2 Electrical, Computer, and Energy Engineering, University of Colorado, Boulder, Colorado, United States, 3 Materials and Manufacturing Directorate, Air Force Research Laobratory, Dayton, Ohio, United States, 4 , Stanford Synchrotron Radiation Lightsource, Menlo Park, California, United States
Show AbstractIn amorphous In-Zn-O (a-IZO), the conductivity can be varied over ~7 orders of magnitude by varying the oxygen partial pressure during deposition or by using a relatively low temperature thermal annealing process (T < 200 °C). There is also a wide composition range in which the films maintain an amorphous state. Here, we report on measurements of the local structural order in various a-IZO thin film samples with the aim of determining the local atomic structures, which control the electrical conductivity in a-IZO. The a-IZO thin film samples were grown by sputtering from ceramic targets with compositions ranging from 70:30 to 90:10 wt.% In2O3:ZnO. The substrate temperature was varied from room temperature to 600 °C to obtain samples that ranged from amorphous to crystalline. We performed extended X-ray absorption fine structure (EXAFS) and grazing incidence wide-angle X-ray scattering (GI-WAXS) experiments at the Stanford Synchrotron Radiation Lightsource. Both of these methods are well suited to studying amorphous systems and we are using the relative strengths of each measurement to compliment the other. We find that the first atomic shell of the In and Zn atoms closely resembles the structure of the pure crystalline oxides of In and Zn. However, the second coordination shell of these films shows almost no local ordering, indicating that these amorphous films are randomly oriented metal-oxygen polyhedra. This experimental finding agrees well with a recent theoretical study [1]. As the substrate temperature is increased, the ordering the second coordination shell becomes much more apparent. We are also attempting to model these oxide films using a model developed for metallic glasses that has proved quite successful in explaining their properties [2]. By using these measurement methods and models, we hope to elucidate how the structure and material properties of these films change with composition while still maintaining an amorphous nature.[1]A. Walsh, J. Da Silva, and S.-H. Wei, "Interplay between Order and Disorder in the High Performance of Amorphous Transparent Conducting Oxides," in Chemistry of Materials. vol. 21, 2009, pp. 5119-5124.[2]D. B. Miracle, T. Egami, K. M. Flores, and K. F. Kelton, "Structural Aspects of Metallic Glasses," MRS Bulletin, vol. 32, pp. 629-634, Jan 1 2007.
9:00 PM - MM9.14
Atom Probe Tomography of Cr-doped In2O3.
Emmanuelle Marquis 1 , David Payne 2
1 Department of Materials, University of Oxford, Oxford United Kingdom, 2 Inorganic Chemistry Laboratory, University of Oxford, Oxford United Kingdom
Show AbstractDilute Magnetic Oxides (DMOs) have recieved widespread attention due to their ability to display room temperature ferromagnetism. Since 2001, when Co-doped TiO2 was found to display this phenomenon,1 a large number of other oxide materials have appeared in the literature claiming similar properties. Unfortunately, there remains considerable doubt as to whether this phenomenon actually exists at all, due to the above mentioned effect only being obeserved in thin films, and not reproducable in "bulk" ceramic samples.In 2006, thin films of Cr- and Cr/Sn-doped In2O3 were shown to display room temperature ferromagnetism.2 It has also been demostrated that no ferromagnetism is observed in materials prepared via traditional solid state (i.e. bulk) synthesis.3 It therefore remains a considerable challenge to fully understand the reasons why these discrepencies occur.Atom Probe Tomography (APT) is a technique which is capable of mapping the 3 dimensional position and chemical identity of individual atoms, with sub-nanometer resolution. This technique therefore provides a unique opportunity to study the spatial dispersion of magnetic dopants in host oxide materials. In this study we have mapped the 3D distribution of Cr atoms in In2O3 from thin film samples, prepared via e-beam evaporation of prepared pellets of Cr-doped In2O3. It is found that there is a considerably lower concentration of Cr present in the films than from the starting material, and that the Cr in the films is found to be located preferentially on grain boundaries. The consequences of these results will be discussed in further detail, as well as a general discussion of the future applications, and impact, that APT will have on the understanding of DMOs.1 Y. Matsumoto et al. Science 291 (2001) 854.2 J. Philip et al. Nature Materials 5 (2006) 298.3 L. Bizo et al. Advance Functional Materials 18 (2008) 777.
9:00 PM - MM9.15
Novel Transparent Conductive Materials: Prediction and Optimization.
