Symposium Organizers
F. (Shadi) Shahedipour-Sandvik State University of New York-Albany
E. Fred Schubert Rensselaer Polytechnic Institute
L. Douglas Bell Jet Propulsion Laboratory
Vinayak Tilak General Electric Global Research Center
Andreas W. Bett Fraunhofer-Institut for Solar Energy Systems
O4: Poster Session: Compound Semiconductor for Lighting, Power and Sensing
Session Chairs
Tuesday PM, April 14, 2009
Exhibition Hall (Moscone West)
6:00 PM - O4.1
Characterization and Fabrication of InGaN-based Blue LED with Underlying AlGaN/GaN SLS Cladding Layer Grown on Si(111) Substrate.
Bin Abu Bakar Ahmad Shuhaimi 1 2 , Pum Chian Khai 1 2 , Takaaki Suzue 1 2 , Yukiyasu Nomura 1 2 , Takashi Egawa 1 2
1 Research Center for Nano-Device and System, Nagoya Institute of Technology, Nagoya, Aichi, Japan, 2 Department of Engineering Physics, Electronics and Mechanics, Nagoya Institute of Technology, Nagoya, Aichi, Japan
Show AbstractSi(111) is a very promising substrate for the growth of low-cost GaN-based energy-saving devices, such as blue and white LEDs. However, the large band-gap InGaN-based light emission is absorbed by the underlying lower band-gap Si substrate, causing a poor efficiency light emission. This paper reports material characterization and fabrication of InGaN-based blue LED with underlying AlGaN/GaN SLS cladding layer grown on Si(111) substrate. The AlGaN/GaN SLS cladding layer is aimed to enhance light reflection towards the upper surface of the LED structure to prevent light from being absorbed by the Si substrate.The sample in this study was grown by MOCVD on Si(111) substrate. Prior to the growth of LED layers, a thin high-temperature AlN layer followed by AlN/GaN multilayers (MLs) has been grown on the substrate. Subsequently, a 400 nm thick AlGaN/GaN SLS cladding layer, a 200 nm thick n-GaN contact layer, an active layer consisting of 15 pairs of 2 nm thick In0.16Ga0.84N well and 9 nm thick In0.08Ga0.92N barrier layers, a 10 nm thick p-AlGaN electron-block layer, and a 50 nm thick p-GaN contact layer was grown for the LED structure. A control sample without the AlGaN/GaN SLS cladding layer was also grown for comparison. Material properties of the samples were evaluated by HR-XRD and PL surface mapping at room temperature. Both samples were also fabricated into LED using standard device processing method to evaluate light emission during current injection.The LED sample with underlying AlGaN/GaN SLS cladding layer shows tremendous enhancement of light emission in PL measurement and in actual current injection, compared to the normal LED.
6:00 PM - O4.11
Reproducible p-type Activation in Acceptor Ion-implanted GaN by High-temperature Ultra-fast Microwave Annealing.
Geetha Aluri 1 , Madhu Gowda 1 , Nadeem Mahadik 1 , Siddharth Sundaresan 1 , Hany Issa 1 , Mulpuri Rao 1 , Jaime Freitas 2 , Syed Qadri 2 , Yong-Lai Tian 3
1 Electrical and Computer Engineering, George Mason University, Fairfax, Virginia, United States, 2 , Naval Research Laboratory, Washington, District of Columbia, United States, 3 , LT Technologies, Fairfax, Virginia, United States
Show AbstractAchieving p-type conduction consistently by acceptor specie ion-implantation into GaN is difficult, because it requires very high annealing temperatures, which is a problem due to a relatively low decomposition temperature of GaN (~ 900°C). This problem can be overcome by using AlN encapsulant films in combination with an annealing technique, which can reach high-temperatures (> 1500°C) with very fast ramping rates (> 500°C/s). Microwave annealing can facilitate such annealing conditions to obtain consistent p-type activation in GaN. In this work, we have attempted to obtain p-type conduction in GaN by Mg ion-implantation or in-situ Be doping followed by high temperature microwave annealing. In this study, single energy (150 keV/5×1014 cm-2) and multiple energy (10-300 keV/2.6×1015 cm-2) Mg implantations were performed at 500°C into undoped 3 μm thick GaN epilayers grown on a-plane sapphire. The samples were capped with 0.6 µm thick PLD AlN. Microwave annealing was performed in pure N2 ambient, after placing the sample on a SiC susceptor, at temperatures ranging from 1300°C-1500 °C for 15 s. Anneals were also performed on AlN capped 1µm thick, in-situ Be-doped (~ 1 x 1019 cm-3) GaN epilayers, grown on 6H-SiC substrates. The annealed samples were characterized by SIMS, x-ray diffraction (XRD), photoluminescence (PL) and Hall measurements. The XRD measurements performed on as-implanted GaN revealed the defect sub-lattice peaks due to the interference of the x-rays from the implanted impurity in the interstitial sites. These sub-lattice peaks disappeared after microwave annealing at temperatures ≥ 1400°C, which confirms that the implanted Mg atoms have taken substitutional positions. The PL spectra of the single-energy Mg-implanted GaN, microwave annealed at temperatures ≥ 1400°C for 15s, have shown Mg activation through the presence of Mg-acceptor related DAP band at 3.27 eV. The intensity of this DAP band increases with increasing annealing temperature. We could not observe p-type activation in single-energy Mg-implanted samples if the anneal duration is < 15s, even for 1500°C annealing. This Mg activation behavior in single-energy Mg-implanted GaN is consistently reproduced. In case of multiple energy Mg-implanted samples, we could not observe p-type conduction even for 15 s annealing due to a large degree of implant lattice damage. Longer (> 15s) duration anneals were not possible due to severe deterioration of the GaN sample surface. In case of in-situ Be-doped GaN, after 1300°C/5s annealing, the Be out-diffused into the AlN layer and also in-diffused toward the GaN/SiC interface. The in- and out- diffusion of the Be increased with increasing annealing temperature with only a small fraction of Be remaining in the GaN layer, revealing the inadequate nature of Be-doping for the GaN technology. Hence, we have not attempted the Be ion-implantation in GaN.
6:00 PM - O4.12
Growth and Characterization of a-plane GaN Grown on r-plane Sapphire.
Yong Gon Seo 1 2 , Moon Suhk Suh 1 , Hyung Do Yoon 1 , Sung Min Hwang 1 , Kyunghwan Oh 2
1 Energy Nano Materials Research Center, KETI, Seongnam-si, Gyeonggi-do, Korea (the Republic of), 2 Institute of Physics and Applied Physics, Yonsei University, Seoul Korea (the Republic of)
Show AbstractGrowth of nonpolar and semipolar GaN films on sapphire substrates has been receiving considerable attention to reduce the spontaneous and strain-induced piezoelectric polarization effects. In this study, nonpolar a-plane GaN epilayer was grown on r-plane sapphire substrates by metalorganic chemical vapor deposition (MOCVD) without a low-temperature GaN buffer layer. Trimethylgallium(TMGa), ammonia, silane were used as Ga, N, Si precursors along with hydrogen carrier gas. The growth procedure consisted of 1) GaN layer of 100nm thickness in N2 atmosphere and 2) near 5.5um thick GaN in H2 atmosphere. The macroscopic surface morphology of a-plane GaN template was very smooth without triangular pits, which was confirmed by Normarski optical microscope imaging. The root mean square of the surface roughness of AFM image (10umx10um) was 2.5nm. From high-resolution X-ray diffraction measurements, the full width at half maximums of a-GaN rocking curve was ~427arcsec aligned to the m-direction and ~768arcsec along c-direction. We investigated the optical property of as-grown a-GaN film analyzed by photoluminescence at different temperatures.