Yaou Song 1 , Julia Medvedeva 1
1 Physics, Missouri University of Science & Technology, Rolla, Missouri, United States
Show AbstractBased on our previous extensive experience in the Transparent Conductive Oxide (TCO) area, we investigated several classes of materials including metal oxides, nitrides, selenides, sulfides and others, using first-principles electronic band structure simulations. In search for novel n-type transparent conductors, we performed electronic band structure calculations for the compounds with same-group next-period cation or anion. For example, vertical chains of compounds Al2O3→Ga2O3→In2O3→Tl2O3, or In2O3→In2S3→In2Se3→In2Te3, etc were systematically considered. First, the electronic band structures were calculated for a number of potential materials. To determine the phase stability, the formation energy was also calculated. Then possible carrier generation mechanisms was modeled for the most promising candidates (sufficient band gap values, small hole effective mass). Meanwhile, a database of electronic band structure properties is created and made available to other researches in the field and the related areas of the materials science.
9:00 PM - MM9.16
Polarity Inversion of Al-doped ZnO Films.
Yutaka Adachi 1 , Naoki Ohashi 1 , Isao Sakaguchi 1 , Shigenori Ueda 1 , Yoshiyuki Yamashita 1 , Hideki Yoshikawa 1 , Keisuke Kobayashi 1 , Hajime Haneda 1
1 , National Institute for Materials Science, Tsukuba Japan
Show Abstract ZnO has received a great deal of attention due to the wide range of its technological applications. Since a ZnO crystal shows spontaneous electrical polarization along the c-axis, it is important for designing devices using ZnO to develop a crystal growth technology to enable the polarity selective growth of ZnO films. Recently, we reported that Al-doping into ZnO led to the polarity inversion of the ZnO films on sapphire substrates. In this work, we examined the effect of Al-doping on the crystalline polarity of ZnO films grown on glass substrates, aiming to achieve the polarity-selective growth of the ZnO films on non-crystalline substrates. We first, demonstrated that hard X-ray photoelectron spectroscopy (HX-PES) is an appropriate method for determining the crystalline polarity of ZnO. Then, we characterized ZnO films grown by pulsed laser deposition (PLD) using HX-PES. The valence band HX-PES spectra of the undoped film on the glass substrate was similar to that of the (000-1) face for the ZnO single crystal, whereas the 1 mol% Al-doped film on the glass substrate showed a similar spectral feature compared with the (0001) face of the ZnO single crystal. This result indicates that the polarity of the ZnO films on the glass substrate changed from the (000-1) face for the undoped film to the (0001) face for the heavily Al-doped film. Since a clear polarity change due to Al-doping was seen at the ZnO/glass structure, we can conclude that the essential parameter governing the polarity of the ZnO films is unlikely lattice matching at the hetero interface between the ZnO films and substrates.
9:00 PM - MM9.17
Reactively Sputtered Thin Films of Amorphous GaCuZnO: The Question of Midgap Trap States.
Mohammad Ebdah 1 , Martin Kordesch 1
1 Department of Physics and Astronomy, Ohio University, Athens, Ohio, United States
Show AbstractAmorphous gallium copper zinc oxide (a-GaCuZnO) thin films were successfully fabricated using radio frequency (RF) reactive magnetron sputtering technique with sputtering targets of polycrystalline Ga, Cu, and Zn species under the flow of Oxygen and Argon and were deposited onto fused silica substrates at room temperature. The structure and composition of the films have been investigated by X-ray diffraction (XRD), and energy dispersive x-ray (EDX) spectroscopy techniques, respectively. The XRD patterns reveal that the sputtered films are amorphous. The optical absorptions of the films were obtained using spectrophometry (SP) technique in the wavelength range (200 - 800) nm. Analysis of the absorption coefficients, using the Tauc linear extrapolation, gives two linear regions; an optical bandgap of 2.30 eV, and a lower gap associated with midgap trap states of 1.70 eV. Annealing the films in air at 600 C for one hour shows that the as-deposited films sustain their amorphous structure after annealing. The corresponding optical bandgap and midgap after annealing are 2.75 and 2.10 eV, respectively. The increase in the observed gaps is attributed to the incorporation of oxygen in the oxidized films. Finally, I-V measurement for characterizing the surface resistivity has been made using a mercury probe of a central dot and ring contacts, and shows a resistivity of 12 ×10-6 (Ω.m) of the as-deposited films, while the annealed films behave as insulators as a result of oxidizing the films, which explains the role of oxygen in increasing the observed gaps after annealing.
9:00 PM - MM9.18
Thermophysical Properties of Al-doped ZnO Films with a Sub-micron Thickness.