6:00 PM - O4.13
Effect of Substrate Engineering of AlN/Si(111) Substrates on Overgrown GaN, AlGaN, and InGaN films.
Mihir Tungare 1
1 College of Nanoscale Science and Engineering, University at Albany, SUNY, Albany, New York, United States
Show AbstractIon implantation has been implemented as a useful technique for strain engineering whether it is to generate crack arresting compressive layers in sapphire and magnesium oxide crystals or to enhance the performance of complementary metal oxide semiconductors (CMOS).1,2 We have demonstrated a simultaneous reduction in crack density and dislocation density in 2µm thick overgrown GaN films using our novel ion-implantation assisted substrate engineering technique.3 Here we present stress analysis of AlN/Si(111) substrates before and after substrate engineering. It is shown that implantation is a key component in the success of this technique. Stress reduction is higher for thinner buffers, with a 36nm AlN buffer showing a decrease in stress by one order of magnitude from >1GPa to 0.1GPa after substrate engineering. The impact of this stress reduction on overgrown GaN, AlGaN, and InGaN films is also presented. Jindal et al. have explained the effect of interfacial strain on growth rate and alloy composition of AlGaN nanostructures.4 Improved quality and compositional control of III-Nitride films is of great technological significance for the realization of better performance devices. Hence, besides the effect of this method on surface morphology, crack density, and dislocation density within the overgrown films, a low temperature photoluminescence (PL) study is also done to better understand the effect of substrate engineering on composition and optical quality. 1 V.N. Gurarie et al., Nucl. Instr. and Meth. in Phys. Res. B 242, 421–423 (2006)2 Shimizu, A. et al. Electron Devices Meeting, IEDM Technical Digest. International, 19.4.1-19.4.4 (2001)3 Jamil et al. Appl. Phys. Lett. 87, 082103 (2005)4 Jindal et al. Mater. Res. Soc. Symp. Proc. Vol. 1087 © 2008
6:00 PM - O4.2
Novel Pulsed Laser Deposited Y-doped BaZrO3 Thin Films for High Temperature Humidity Sensors.
XiaoXin Chen 1 , Loren Rieth 1 , Mark Miller 1 , Florian Solzbacher 1 2 3
1 Dept of Electrical and Computer Engineering, University of Utah, Salt Lake City, Utah, United States, 2 Dept of Material Science and Engineering, University of Utah, Salt Lake City, Utah, United States, 3 Dept of Bioengineering, University of Utah, Salt Lake City, Utah, United States
Show AbstractThere are no reliable humidity microsensors with long-term stability at high operating temperatures (> 500 °C) as required for in-situ point of source emissions control as used in power plant combustion processes. Defect chemistry based models and initial experimental results in recent fuel cell literature indicate that bulk Y-doped BaZrO3 could be suitable for use in highly selective, high-temperature compatible humidity sensors. In order to accomplish faster response and leverage low cost batch microfabrication technologies we have developed thin film deposition processes, characterized layer properties and fabricated and tested high temperature humidity micro sensors using these thin films. Previously published results on sputtering BaZrO3 thin films have confirmed the principle validity of our approach. (1) the difficulty in controlling the stoichiometry of the films and their electrical properties as well as (2) mud flat cracking of the films however have rendered sputtering an unsuitable process for reproducible and reliable thin film humidity microsensor fabrication. In the work presented here, thin films of BaZrO3 doped with 20% Y were deposited using pulsed laser deposition PLD, which allowed us to successfully eliminate the above mentioned problems while maintaining the advantageous humidity sensing characteristics of the films. Films were characterized using X-Ray Diffraction XRD, Atomic Force Microscopy AFM, and X-Ray Photoelectron Spectroscopy XPS to determine microstructure, surface morphology, and composition. Films of 200 and 500 nm were deposited onto oxidized Silicon substrates using PLD at various substrate temperatures (RT-700°C) in a VAC PLD system. Films were annealed at up to 800°C. PLD samples were polycrystalline (compared to previously published, amorphous sputtered films) with substrate temperature having a large effect on the structure of the films. AFM results revealed that PLD samples have a bigger hillock size and more than 10 times larger RMS roughness than sputter deposited samples. X-ray Photoelectron Spectroscopy (XPS) data were collected from as deposited and annealed samples at the sample surface as well as after 4 min of Ar+ etching. PLD samples were close to the desired stoichiometry and a correlation could be found between deposition and annealing parameters and layer composition. We will present film composition data, electrical conductivity data and humidity measurements for 200 and 500 nm thick films at 500 and 650°C at vapor pressures of between 0.05 and 0.5 atm, with good repeatability and hysteresis during cycling for several hours. Sensitivities of up to 5.7 atm-1 were measured. Cross sensitivities towards H2, O2, NH3, NOX in typical concentrations found in the exhaust gas stream of power plant combustion processes were determined and found to be orders of magnitude smaller than the humidity sensitivity.
6:00 PM - O4.3
Nonthermal Plasma Synthesis of Luminescent InP Nanocrystals
Ryan Gresback 1 , Uwe Kortshagen 1
1 Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractGroup III-V nanocrystals exhibit unique properties due to quantum confinement that can be exploited for numerous applications, most notably in optoelectronics and biomedical fields. A variety of solution-based synthesis methods have been developed for InP nanocrystals which generally require high temperatures or long reaction times. In contrast, a flow-through, low-pressure plasma approach allows for a variety of gas, liquid, and solid precursors, and provides a scalable, continuous synthesis route. This work reports on colloidal core-shell InP/ZnS nanocrystals made via plasma synthesis of InP nanocrystals and subsequent growth of a ZnS shell by traditional liquid phase methods.InP nanocrystals were synthesized using a nonthermal radio-frequency (13.56 MHz) flow-through plasma operated between 2 and 4 torr with 10-70 watts of input power. Phosphine (PH3) and trimethylindium (In(CH3)3) vapor were used as precursors to synthesize bare, free-standing nanocrystals with diameters between ~2-8 nm. The size of the nanocrystals is controlled by the residence time and plasma conditions. The dry nanocrystals were reacted with myristic acid in octadecene to form a colloid. In order to enhance the PL of the nanocrystals, a ZnS shell is then grown. The InP colloid was injected with repetitions of zinc stearate and sulfur in octadecene at 220 C for 15 minutes for each layer to grow the ZnS shell. The InP/ZnS colloids exhibit strong PL with quantum yields greater than 10%. The wavelength of the PL can be tuned from green to red by altering the InP core size. High resolution transmission electron microscopy and X-ray diffraction confirm the size and morphology of the core-shell structure. This work was supported by the UMN Center for Nanostructure Applications and by the MRSEC Program of the National Science Foundation under Award Number DMR-0819885.
6:00 PM - O4.5
In-situ Study of the Kinetics and Durability of Semiconducting Pd-hydride Formation for Integrated Hydrogen Sensor Applications.
Renaud Delmelle 1 , Marie-Stephane Colla 1 , Thomas Pardoen 1 , Joris Proost 1
1 Division of Materials and Process Engineering, Université catholique de Louvain, Louvain-la-Neuve Belgium
Show AbstractThe hydrogen sensitivity and durability of Pd thin film systems has been studied in-situ by monitoring its hydriding kinetics. The experimental technique used for this work consists of a high resolution curvature measurement setup, which continuously monitors the reflections of multiple laser beams coming off a cantilevered thin film sample [1]. After mounting the sample inside a vacuum chamber, a H-containing gas mixture is introduced to instantaneously generate a given pH2 inside the chamber. The resulting interaction of H with the Pd layer then leads to the formation of semiconducting Pd-hydride (Pd-H), inducing a volume expansion of the thin film system. This leads in turn to changes in the sample curvature as a result of internal stresses developing in the film during the hydriding cycle. Such curvature changes can be used efficiently in integrated H-sensor applications, based for instance on the capacitive detection of micro-cantilever bending [2]. In order to quantify the sensitivity of the Pd films to the imposed pH2, a two-step model for the hydriding kinetics has first been proposed from our in-situ curvature data. Theoretical expressions for the two rate-limiting steps of Pd-hydride formation (H surface and bulk absorption) have been derived, showing characteristic dependencies on pH2. The influence of different material parameters on the adsorption and absorption kinetics has then been evaluated. A strong influence of the Pd impurity level and the cantilever film thickness on its curvature sensitivity and hence the underlying hydriding kinetics has been identified. On the other hand, the same parameters turned out to have a negligible effect on the equilibrium H-uptake. These equilibrium values were found to be mainly influenced by the fraction of crystalline α and β phases in the Pd-H film, the latter depending only on the imposed pH2. Finally, the durability of the Pd-H system has been evaluated by monitoring the curvature evolution during multiple hydriding/dehydriding cycling. Significant residual curvature has been observed during such cycling, although its level remained constant from cycle to cycle. A possible mechanical origin of this irreversibility will be proposed.[1] J. Proost and F. Spaepen, J. Appl. Phys. 91 (2002) 204-216.[2] Zhiyu Hu, T. Thundat and R. J. Warmack, J. Appl. Phys. 90 (2001) 427-431.
6:00 PM - O4.7
Growth by Molecular Beam Epitaxy of GaNAs Alloys with High As Content for Potential Photoanode Applications in Hydrogen Production.
Sergey Novikov 1 , C. Staddon 1 , A. Akimov 1 , R. Campion 1 , N. Zainal 1 , A. Kent 1 , C. Foxon 1 , C. Chen 2 , K. Yu 3 , W. Walukiewicz 3
1 School of Physics and Astronomy, University of Nottingham, Nottingham United Kingdom, 2 Photovoltaics Technology Center, Industrial Technology Research Institution, Taiwan 310 Taiwan, 3 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractPhotoelectrochemical (PEC) cells, illuminated by sunlight, have the ability to split water into hydrogen and oxygen. Such cells use photoactive electrodes immersed in an aqueous electrolyte or water. The choice of material for the photoanode is crucial for efficient hydrogen production using the PEC method. An ideal photoelectrode must meet three criteria: corrosion stability, a band gap near 2eV to have sufficient potential to drive the water-splitting reactions while still absorbing a significant fraction of the solar spectrum and band edges must “straddle” the H+/H2 and O2/H2O potentials for spontaneous water splitting reactions.In III-V compounds the above requirements point towards group III–nitrides. Gallium nitride (GaN) has a band gap ~3.4eV, high mechanical hardness and excellent corrosion resistance. The band gap of GaN can be potentially adjusted and decreased due to strong negative bowing in the GaN-based solid solutions with group V elements such as As. Our theoretical calculations suggest that the GaN1-xAsx material system is one of the most promising materials for the photoanodes. If As is added to GaN, it forms an isoelectronic impurity band within the band gap of GaN due to an anticrossing interaction with the valence band. This effectively reduces the band gap and pushes the valence band edge up toward the O2/H2O potential, while leaving the conduction band above the H+/H2 potential. However, a large miscibility gap was theoretically predicted and experimentally confirmed for the Ga-N-As system. GaN1-xAsx alloys at the N-rich end of the phase diagram have been grown by molecular beam epitaxy (MBE) and metal-organic chemical vapour deposition (MOCVD). It was difficult to obtain a high concentration of As in the alloy before phase separation occurs. GaN1-xAsx layers with x up to ~1% and 8% have been reported for MBE and MOCVD grown materials, respectively.We will present results on a systematic investigation on the low-temperature growth of GaNAs layers on sapphire substrates by plasma-assisted MBE. GaNAs films with high (up to 50%) incorporation of As have been grown. However, with increasing As incorporation the GaNAs layers became amorphous. We have studied the structural, optical and electrical properties of these GaNAs layers using X-ray diffraction, ion beam analytical techniques, atomic force microscopy, optical absorption and reflection, photoluminescence and modulation spectroscopy. With increasing As incorporation we have observed a gradual shift of the absorption edge from the band-gap of GaN at ~3.4eV to the values as low as ~1.5eV.
6:00 PM - O4.8
Ultrafast Electron-Hole Dynamics in Core/Shell CdSe/CdS Dot/Rod Nanocrystals.