Nobuto Oka 1 , Kentaro Kimura 1 , Takashi Yagi 2 , Naoyuki Taketoshi 2 , Tetsuya Baba 2 , Yuzo Shigesato 1
1 , Graduate School of Science and Engineering, Aoyama Gakuin University, Sagamihara, Kanagawa Japan, 2 , National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki Japan
Show AbstractAl-doped ZnO (AZO) thin film has recently attracted much attention as possible substitutes for Sn-doped In2O3 (ITO) and In2O3-ZnO (IZO) films which have been widely used as transparent conductive electrodes in various optoelectronic applications [1]. A thermal design for such devices is quit important since heat generation in the device could damage itself [2]. It needs the highly accurate thermophysical properties for transparent conductive electrodes. In this paper, the thermal diffusivity of 200-nm-thick AZO films was investigated using a “rear heating/front detection type” nanosecond thermoreflectance system [2, 3]. AZO films and Mo/AZO/Mo three-layered films were fabricated on fused silica substrates heated at 200°C or 300°C by dc magnetron sputtering using a high dense ceramic AZO target (Al2O3: 2.5 wt.%) and Mo metal target. During the deposition of the AZO film, Ar-O2 or Ar-H2 mixture gases were used in order to control the electrical conductivity of the AZO films. The resistivity, carrier density, and Hall mobility of the AZO films ranged from 5.9×10-4 to 5.5×10-1 Ω cm, from 3.2×1018 to 5.0×1020 cm-3, and from 3.5 to 22 cm2 / V s, respectively. The thermal diffusivity was (2.4 - 1.6)×10-6 m2/s and the corresponding thermal conductivity was found to be proportional to the electrical conductivity. The contribution of electrons to the thermal transport can be described by the Wiedemann-Franz law. On the other hand, the thermal conductivity contributed by phonons was almost constant against the electrical conductivity, larger than that in polycrystalline ITO and amorphous ITO, IZO films.[1] T. Minami, Y. Ohtani, T. Miyata, T. Kuboi, J. Vac. Sci. Technol. 1172, A25 (2007).[2] T. Baba, Proceedings of the Tenth International Workshop on Thermal Investigations of ICs and Systems, Sophia Antipolis, France (2004) 241.[3] T. Yagi, K. Tamano, Y. Sato, N. Taketoshi, T. Baba, and Y. Shigesato, J.Vac. Sci. Technol. A 23 (2005) 1180.ACKNOWLEDGMENTSThis work was supported by the New Energy and Industrial Technology Development Organization (NEDO) as a project of “Development of High-efficiency Lighting Based on the Organic Light-emitting Mechanism.”
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Analyses on IGZO Films Deposited by dc Magnetron Sputtering with H2O Introduction Using Hard X-ray Photoelectron Spectroscopy (HAXPES).
Nobuto Oka 1 , Takafumi Aoi 1 , Ryo Hayashi 2 , Hideya Kumomi 2 , Takeharu Sugiyama 3 , Eiji Ikenaga 3 , Yuzo Shigesato 1
1 , Graduate School of Science and Engineering, Aoyama Gakuin University, Sagamihara, Kanagawa Japan, 2 , Canon Research Center, Canon Inc., Tokyo Japan, 3 , Japan Synchrotron Radiation Research Institute (JASRI), Sayo-gun, Hyogo Japan
Show AbstractRecently, flexible thin film transistors (TFTs) have been studied powerfully. The thin film layers for the flexible TFTs including semiconductor layers should be formed at a temperature which is lower than the softening temperature of a flexible polymer substrate. In addition, in order to achieve high stability of TFT characteristics, carrier density of semiconductor layer should be controlled precisely. It was reported that the carrier density could be controlled to be less than 1016 cm-3 for amorphous indium-gallium-zinc-oxide (IGZO) deposited by magnetron sputtering at room temperature and the high performance TFTs was already demonstrated using amorphous IGZO films as the semiconductor layers deposited on plastic substrates by the sputtering method. [1] We introduced H2O into the magnetron sputter deposition process and analyzed how the H2O partial pressure during the deposition affects the electrical properties of the IGZO films. IGZO films were deposited on unheated fused silica glass substrates by dc magnetron sputtering using an InGaZnO4 target. Sputtering power was kept at 50 W. Total gas pressure of the mixture of Ar and H2O gases was maintained at 0.5 Pa during the deposition. H2O partial pressure was varied quantitatively using a precise needle valve with monitoring partial pressure of H2O using quadrupole mass spectrometer (QMS). Incorporation of H in the films was investigated by depth profiles of the hydrogen content, which was analyzed in detail by a secondary ion mass spectroscopy (SIMS). Electrical properties of the films was analyzed by the four-point probe method and hall effect measurement. As a result, it is demonstrated that electrical conductivity of IGZO films can be controlled precisely by controlling the H2O partial pressure during the depositions. It should be noted that, the carrier density of the IGZO films was largely reduced at H2O partial pressure above 1.0×10-2 Pa, indicating the possibility for controlling device characteristics. The TFT using the IGZO film showed the sufficient operations with on-to-off current ratios larger than 105. [2] The chemical state of In, Ga, Zn, O and the valence band state were observed in detail by hard X-ray photoelectron spectroscopy (HAXPES), where the effects of the H2O introduction were analyzed clearly. The synchrotron radiation experiments were performed at the BL47XU in the SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No.2009B1023).[1] H.Yabuta, M.Sano, K.Abe, T.Aiba, T.Den, H.Kumomi K.Nomura, T.Kamiya, H.Hosono, Appl. Phys. Lett. 89 (2006) 112123.[2] T. Aoi, N. Oka, Y. Sato, R. Hayashi, H. Kumomi, and Y. Shigesato, Thin Solid Films 518 (2010) 3004.
9:00 PM - MM9.20
Epitaxial ZnO Film Growth on Si Using MgO Buffer Layer.
Hak Ki Yu 1 , Jong-Lam Lee 1
1 Material Science & Engineering, POSTECH, Pohang, Kyungbuk, Korea (the Republic of)
Show AbstractA ZnO based semiconductors have been viewed as a promising candidate for developing UV/blue light emitting diodes and laser diodes in the next generation due to its large direct band gap of 3.37 eV and exciton binding energy of 60 meV at room temperature (RT). Because the Si substrate has advantage in low costs, high crystal quality, and the opportunity of integrating Si technology with ZnO based optoelectronic devices, a lot of studies have been tried to grow high quality ZnO film on Si. However, large lattice mismatch (15.4%), large difference in thermal expansion coefficients, easy oxidation of Si surface, and the formation of silicides even at RT cause difficulty in epitaxial growth of ZnO on Si. In other to improve the crystallinity of ZnO on Si, intermediate layers have been used as buffer. Firstly, non-oxide buffer layers including wurtzite (GaN, AlN) and zinc blende (ZnS) had been tried due to similar lattice structure with ZnO. However, those materials have demerits in price competitiveness and stability as a buffer layer for oxide growth. Recently, cubic based bixbyite crystals such as Y2O3, Sc2O3, and Lu2O3 are extensively studied due to strong formation enthalpy than SiO2. However, bixbyite crystal has 1/4 natural tetrahedral vacancies and these empty sites could act as a diffusion path of oxygen during high temperature ZnO growth. MgO is a simple ionic oxide with the rock-salt structure, high chemical resistance, good insulating properties, and abundance in nature. Due to these merits, MgO has been used as a buffer for functional oxide; especially MgO has low lattice mismatch with ZnO about 9%. Strong polarity of MgO (111) orientation induced by alternating Mg2+ plane and O2- plane could enhance not only ZnO growth with strong polarity; (002) orientation but also the atomic arrangement Si-O-Mg-O sequence to achieve stable energy configuration, resulting in clear interface with Si.In this letter, we report the growth and properties of epitaxial ZnO films on Si (111) substrates using MgO as a buffer layer. Although Si (111), MgO (111), ZnO (002) have close packed atomic structures commonly, MgO (111) on Si (111) has large lattice mismatch about 22.4 %. In systems with large lattice mismatch, epitaxial growth of thin films is made by matching domains where integral multiples of major lattice planes match across the interface. In our result, domain matched structure with 9/7 of MgO/Si (equal mixture of 4/3 and 5/4 domains) is formed at the interface to achieve the lowest domain mismatch; experimentally confirmed by HR-TEM and Fourier filtering. In theta-2theta, no impurity or polycrystalline ZnO peaks were detected except the epitaxial ZnO (002) for the film on MgO buffered Si, whereas additional ZnO (102) peak was shown for the film on bare Si substrate. Phi-scan analysis across the off-normal ZnO {101}, MgO {200}, and Si {004} was fulfilled and confirmed the epitaxial relationships as (0001)[1-210]ZnO∥(111)[1-10]MgO∥(111)[1-10]Si.
9:00 PM - MM9.21
Low Temperature Processed Highly Conducting, Transparent, and Wide Bandgap Gadolinium Doped CdO Thin Films for Optoelectronics.