Maria Grazia Lupo 2 1 , Fabio Della Sala 2 , Luigi Carbone 2 , Margherita Zavelani-Rossi 1 , Roberto Cingolani 2 , Liberato Manna 2 , Guglielmo Lanzani 1
2 , National Nanotechnology Laboratory of CNR-INFM, Lecce Italy, 1 , Politecnico di Milano, Milano Italy
Show AbstractColloidal semiconductor nanocrystals are nanoscale materials whose optical, electronic and transport properties, due to their strong dependence on size and shape, can be finely tuned by advanced chemical synthesis approaches. Among various types of semiconductor nanocrystals, core-shell nanostructures comprised of a semiconductor core that is covered by a thin shell of another type of semiconductor material have been extensively investigated.Elaborate core-shell systems have been reported, such as rod-shaped CdS embedding a spherical CdSe domain located closer to one end of the rods across the rod length[1,2]. These dot/rod core/shell CdSe/CdS nanocrystals have been recently synthesized with a seeded growth approach at high temperature, which yields highly uniform nanocrystals in terms of distributions of rod lengths and diameters. These rods have high crystallinity, they spontaneously self assemble on substrates and can be easily organized in close-packed ordered arrays over large areas. The photoluminescence quantum yield is very high, up to 70% for rods with aspect ratio lower than 10 [1].In these rods holes are confined inside the CdSe core whereas electrons can penetrate the elongated CdS shell: this behaviour makes these nanocrystals particularly interesting both for technological applications and for a better understanding of physics in semiconductor nanoparticles.We use femtosecond pump-probe transient spectroscopy in the visible spectral range to analyze ultrafast carrier dynamics of these new heterostructures. Upon tuning the pump photon energy and intensity we can selectively excite the core or the shell: the electronic structure and the wavefunction distribution at the rod/dot interface and the electron-hole dynamics within the nanostructure have been investigated. We measured a time constant of 650±80 fs associated to hole localization into the CdSe dot. Furthermore we found evidence that, after charge carrier relaxation into the lower energy levels associated to the CdSe dot, the electron wave function extends into the rod: upon exciting the CdSe core we found an intense bleaching signal in the CdS spectral region, which we assign to the delocalization of the electronic wavefunction on the basis of envelop-function theoretical calculations[3].(1)L. Carbone, C.Nobile, M. De Giorgi, F. Della Sala, G. Morello, P. Pompa, M. Hytch, E. Snoeck, A. Fiore, I. R. Franchini, M. Nadasan, A. F. Silvestre, L. Chiodo, S. Kudera, R. Cingolani, R. Krane, L. Manna, Nano Lett. 7, 2942-2950, (2007).(2)J. Muller, M. Lupton, P. G. Lagoudakis, F. Schindler, R. Koeppe, A.L. Rogach, J. and Feldmann, D.V. Talapin, and H. Weller, Nano Lett. 5, 2044-2049,(2005).(3)M.G. Lupo, F. Della Sala, L.Carbone,M. Zavelani-Rossi, A. Fiore, L.Luer, D. Polli, R.Cingolani, L.Manna, G. Lanzani, Nano Letters, accepted for publication.
Symposium Organizers
F. (Shadi) Shahedipour-Sandvik State University of New York-Albany
E. Fred Schubert Rensselaer Polytechnic Institute
L. Douglas Bell Jet Propulsion Laboratory
Vinayak Tilak General Electric Global Research Center
Andreas W. Bett Fraunhofer-Institut for Solar Energy Systems
O6: Compound Semiconductor for Sensing
Session Chairs
L. Douglas Bell
F. (Shadi) Shahedipour-Sandvik
Wednesday PM, April 15, 2009
Room 2024 (Moscone West)
2:30 PM - **O6.1
Nanostructuring Bulk Semiconductors for Solid State Energy Conversion
Jean-Pierre Fleurial 1 , Pawan Gogna 1 , Gang Chen 2 , Mildred Dresselhaus 2 , Hohyun Lee 2 , Zhifeng Ren 3 , Dezhi Wang 3 , Sabah Bux 4 , Daniel King 4 , Richard Kaner 4
1 Power Systems Section, Jet Propulsion Laboratory, Pasadena, California, United States, 2 Mechanical Engineering Department, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Physics Department, Boston College, Chestnut Hill, Massachusetts, United States, 4 Department of Chemistry and Biochemistry and California NanoSystems Institute, University of California at Los Angeles, Los Angeles, California, United States
Show AbstractThe search for bulk materials with high dimensionless thermoelectric figures of merit, ZT, has investigated novel complex structure compounds and phases that combine the electrical transport properties of heavily doped semiconductors or even semimetals with the low thermal conductivity of glassy materials. Low dimensional structures have emerged over the last decade as a promising approach to realize significant increases in thermal-to-electric energy conversion efficiency, as experimentally demonstrated in Bi2Te3-based superlattices and PbTe-based quantum-dots superlattices. Instead of quantum carrier confinement effects, as originally predicted, large reductions in thermal conductivity values have been experimentally found mostly responsible for the reported 100 to 200% increases in ZT. However, a number of technical challenges remain for such materials to be of practical use, especially for medium to large-scale high temperature applications. As a result, some of the most recent research has focused on engineering the low-dimensionality into bulk compounds by forming nanoscale features during the synthesis process. Nanostructuring leads to quantum confinement effects and boundary scattering at the nanoparticle interfaces. Quantum confinement enhances the density of states, and thus increases the Seebeck coefficient without reducing the electrical conductivity. Phonon scattering at the interfaces leads to very significant reductions in the lattice thermal conductivity. “Electronics” semiconductors such as Si, Ge, III-V and II-VI compounds are attractive candidates for this approach since they possess excellent electrical transport properties but are poor thermoelectric materials due to their high thermal conductivity values. Various techniques were successfully developed to produce doped and undoped nanoparticles of these semiconductors in sufficient quantities to consolidate into highly dense bulk samples suitable for high temperature electrical and thermal transport property measurements. Results obtained on most of the investigated nanobulk materials have shown that the lattice thermal conductivity could be reduced by as much as 90% compared to single crystalline material and that some of the most refractory materials exhibited excellent thermal stability. After an overview of the most recent data, we discuss the potential of such nano-engineered bulk semiconductors for large scale power generation applications such as waste heat recovery or to contribute to the development of inexpensive and moderately efficient solar cells.
3:00 PM - **O6.2
Quantum Structure MBE Growth for Infrared Sensor Applications at the Jet Propulsion Laboratory.
Cory Hill 1
1 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, United States
Show AbstractAt the Jet Propulsion Laboratory (JPL), we produce a variety of custom hardware for infrared sensing applications. In this presentation, we will briefly introduce the atmospheric transmission and fundamental spectroscopic science applications driving the hardware development. We will then discuss large-area infrared focal plane arrays produced at JPL and the molecular beam epitaxial crystal growth techniques involved. Some of the specific imaging technologies we will discuss are the quantum well infrared photodetector (QWIP), antimonide-based superlattices, and the quantum dot in a well (DWELL). These technologies span the range from small format 256x256 research arrays to simultaneously imaging, dual color, pixel collocated megapixel arrays. In addition to the passive sensor technologies mentioned above, our materials effort also produces infrared laser sources for use in highly sensitive atmospheric measurements. The materials issues related to the interband cascade laser (ICL) at the heart of these measurements will also be presented.
3:30 PM - O6.3
A III-nitride Layered Barrier Structure for Hyperspectral Imaging Applications.
Douglas Bell 1 , Neeraj Tripathi 2 , Fatemeh Shahedipour-Sandvik 2 , Vibhu Jindal 2 , James Grandusky 2
1 M/S 302-231, Jet Propulsion Laboratory, Pasadena, California, United States, 2 College of Nanoscale Science and Engineering, University at Albany, Albany, New York, United States
Show AbstractWe report on a novel photodetector structure based on III-nitride materials. A layered barrier configuration is used to create a barrier with voltage-tunable height. Such a barrier can be used as a filter on photoexcited holes and electrons, with a tunability range of as much as ~3eV. Thus the detector can function as a continuously tunable pixel in a hyperspectral imaging array. This would eliminate the need for external gratings and filters; in addition to this advantage, the tunability of pixels allows decrease of the array dimension by one. Expected range of wavelength tunability for this detector will be as much as a factor of three (e.g. 260-1200 nm), with a wavelength resolution Δλ/λ of about 0.005. Unlike conventional hyperspectral detectors, the number and wavelength spacing of spectral channels will not be fixed, but can be adjusted dynamically.The III-nitride materials family is a good candidate for this device, combining large band offsets with the ability for epitaxial growth. We have demonstrated the feasibility of using III-nitride materials to fabricate layered tunnel barriers, and have demonstrated tunability of photodetection using these structures. Metal / barrier / semiconductor configurations have been used to demonstrate barrier lowering as a function of voltage, and a consequent change in wavelength sensitivity. Internal quantum efficiencies of > 12% have been achieved with prototype devices.