R. Gupta 1 , K. Ghosh 1 , R. Patel 2 , P. Kahol 1
1 Physics, Astronomy, and Materials Science, Missouri State University, Springfield, Missouri, United States, 2 Roy Blunt Jordan Valley Innovation Center, Missouri State University, Springfield, Missouri, United States
Show AbstractGadolinium (Gd) doped cadmium oxide (CdO) thin films are grown at low temperature (100 °C) using pulsed laser deposition technique. These films are deposited by ablating the sintered target of cadmium oxide containing 2 at% of gadolinium with a KrF excimer laser (λ = 248 nm and pulsed duration of 20 ns). The effect of oxygen partial pressure on structural, optical, and electrical properties are studied. X-ray diffraction studies revealed that these films are polycrystalline in nature with preferred orientation along (111) direction. Atomic force microscopy studies showed that these films have very smooth surface with maximum root mean square roughness of 0.77 nm. The films are highly transparent and transparency of the films increases with increase in oxygen partial pressure. We observe an increase in optical band gap of cadmium oxide films by gadolinium doping. The maximum optical band gap of 3.4 eV is observed for films grown at 1 × 10-5 mbar of oxygen pressure. Oxygen partial pressure strongly affects the electrical properties too. We observed that the electrical resistivity of the films first decreases and then increases with an increase in oxygen partial pressure. Similar trend is observed for mobility of the films. On the other hand, carrier concentration of the films decreases with an increase in oxygen partial pressure. The lowest electrical resistivity of 2.71 × 10-5 Ω.cm and highest mobility of 258 cm2/Vs is observed. These low temperature processed highly conducting, transparent, and wide bandgap semiconducting films could be used for flexible optoelectronics devices. Detailed study based on effect of partial oxygen pressure on structural, optical, and electrical properties of gadolinium doped cadmium oxide films will be presented.
9:00 PM - MM9.3
Effect of Growth Conditions on Electronic and Structural Properties of GZO Films Grown by Plasma-enhanced Molecular Beam Epitaxy on p-GaN(0001)/Sapphire Templates.
Huiyong Liu 1 , Vitaliy Avrutin 1 , Natalia Izyumskaya 1 , Michael Reshchikov 2 , Stephen Wolgast 3 , Cagliyan Kurdak 3 , Andrew Yankovich 4 , Alexander Kvit 4 , Paul Voyles 4 , Umit Ozgur 1 , Hadis Morkoc 1 2
1 Electrical and Computer Engineering, Virginia Commonwealth University, Richmond, Virginia, United States, 2 Physics , Virginia Commonwealth University, Richmond, Virginia, United States, 3 Physics, University of Michigan, Ann Arbor, Michigan, United States, 4 Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States
Show AbstractZnO heavily doped with Ga (GZO) is a promising material for transparent contacts in GaN-based devices, such as light emitting diodes (LEDs) and vertical-cavity surface-emitting-laser (VCSEL). For these applications high conductivity and high optical transmittance of GZO films are required. In addition, it is important to control surface morphology of GZO films, since rough surface is desired for better light extraction in LEDs, while flat GZO surface is needed for VECSEL applications. This was a motivation to study the effect of growth conditions to achieve desired physical properties and surface morphology of GZO films.In this work, GZO films were grown on (0001)GaN/c-sapphire templates by plasma-enhanced molecular beam epitaxy. Crystal structure of the layers was investigated by transmission electron microscopy (TEM) and x-ray diffraction (XRD). Effect of substrate temperature and surface roughness of the GaN template on surface morphology of GZO films was studied atomic force microscopy (AFM). Electrical properties of the layers were studied by temperature-dependent Hall effect measurements using the van der Pauw configuration, and optical properties were examined by photoluminescence (PL) and transmittance measurements. The room-temperature Hall mobility as high as 90 cm2/V-s was achieved for a GZO layer with carrier concentration of ~5x10^20 cm^-3 grown in two-dimensional mode. The temperature dependence of Hall mobility of GZO/p-GaN was found to be not consistent with those reported in the literature. The unusual temperature dependence of the Hall mobility was investigated by numerical simulations assuming different scattering mechanisms. The optimized growth conditions were utilized to fabricate GZO current spreading layers for InGaN LEDs and their current-voltage characteristics and electroluminescence were tested.
9:00 PM - MM9.4
Spin Electronics and Spin Photonics on Transparent Conducting Oxides -Surface Plasmons and Frequency Matching in ZnO Quantum Wells.
Hitoshi Tabata 1 , Hiroaki Matsui 1
1 Graduate School of Engineering, The University of Tokyo, Tokyo Japan
Show Abstract Heterointerfaces of metals (organics) and oxides have shown linear Schottky behavior due to a difference of energy band potentials. Moreover, resistance switching at metal/oxide interfaces has attracted much attention. In this work, we pay attention to surface plasmons (SP) induced at metal/oxide heterointerfaces combined with a negative and positive dielectric constant. SP represents one of the interfaces phenomena and is possibly superior to the existing material character. Therefore, SP has been recognized as an important technology and dynamic physics. Our research group is aiming to establish a new interface technology by coupling the SP with oxides, which is applied for novel surface and optical sensors. Large density of states (DOS) at plasma freqency introduces a high electromagnetic field, which is attenuated within several tens of nanometers from an Ag/ZnO interface. To induce SP-coupling, carriers in quantum wells (QW) must be rigidly set near the interface since a dynamic carrier interaction between Ag and QWs is generated by SP. Moreover, the recombination energy of excitons must be matched with plasma frequency, which was realized with ZnO/CdZnO single-QW fabricated by controlling band gap and polarity engineering. The singe-QW just have a functionality of SP-coupled quantum emitters. It is known that Ag/ZnO and Au/ZnO interfaces act as Ohmic and Schottky junctions on the basis of work function, respectively. As new interface function, we introduce an engineering of single-QWs and discuss a mechanism of SP coupling at the metal/ZnO interface that is dominated by dielectric constant. This study recognizes as a first step for “Oxide Plasmonics” based on control of the dielectric constant at metal/oxide interface. Ref. H. Matsui and H. Tabata, Appl.Phys.Lett. 94 (2009) 161907, J.Appl.Phys. (2010) accepted, Phys.Rev.B 75, 014438 (2007).
9:00 PM - MM9.5
Fabrication of Highly Crystalline In2O3 Oxide Semiconductor Thin Films on YSZ (111) Substrates.
Kentaro Kaneko 1 2 , Shizuo Fujita 2
1 Electronic Science and Engineering, Kyoto Univ., Kyoto Japan, 2 Photonics and Electronics Science and Engineering Center, Kyoto Univ., Kyoto Japan
Show AbstractIndium oxide is a transparent conducting semiconductor which is well known for tin-doped oxide (indium tin oxide : ITO). ITO is widely used as transparent electrodes for silicon solar cells and liquid crystal display. But in recent studies, there are some attempts to use non-doped In2O3 as a new transparent oxide semiconductor by making of highly crystalline In2O3 for the purpose of improving its mobility and carrier concentration (T. Koida et al., JAP 99 (2006) 123703 M. Oda et al., (70th Autumn Meet., 2009); Japan Society of Applied Physics and Related Societies, 10p-J-14[in Japanese] O. Bierwagen et al., JAP 107 (2010) 113519). In this symposium, we report the fabrication and characterization of In2O3 thin films on (111) yttria-stabilized zirconia (YSZ) substrates. The growth method is mist Chemical Vapor Deposition which is originally developed by our laboratory(K. Kaneko et al., APEX 2 (2009) 075501). The precursor source is indium acetylacetonate [(C5H8O2)3In] and it was solved in deionized water. A slight amount of hydrochloric acid (HCl) was added to dissolve the sources completely. The 2θ-θ scanning profiles of X-ray diffraction (XRD) results showed (111) orientation growth of In2O3 thin film on (111) YSZ substrate. In order to estimate crystalline quality of the film, XRD rocking curve (ω-scanning) measurements were conducted. The full-widths at half maximum (FWHM) of the XRD rocking curves were 109.1 arcsec (= 0.03°). These results indicate the successfully growth of highly crystalline In2O3 thin films. One of the promising potentials of highly crystalline In2O3 thin film is high mobility and low carrier concentration. To investigate these properties, Van-der-Pauw-Hall measurements were performed. The resistivity of this sample was 8.11E(-3) Ωcm, the carrier concentration was 2.85E(19) /cm3 and the mobility was 27 cm2/Vs . Comparing with recent other works, the resistivity and carrier concentration were larger and the mobility was lower even though its high crystallinity. A plausible reason for these results is that the In2O3 is thin and contains oxygen vacancies. Detailed discussions, together with the improvement of electronic properties will be presented at the symposium.
9:00 PM - MM9.6
Optical Properties and Carrier Transport of CdO.