4:15 PM - O6.4
AlGaN/GaN HEMT Devices for Biosensing Applications.
Vibhu Jindal 1 , Neeraj Tripathi 1 , Xiobin Xu 1 , Hua Shi 2 , Magnus Bergkvist 1 , Nathaniel Cady 1 , Fatemeh Shahedipour-Sandvik 1
1 College of Nanoscale Science and Engineering, University at Albany, Albany, New York, United States, 2 Department of Biological Sciences, University at Albany, Albany, New York, United States
Show AbstractGroup III-nitride materials including AlGaN and GaN are currently being explored for a variety of sensitive, charge-based biosensing applications. AlGaN/GaN based HEMT devices offer an exciting opportunity for label-free detection of charged biomolecules and whole cells, which can interact with the 2DEG (two dimensional electron gas) formed at AlGaN/GaN interface. This work focuses on some of the immobilization strategies developed for the attachment of biological materials directly onto AlGaN surfaces. It has been demonstrated that variety of biomolecules such as nucleic acids, poly-L-lysine, protein A, and immunoglobulins (IgG) can be directly adsorbed onto group-III nitride materials and that adsorbed species can retain functionality for biosensing applications. Furthermore, the successful capture of bacterial cells suggests that direct immobilization strategies can be used for biosensing applications on AlGaN/GaN HEMT structures. Nucleic acid interaction studies have demonstrated that DNA molecules are firmly bound to the AlGaN surfaces over different ionic strengths and a wide range of pH conditions and that immobilized molecules are available for sequence-specific hybridization to single-stranded target DNA. Surface-bound DNA-DNA hybrids can be observed via fluorescent staining. Hybridization reactions were observed for target DNA concentrations ranging from 2μM to 2nM, allowing this effective immobilization approach to be used for biosensing applications. DNA stability on AlGaN surfaces treated with varying pH and ionic strength conditions further suggests complex DNA-AlGaN interactions. The work also investigates AlGaN/GaN HEMT device properties under the influence of adsorbed biomolecules. The investigated device structure is further modeled by SENTAURUS device simulator to predict device properties and correlate with experimental observations.
4:30 PM - O6.5
Passivation and Functionalization of III-V Semiconductors.
Rory Stine 1 , Edward Aifer 1 , Lloyd Whitman 1 , Dmitri Petrovykh 1 2
1 , Naval Research Laboratory, Washington, District of Columbia, United States, 2 , University of Maryland, College Park, Maryland, United States
Show AbstractWe describe chemical passivation of GaSb and InAs surfaces by thioacetamide (TAM) and deposition of bi-functional self-assembled monolayers (SAMs) of cysteamine on InAs. Sulfur-based wet chemical treatments have been extensively explored for electronic and chemical passivation of III-V semiconductors. In most cases, however, the passivation effects have only been examined indirectly, via changes in electronic properties of treated devices. In contrast, we use x-ray photoelectron spectroscopy (XPS) to directly characterize the surface chemistry and structure of the chemically passivated and functionalized III-V semiconductors.We examined passivation of GaSb and InAs surfaces by TAM in both acidic and basic aqueous solutions and found that the resulting sulfide layers dramatically depend not only on the solution chemistry and individual elements, but also on the specific compound semiconductor. The layers produced by base-TAM appear to be of self-limited thickness <1 nm, whereas acid-TAM creates considerably thicker (1–2 nm) sulfide layers. Passivation by both acid- and base-activated TAM offers significant protection against reoxidation in ambient air for up to one day.In order to functionalize InAs(001) using a similar thiol-based chemistry, we deposited cysteamine, a small molecule with thiol and amine termini, from a basic aqueous solution. Analysis of the deposited films using XPS reveals that cysteamine forms a monolayer in which molecules are oriented and attached to the InAs surface exclusively via their thiol termini. The free amine ligands presented at the interface of the resulting bi-functional SAM should provide a convenient pathway for subsequent surface functionalization, e.g., for applications in chemical or biological sensing.The general overarching conclusion of our systematic studies of chemical passivation and functionalization of III-V semiconductors is that while the sulfur- and thiol-based wet treatments often are effective and practical, the details of the resulting surface chemistry and structure need to be directly examined and cannot be reliably inferred from results for “similar” or “related” compound semiconductors.
4:45 PM - O6.6
High-Temperature Detection of Nitrogen Oxides by Indium Oxide Solid State Sensors with Gold Promoter Layers.
Srinivasan Kannan 1 , Loren Rieth 1 , Florian Solzbacher 1
1 Department of Electrical and Computer Engineering, University of Utah, Salt Lake City, Utah, United States
Show Abstract New emissions regulations, as well as the need for higher efficiency in combustion-based power production make exhaust gas control a necessity for manufacturers and users alike. Indium Oxide (In2O3) has a high melting temperature (Tm=1910 °C) and low vapor pressure suggesting that the material is stable at the required operating temperatures (> 500 °C). In2O3 thin films with sensitivity to NOx at operating temperatures of 500 °C and greater have not been reported in literature. In2O3 thin films were sputter deposited with thickness of 35, 150 and 275 nm respectively. Gold promoter layers (~5 nm) were sputter deposited on some of the In2O3 layers to enhance response to NOx. Promoter layers are additives that modify a catalyst to gain enhanced sensor response (S) to a target gas. All In2O3 thin films with promoter layers and 150 nm films without promoter layers were investigated with and without annealing. The films were annealed at 700, 800 and 900 °C in an ambient of 2% H2 in Ar, N2 and O2. Sensors were tested for NOx in the region of 5 to 50 ppm NOx, which are relevant to high temperature combustion processes. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Atomic force microscopy (AFM) revealed the microstructure, composition and particle size of thin film In2O3. In2O3 films (150 nm) with Au promoter layers displayed enhanced sensor response (S~25) in comparison with as-deposited In2O3 films (S~5) to 25 ppm NOx at 500 °C in ambient N2. The particle size of In2O3 films increased with deposition thickness from ~ 3 to 60 nm for films from 35 to 275 nm thick, respectively. Sensor response decreased with increase in particle size and film thickness (S~200 for 35 nm films, particle size~3nm to S~25 for 150 nm films, particle size~35 nm). All annealed films increased in particle size, increasing from 30 to > 45 nm for a 150 nm thick film. Annealed films (150 nm thick) exhibit a decrease in sensor response from S of 25 to 5 for 25 ppm NOx when annealed at 900 °C for 5 hours in ambient N2. The selectivity of In2O3 was poor in presence of NH3 (100 ppm) and H2 (5000 ppm) with a reduced NOx response(S~0.1) to 25 ppm NOx. XRD spectra of the In2O3 thin films reveals as-deposited film and annealed films to have predominantly (222) and (400) crystallite orientation. Gas phase Fourier transform infrared spectroscopy (FT-IR) was used to understand the gas phase reactions occurring between NOx and NH3 as well as NOx and H2 at operating temperatures between 400 and 600 °C. The NO2 absorbance peak at 1600 cm-1 and 1630 cm-1 was observed to decrease in the presence of 100 ppm NH3 or 2275 ppm of H2. H2 etched away the surface of the In2O3 rendering the sensor unsuitable for use in hydrogen rich environments as confirmed by XPS measurements. In2O3 thin films demonstrate excellent sensitivity to NOx at temperatures greater than 500 °C with particle size, film thickness and anneal conditions having an effect on gas sensitivity.