Sepehr Vasheghani Farahani 1 , Timothy D. Veal 1 , Philip David C. King 2 , Jesus Zuniga-Perez 3 , Vicente Munoz-Sanjose 4 , Chris F. McConville 1
1 Department of Physics, University of Warwick, Coventry United Kingdom, 2 School of Physics & Astronomy, University of St. Andrews, St. Andrews United Kingdom, 3 Centre de Recherche sur l’Hétéro-Epitaxie et ses Applications, Centre National de la Recherche Scientifique, Valbonne France, 4 Departamento de Fisica Aplicada y Electromagnetismo, Universitat de Valéncia, Valéncia Spain
Show AbstractCdO is a transparent oxide which crystallizes in the rocksalt structure and exhibits high conductivity. It has extensive applications in photovoltaics and is advantageous for the production of electrical contact to materials. In particular, the related compound, CdTe, has recently become widely used in terrestrial photovoltaic energy conversion in solar cells with fairly good properties. CdO is chemically compatible with CdTe and is transparent over the range of photon wavelengths absorbed by CdTe. In this work, the cadmium oxide layers were grown on r-plane sapphire at atmospheric pressure by metal-organic vapour phase epitaxy in which the precursors for oxygen and cadmium are tertiary butanol and dimethylcadmium respectively. The optoelectronic properties of CdO have been studied after annealing the samples in vacuum and atomic hydrogen, and irradiating with helium ions. Hall effect measurements and Fourier transform infrared (FTIR) reflectance spectroscopy in the mid-IR range were used to obtain the carrier concentration, plasma resonance frequency and carrier life time. The optical gap was determined from the absorption spectra derived from the measured transmission and reflectance data in the visible range. Simulation and analysis of the optical data regarding the non-parabolicity of the conduction band and band gap renormalization enabled the band gap and band edge effective mass of CdO to be evaluated as 2.20 eV and 0.24m0 respectively. Transport and optical mobilities obtained from Hall effect measurements and carrier lifetime respectively, have been determined as a function of carrier concentration. The results show that the latter is significantly higher than the former for the as-grown samples. The relative contributions of grain boundaries and ionized impurities to the carrier scattering will be discussed.
9:00 PM - MM9.7
Charge State of Manganese and Its Correlation with the Electrical Resistivity of Doped Indium Tin Oxide Films.
Sarath Kumar 1 , Mohamed Hedhili 1 , Subbiah Kasiviswanathan 2 , Husam Alshareef 1
1 Materials Science and Engineering, King Abdullah University of Science and Technology, Thuwal 23955-6900 Saudi Arabia, 2 Department of Physics, Indian Institute of Technology Madras, Chennai 600036 India
Show AbstractTransparent conducting oxides such as indium tin oxide (ITO) doped with transition elements are being pursued for spintronics applications [1]. In order to optimize the properties of doped ITO films for such applications, an understanding of the chemical environment of the dopant is important. Here, we present the X-ray photoelectron spectroscopy (XPS) studies on Mn doped ITO (Mn:ITO) films grown by dc reactive sputtering and the correlation between charge state of Mn and the electrical resistivity of the films. The lattice constant of Mn:ITO films with 1.6% and 4.3% Mn has been calculated to be smaller to that of ITO and is attributed to the lower ionic radius of Mn ions compared to that of In3+ ions. XPS analysis revealed the oxidation state of Mn in the films. The Mn 2p3/2 core level spectra centered at ~641.2 eV was fitted using three components: one centered at ~640.8 eV (corresponding to Mn2+ as in MnO), second at ~642.7 eV (corresponding to Mn4+ as in MnO2) and a third satellite ~5.4 eV from the first component. The satellite excitation is typical of MnO confirming the presence of Mn2+ [2]. Electrical resistivity measurements performed as a function of temperature revealed that the films showed metal-like behavior. The observed increase in the resistivity of the films (from 0.43 to 1.89 mΩ cm) with increasing Mn concentration is explained by considering the predominant +2 charge state of Mn. Mn2+ ions substituting for In3+ ions reduce the carrier concentration in ITO, which directly results in the increase in resistivity with increasing Mn concentration. References[1] M. Venkatesan, R. D. Gunning, P. Stamenov, and J. M. D. Coey, J. Appl. Phys., 103, 07D135 (2008).[2] F. Müller, R de Masi, D. Reinicke, P. Hufner, and K. Stowe, Surf. Sci. 520, 158 (2002).
9:00 PM - MM9.8
Combinatorial Research Approach to P-type Transparent Conducting Zn-Co-Ni-O Thin Films.
Andriy Zakutayev 1 , Jennifer Leisch 1 , Paul Ndione 1 , Phil Parilla 1 , John Perkins 1 , David Ginley 1
1 National Center for Photovoltaics, National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractP-type transparent conducting thin films are needed for photovoltaic applications. However, the variety and the functional quality of such thin films are currently limited, partly due to the chemical complexity of the materials. Most of the known p-type transparent conductors are the Cu-based ternary (CuMO2, SrCu2O2) or quaternaty (MCuOCh, BaCuChF) compounds, where M is a trivalent metal and Ch is a chalcogen. Some rare exceptions are the Cu-free binary p-type transparent conductors SnO and NiO.A combinatorial approach is well suited for quick screening of chemically complex materials. This approach involves deposition and characterization of compositionally-graded thin films, denoted “libraries”. The libraries are prepared using co-deposition from multiple targets and characterized as a function of position on the sample. Such a spatially resolved automated characterization is referred to as “mapping”.We applied the combinatorial approach to (Zn,Ni)Co2O4 p-type transparent conducting thin films. The Zn-Co-Ni-O libraries were deposited using the radio-frequency magnetron sputtering from the ZnO, Co3O4 and NiO targets positioned at the 45-degree angle with respect to the stationary glass substrate. The content of Ar, O2 and other gasses in the chamber during the deposition was monitored using a residual gas analyzer. The Zn/Co ratio and the level of Ni doping in the resulting samples was determined using x-ray fluorescence mapping and the phase of the films was mapped using x-ray diffraction. The electrical conductivity of the libraries was measured using four-point-probe sheet resistance mapping and transparency of the samples was mapped using optical transmittance measurements.We found that the crystallographic structure of the Zn-Co-O libraries depends on the chemical composition. However, the conductivity and the transparency of the samples are independent of Zn/Co ratio and O2 partial pressure during the growth, which we were able to conclude from studying only eight libraries. The small number of the samples required to draw these conclusions shows that the combinatorial research approach is an effective high throughput method to study new materials in general and Zn-Co-O p-type transparent conductors in particular. The results of growth and characterization of the Zn-Co-Ni-O system will also be presented.
9:00 PM - MM9.9
Polymorphism and Phase Transitions in Indium Oxides and Hydroxides.
Aleksander Gurlo 1 2
1 Faculty of Materials- and Geosciences, Technische Universitaet Darmstadt, Darmstadt Germany, 2 Harvard School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States
Show AbstractDespite progress in theoretical prediction as well as in experimental realization of new In2O3 polymorphs, a complete pressure-temperature diagram of indium oxide is still lacking. Even less is known about structural chemistry and phase transitions of indium hydroxide and oxohydroxide. Our recent works deal with the synthesis and characterization of known and new polymorphs in indium-oxygen system. In this way (i) a new orthorhombic In2O3 polymorph has been synthesized under high-pressure high-temperature conditions [1], (ii) the metastability of corundum-type In2O3 have been proved both theoretically and experimentally [2], (iii) new sol-gel methodologies to synthesize high pressure In2O3 polymorphs under ambient pressure conditions have been developed [3], (iv) the stabilisation of pseudo-cubic {012} morphology in corundum-type In2O3 over several length scales have been verified [4], (v) the mobility and carrier concentration of well-defined corundum- and bixbyite-type In2O3 nanocrystals have been measured at different temperatures and in different gas atmospheres [5], (vi) a synthetic methodology for hierarchically organized hollow spheres has been developed [6], (vii) the crystallization of bixbyite-type In2O3 has been proven using in-situ time-resolved synchrotron radiation [7], and (viii) the stabilization of high-pressure corundum-type In2O3 polymorph in nanocrystals have been explained [8] . In our presentation we will specifically address several selected topics. References[1] A. Gurlo, D. Dzivenko, P. Kroll, R. Riedel, High pressure - high temperature synthesis of Rh2O3(II) -type In2O3 polymorph, Physica Status Solidi - RRL, 2 (2008) 269 [2] A. Gurlo, P. Kroll, R. Riedel, Metastability of corundum-type In2O3, Chem.- Eur. J., 14 (2008) 3306[3] M. Epifani, P. Siciliano, A. Gurlo, N. Barsan, U. Weimar. Ambient pressure synthesis of corundum-type In2O3, J. Amer. Chem. Soc. 126 (2004) 4078[4] A. Gurlo, S. Lauterbach, G. Miehe, H.-J. Kleebe, R. Riedel, Nanocubes or nanorhombohedra? Unusual crystal shape of corundum-type oxides, J .Phys. Chem. C, 112 (2008) 112, 9209[5] A. Oprea, A. Gurlo, N. Barsan, U. Weimar, Transport and gas sensing properties of In2O3 nanocrystalline thick films: a Hall effect based approach, Sens. Actuators B, 139 (2009) 322[6] A. Gurlo, G. Miehe, R. Riedel, Surfactant-free self-assembly route to hollow In2O3 microspheres, Chem. Commun. , (2009) 2747[7] L. Schlicker, R Riedel, A. Gurlo, Indium hydroxide to bixbyite-type indium oxide transition probed in-situ by time-resolved synchrotron radiation, Nanotechnology, 20 (2009) 495702 [8] A. Gurlo, Structural stability of high pressure polymorph in In2O3 nanocrystals: evidence of stress-induced transition? Angew. Chem. Int. Edition, (2010), 10.1002/anie.201000488