5:00 PM - O6.7
Polarization Effects on III-nitride Based Tunnel Barriers.
Neeraj Tripathi 1 , Vibhu Jindal 1 , Fatemeh Shahedipour-Sandvik 1 , Lloyd Bell 2
1 College of Nanoscale Science and Engineering, University at Albany, State University of New York, Albany, New York, United States, 2 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, United States
Show AbstractIII-nitride alloys have been under investigation for a wide range of applications including LEDs, LDs, HEMTs, photodetectors, photocathodes and sensors. We have recently envisioned and reported a promising application of III-nitrides in development of multil-heterojunction tunnel barriers. Such tunnel barriers enable a novel approach of performing energy spectroscopy of hot photoexcited carriers, equivalent to wavelength spectroscopy of incident radiation, providing a compact and dynamically tunable hyperspectral detector. Hyperspectral imaging is a critical composition analysis technology that serves as an important tool to understand various Earth atmosphere characteristics like changes in Ozone layer, air quality, land cover change, terrestrial and marine productivity and thus provides a vital feedback about effect of human actions on atmosphere. Proposed III-nitride based hyperspectral detectors are potentially much superior compared to the conventional hyperspectral imagers and hence deserve further investigation.Presence of spontaneous and piezoelectric polarization in III-nitrides make electron transport in multi-heterojunction devices a complex problem, that requires a thorough understanding of energy and electric field profile across the device layers. AlGaN/GaN heterojunctions have been shown to induce high density of polarization charges that govern the electrical characteristics of the device. Effect of polarization induced charges on the energy band profile plays an important role in the device design of the proposed tunnel barriers that consist of 6 AlGaN/GaN heterojunctions. Here we report on theoretical simulations that provide important insight on the effect of polarization induced charges and multiple 2-dimensional carrier gases on the performance of the nitride tunnel barriers. SENTAURUS TCAD device simulator has been utilized to develop simulation models that incorporate spontaneous and piezoelectric polarization in nitride layers. Results show presence of 2DEG at the first two and 2DHG at the last two AlGaN/GaN interfaces. These multiple 2 dimensional charges greatly influence the local energy band profile, leading to a complex dependence of the photoemission threshold on applied bias and thickness of the tunnel barrier epilayers. Internal photoemission measurements have been performed on different thickness tunnel barriers. Simulation results provide possible explanation of some features of the experimentally observed internal photoemission spectrum that couldn’t be explained with simulation models without polarization. Experimental observations supported by simulation results will be presented. These results provide critical feedback towards optimum tunnel barrier device design as well as demonstrate the significance of spontaneous and piezoelectric polarization in III-nitride heterojunction devices.
Symposium Organizers
F. (Shadi) Shahedipour-Sandvik State University of New York-Albany
E. Fred Schubert Rensselaer Polytechnic Institute
L. Douglas Bell Jet Propulsion Laboratory
Vinayak Tilak General Electric Global Research Center
Andreas W. Bett Fraunhofer-Institut for Solar Energy Systems
O7: Materials Growth and Characterization
Session Chairs
Kris Bertness
James Grandusky
Thursday AM, April 16, 2009
Room 2024 (Moscone West)
9:15 AM - **O7.1
In-situ Measurement of Tensile Stress Generation from Dopant-induced Dislocation Inclination in MOCVD-grown AlGaN Thin Films.
Joan Redwing 1 3 , Ian Manning 1 2 , Jeremy Acord 2 , Xiaojun Weng 2 3 , Mark Fanton 2 , David Snyder 2
1 Materials Science and Engineering, Penn State University, University Park, Pennsylvania, United States, 3 Materials Research Institute, Penn State University, University Park, Pennsylvania, United States, 2 Electro-Optics Center, Penn State University, Freeport, Pennsylvania, United States
Show AbstractThe introduction of Si dopants into GaN and AlxGa1-xN increases the tensile stress in the films, which can lead to film cracking in thick and/or highly doped layers even in films that initiate growth under a compressive epitaxial stress. Several mechanisms have been proposed to explain the tensile stress including crystallite coalescence resulting from Si-induced surface roughening and the inclination of threading dislocations. In the latter mechanism, the propagation of inclined threading edge-type dislocations with effective misfit components in the basal plane is predicted to give rise to a strain gradient in the film. This model, known as dislocation effective climb, assumes a constant threading dislocation density throughout the layer thickness. In this study, in-situ wafer curvature measurements were used to directly study the impact of Si doping from SiH4 on intrinsic growth stress during MOCVD growth of AlxGa1-xN (x=0.40-0.45). Post-growth transmission electron microscopy (TEM) characterization was used to correlate measured changes in stress with changes in film microstructure. Nominally undoped AlxGa1-xN layers grown on (0001) SiC using a thin AlN buffer layer were observed to initiate growth under a compressive stress which relaxed with increasing film thickness. When SiH4 was added during layer growth, however, the stress was observed to transition from compressive to tensile. The compressive-to-tensile stress transition occurred more rapidly as the Si dopant concentration was increased from 4x1018 cm-3 to 3x1019 cm-3. Cross-sectional TEM measurements revealed an abrupt change in the angle of inclination of threading dislocations in the films at the point of Si addition. The magnitude of the in-situ measured stress gradient was comparable to that predicted by the dislocation effective climb model suggesting that dislocation inclination is the dominant mechanism responsible for tensile stress generation in the Si-doped AlxGa1-xN layers. The dislocation density of a 380 nm thick Si-doped AlxGa1-xN layer with inclined dislocations was 3.69±0.78 x1010 cm-2, as measured by plan-view TEM, compared to 5.85±0.69 x1010 cm-2 for an undoped AlxGa1-xN layer of similar thickness. The inclination of threading dislocations is believed to result in increased dislocation interaction and annihilation leading to a reduction in dislocation density. Based on these results, the dislocation effective climb model was modified to account for the variation of dislocation density with film thickness and was found to accurately predict the experimentally observed compressive-to-tensile stress transition in the Si-doped AlxGa1-xN layers.
9:45 AM - O7.2
Growth and Characterization of a-plane AlGaN Films on r-plane Sapphire by Metalorganic Chemical Vapor Deposition.
Huei-Min Huang 1 , Jun-Rong Chen 1 , Shih-Chun Ling 1 , Tsung-Shine Ko 1 , Tien-Chang Lu 1 , Hao-Chung Kuo 1 , Shing-Chung Wang 1
1 Photonics, Nation ChiaoTung university, Hsinchu Taiwan
Show Abstract III-nitrides and their alloys are promising materials and widely used in the development of blue-ultraviolet optoelectronic devices such as light-emitting diodes (LEDs) and laser diodes (LDs). In particular, AlGaN alloys have attracted much attention since the composition-dependent bandgap can cover a wide spectral range from 3.4 to 6.3 eV. In most nitride-based light-emitting devices, the active region with single or multiple-quantum-well structure is grown on the conventional (0001)-oriented sapphire substrates. Under this condition, the strong built-in electrostatic fields appear because of the spontaneous and piezoelectric polarization in nitride-based materials. The built-in electrostatic fields result in the spatial separation of electron and hole wave functions, which leads to a reduced oscillation strength and lower quantum efficiency of radiative transitions in the quantum wells. The polarization-induced internal electric fields limit the performances of nitride-based devices. The growth of group III nitride epilayer along non-polar orientations provides an efficient approach to eliminate such polarization effects that has been respectively explored for a- or m-oriented films of wurtzite GaN on r-plane sapphire and γ-LiAlO2. Growth of non-polar III-nitride could overcome the presence of large built-in electrostatic fields and improve the quantum efficiency of light emitting devices. In this study, the non-polar a-plane AlGaN films were grown by using low-pressure metal organic chemical vapor deposition (MOCVD).For all AlGaN samples, a 30-nm-thick AlN nucleation layer was deposited on r-plane sapphire substrate, followed by a 2-μm-thick GaN layer. Furthermore, an AlGaN film with the thickness of about 0.35 μm was grown on the 2-μm-thick GaN layer. In order to improve the crystal quality of the AlGaN samples, we inserted a SiNx interlayer between the substrate and the AlN nucleation layer to suppress dislocations caused by lattice mismatch between sapphire and epitaxial layers. From the room-temperature PL results, the emission wavelengths of the a-plane AlxGa1-xN films can be shorten from 360 to 320 nm with increasing Al composition. Moreover, the Al composition-dependent properties including energy bandgap, crystal quality, and degree of emission polarization will be investigated in this study as well.
10:00 AM - O7.3
Property Enhancement in a-plane GaN having Multiple SiNx or InN Interlayers.
Hsin-Hsien Wu 1 , Jyh-Ming Ting 1
1 Department of Materials Science and Engineering, National Cheng Kung University, Tainan Taiwan
Show AbstractA-plane GaN (11-20) epilayers having multiple SiNx or low-temperature grown InN interlayers were grown on r-plane (1-102) sapphire using a metal-organic chemical vapor deposition process. The resulting samples were characterized using high-resolution X-ray diffractometry (HRXRD), atomic force microscopy (AFM), Hall measurements, and Raman spectroscopy. HRXRD analysis shows that the full width at the half maxima of the (101) and (110) rocking waves are extremely small and equal to 0.25° and 0.056°, respectively. This indicates very low dislocation densities and stacking faults. AFM images also show a significant decrease in the root-mean-square roughness. Due to the low dislocation densities, , the mobility at the wafer center was found to be as high as 202 cm2/V/s at a carrier concentration of only 6.55e17/cm3.
10:15 AM - O7.4
Crystalline Perfection of Epitaxial Structure: Correlations with Growth Rate, Thickness, and Elastic Strain of Epitaxial Layers.
Balakrishnam Jampana 1 , Nikolai Faleev 2 , Ian Ferguson 4 , Robert Opila 1 , Christiana Honsberg 3
1 Materials Science and Engg., University of Delaware, Newark, Delaware, United States, 2 Electrical and Computer Engg., University of Delaware, Newark, Delaware, United States, 4 School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States, 3 Electrical Engineering, Arizona State University, Tempe, Arizona, United States
Show AbstractThe III-Nitride materials have become prominent in the production of blue and green LED and lasers. Recently, wide band gap (~2.4eV) InGaN has been studied for developing solar cells. InGaN is grown on a standard GaN template. In general, growth of lattice mismatched epitaxial layers reduces the crystalline perfection of epitaxial material, with typical areal densities of 1010 cm-2. The increase of crystalline defects, compared to other III-V photovoltaic materials, increases the recombination of photo-generated carriers and hence degrades the performance of InGaN solar cell. In this paper a correlation between crystalline perfection of InGaN epitaxial layer with growth rate, thickness and elastic strain is presented. A model, based on diffusion, accumulation and structural transformation of point defects is presented which explains the types and spatial distribution of crystalline defects at epitaxial layers.In0.12Ga0.88N epi-layers are grown on a standard GaN template and studied for crystalline perfection. The GaN template is a standard two step MOCVD grown GaN (~2um) on sapphire (0001) substrate. Two parameters are controlled in the growth of InGaN epi-layers, growth rate and layer thickness. High resolution x-ray diffraction (HRXRD) rocking curves and reciprocal space maps are utilized in characterizing the crystalline perfection. HRXRD of In0.12Ga0.88N/GaN epitaxial layers reveals strong correlations between crystalline perfection of epitaxial layers and growth rate, thickness, and elastic strain. Non-zero elastic strain leads to deterioration in accommodation of elastic strain and the apparent creation of point defects on the growth surface. Created at the surface they diffuse inward to accumulate and structurally transform forming extended crystalline defects under the action of secondary elastic stress. Deterioration in accommodation of elastic strain and increase in amount of created point defects are observed with increase in growth rate and initial elastic strain (at InGaN-GaN interface). Thickness of InGaN epitaxial layer significantly affects the spatial distribution of diffused point defects. The thickness also affects types and spatial distribution of extended crystalline defects, formed by structural transformation of point defects, in the volume of epitaxial layers. The initial stage of relaxation of induced elastic stress observed from reciprocal space maps in thick epilayers (25 – 50 critical layer thicknesses) cannot be explained by existing models. Our model, based on diffusion, accumulation and structural transformation of point defects, allows correctly explain observed types and spatial distribution of crystalline defects at epitaxial layers.The formation of crystalline defects in lattice mis-matched growth cannot be eliminated completely, but their minimization predicted by our model will aid in growth of good crystalline material to be utilized in achieving a wide band gap InGaN solar cell.
10:30 AM - O7.5
Investigation of Composition-dependent Optical Phonon Modes in AlxGa1-xN Epitaxial Layers Grown on Sapphire Substrates.
Jun-Rong Chen 1 , Tien-Chang Lu 1 , Hao-Chung Kuo 1 , Shing-Chung Wang 1
1 Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu Taiwan
Show AbstractHexagonal GaN and AlN semiconductors, and AlGaN alloys, have attracted considerable attention due to their successful applications in the fabrication of high-performance electronic and optoelectronic devices, such as heterostructure field-effect transistors, ultra-violet light-emitting diodes, and laser diodes. By varying the alloy composition, different electrical and optical properties can be obtained in a wide spectral range from 3.4 to 6.3 eV. In order to further engineer these alloys and related optoelectronic devices, it is necessary to work on the fundamental properties of these materials. The infrared optical response of these alloys is important for the determination of crystal quality and phonon properties. In general, GaN and its related compounds grown on (0001)-oriented sapphire substrates have the hexagonal wurtzite structure. Therefore, they are natural optically anisotropic uniaxial crystals. The investigation of anisotropic AlGaN phonon modes and composition-dependent phonon energies will be beneficial for the design and develop of nitride-based quantum cascade lasers since the carrier dynamics and temperature-dependent performance of quantum cascade lasers are closely related to the phonon energies.In this study, we reported the systematical study of optical anisotropy of hexagonal AlxGa1-xN epitaxial films grown on (0001)-oriented sapphire substrates using Fourier transform infrared reflectance (FTIR) measurements. In order to probe the anisotropic phonon modes, the incident angle with respect to the plane of incidence were kept at 30 and 75 degree, respectively. The experimental data can be theoretically fitted by employing a four-phase layered system model, which takes the crystal anisotropic properties into account by using a 4x4 matrix method. The Al composition dependence of phonons, including A1(TO), E1(TO), A1(LO), and E1(LO) were determined from the theoretical fitting by using the Lorentz oscillator dielectric function model. From the experimental FTIR data, several specific absorption dips were observed and shifted with increasing aluminum composition. This observed phenomenon and physical mechanism will be investigated and discussed.
11:30 AM - **O7.7
Defects in Bulk ZnO.
Matt McCluskey 1
1 , Washington State University, Pullman, Washington, United States
Show AbstractZinc oxide (ZnO) has attracted resurgent interest as an active material for energy-efficient lighting applications. An optically transparent crystal, ZnO emits light in the blue-to-UV region of the spectrum. The efficiency of the emission is higher than more “conventional” materials such as GaN, making ZnO a strong candidate for solid-state white lighting. Despite its advantages, however, ZnO suffers from a major drawback: as grown, it contains a relatively high level of donors. These unwanted defects compensate acceptors or donate free electrons to the conduction band, thereby keeping the Fermi level in the upper half of the band gap. One such defect is hydrogen, a common impurity in ZnO. We have studied hydrogen donors using infrared (IR) spectroscopy and Hall-effect measurements. One donor species results in an IR absorption peak corresponding to O-H bond-stretching vibrations. This species is unstable, decaying in a few weeks at room temperature. By correlating the free-electron concentration with the IR absorption strength, we established that hydrogen acts as a shallow donor.Novel schemes for introducing acceptor dopants must be investigated if ZnO-based devices are to become technologically and economically feasible. Toward that end, we formed nitrogen-hydrogen (N-H) complexes in ZnO during chemical vapor transport (CVT) growth, using ammonia as an ambient. The N-H bond-stretching mode gives rise to an IR absorption peak at 3150.6 cm-1. Isotopic substitutions for hydrogen and nitrogen result in the expected frequency shifts, thereby providing an unambiguous identification of these complexes. The N-H complexes are stable up to ~700°C. The introduction of neutral N-H complexes could prove useful in achieving reliable p-type conductivity in ZnO. Work supported by the Department of Energy and the National Science Foundation.
12:00 PM - O7.8
Defect and Interface Properties of Epitaxial ZnO on (111) Si Substrates.
Wei Guo 1 , Michael Katz 1 , Christopher Nelson 1 , Tassilo Heeg 2 4 , Darrell Schlom 2 4 , Bing Liu 3 , Yong Che 3 , Xiaoqing Pan 1
1 Material Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Material Science and Engineering, Pennsylvania State University, University Park, Pennsylvania, United States, 4 Department of Materials Science and Engineering, Cornell University, Ithaca, New York, United States, 3 Materials Research Group, IMRA America, Inc., Ann Arbor, Michigan, United States
Show AbstractThe potential for integrating ZnO-based multi-functional devices into Si electronics has motivated the growth of epitaxial ZnO films on Si substrates. Employing RE2O3 (RE: Gd, Lu, or Sc) buffer layers can help to compensate the large lattice and thermal mismatches between ZnO and Si, and lead to epitaxial ZnO film with high crystallinity. The epitaxial orientation relationships are (0001)ZnO//(111)RE2O3//(111)Si and [1120]ZnO//[110]RE2O3//[110]Si. The ZnO films show high Hall moblities at room temperature, narrow widths of x-ray diffraction rocking curves, and high quality photoluminescence (PL) characteristics comparable to those of the ZnO single crystal. Defect related room temperature electroluminescence of the n-ZnO/RE2O3/p-Si heterostructures were studied by high resolution transmission electron microscopy and electron energy-loss spectroscopy.
12:15 PM - O7.9
ZnO Epitaxial Film Growth on Sapphire Substrates by Chemical Vapor Deposition.
Zhuo Chen 1 , Tom Salagaj 2 , Christopher Jsesen 2 , Karlheinz Strobl 2 , Mim Nakarmi 1 , Kai Shum 1
1 Physics, Brooklyn College - CUNY, Brooklyn, New York, United States, 2 , First Nano, a Division of CVD Equipment Corp., Ronkonkoma, New York, United States
Show AbstractZnO thin films with thickness around 200 nm were deposited on a-plane sapphire substrates by Chemical Vapor Deposition (CVD) method with a mixed ZnO-powder/C-powder solid source. These films were characterized by Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), and photoluminescence (PL) spectroscopy. The correlation between surface structural properties of ZnO thin films and their optical signature measured by temperature dependence of PL is investigated for various growth conditions such as flow rates of carrier gas and O2 injection gas, growth temperature, and the ZnO/C source mixing ratios. At room temperature, the excitonic absorption edge of 3.305 eV of these films was determined by optical absorption measurements.
12:30 PM - O7.10
The Oxygen Terminated Epitaxial ZnO Ultra-thin Film on c-plane Al2O3 by Atomic Layer Deposition.
Song Yang 1 , Bi-Hsuan Lin 1 2 , Wei-Ren Liu 1 2 , Jian-Huei Lin 1 , Chia-Hong Hsu 2 1 , Cheng-Siung Chang 1 , Wen-Feng Hsieh 1
1 Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung, Hsinchu Taiwan, 2 , National Synchrotron Radiation Research Center, Hsinchu Taiwan
Show AbstractWe have grown ZnO ultra-thin films on c-plane sapphire (Al2O3) by atomic layer deposition (ALD) at 200°C. Even for samples without thermal annealing, well defined relative in-plane orientation between the as-deposited ZnO film and Al2O3 substrate can be resolved. The in-plane epitaxial relationship, characterized by x-ray azimuthal scans across off-normal reflections, is {10-10}ZnO||{10-10}Al2O3. The surface morphology of the samples upon chemical etching indicates the surface of the as-deposited ZnO film is oxygen terminated. The small width of ZnO (0002) rocking curves, 0.017° and 0.015° for the as-deposited and 800°C annealed samples, respectively, evidences the excellent crystalline quality of the grown layers. An increase of the layer thickness accompanied by a decrease of electron density of the ZnO layers after annealing was observed from the reflectivity data. The cause of this change will be discussed.