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At least 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

In-situ spectroscopic ellipsometry and structural study of HfO{sub 2} thin films deposited by radio frequency magnetron sputtering

We have investigated the reduction of unwanted interfacial SiO{sub 2} layer at HfO{sub 2}/Si interface brought about by the deposition of thin Hf metal buffer layer on Si substrate prior to the deposition of HfO{sub 2} thin films for possible direct contact between HfO{sub 2} thin film and Si substrate, necessary for the future generation devices based on high-κ HfO{sub 2} gate dielectrics. Reactive rf magnetron sputtering system along with the attached in-situ spectroscopic ellipsometry (SE) was used to predeposit Hf metal buffer layer as well as to grow HfO{sub 2} thin films and also to undertake the in-situ characterization of the high-κ HfO{sub 2} thin films deposited on n-type 〈100〉 crystalline silicon substrate. The formation of the unwanted interfacial SiO{sub 2} layer and its reduction due to the predeposited Hf metal buffer layer as well as the depth profiling and also structure of HfO{sub 2} thin films were investigated by in-situ SE, Fourier Transform Infrared spectroscopy, and Grazing Incidence X-ray Diffraction. The study demonstrates that the predeposited Hf metal buffer layer has played a crucial role in eliminating the formation of unwanted interfacial layer and that the deposited high-κ HfO{sub 2} thin films are crystalline although they were deposited at room temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Layer dependence of graphene-diamene phase transition in epitaxial and exfoliated few-layer graphene using machine learning

The study of the nanomechanics of graphene—and other 2D materials—has led to the discovery of exciting new properties in 2D crystals, such as their remarkable in-plane stiffness and out of plane flexibility, as well as their unique frictional and wear properties at the nanoscale. Recently, nanomechanics of graphene has generated renovated interest for new findings on the pressure-induced chemical transformation of a few-layer thick epitaxial graphene into a new ultra-hard carbon phase, named diamene. Here in this work, by means of a machine learning technique, we provide a fast and efficient tool for identification of graphene domains (areas with a defined number of layers) in epitaxial and exfoliated films, by combining data from atomic force microscopy (AFM) topography and friction force microscopy (FFM). Through the analysis of the number of graphene layers and detailed Å-indentation experiments, we demonstrate that the formation of ultra-stiff diamene is exclusively found in 1-layer plus buffer layer epitaxial graphene on silicon carbide (SiC) and that an ultra-stiff phase is not observed in neither thicker epitaxial graphene (2-layer or more) nor exfoliated graphene films of any thickness on silicon oxide (SiO 2 ).

36 MATERIALS SCIENCE↗
DOE OSTI2015

Polycrystalline ferroelectric or multiferroic oxide articles on biaxially textured substrates and methods for making same

A polycrystalline ferroelectric and/or multiferroic oxide article includes a substrate having a biaxially textured surface; at least one biaxially textured buffer layer supported by the substrate; and a biaxially textured ferroelectric or multiferroic oxide layer supported by the buffer layer. Methods for making polycrystalline ferroelectric and/or multiferroic oxide articles are also disclosed.

36 MATERIALS SCIENCE↗
NASA NTRS1988

New MBE buffer for micron- and quarter-micron-gateGaAs MESFETs

A new buffer layer has been developed that eliminates backgating in GaAs MESFETs and substantially reduces short-channel effects in GaAs MESFETs with 0.27-micron-long gates. The new buffer is grown by molecular beam epitaxy (MBE) at a substrate temperature of 200 C using Ga and As sub 4 beam fluxes. The buffer is crystalline, highly resistive, optically inactive, and can be overgrown with high quality GaAs. GaAs MESFETs with a gate length of 0.27 microns that incorporate the new buffer show improved dc and RF properties in comparison with a similar MESFET with a thin undoped GaAs buffer. To demonstrate the backgating performance improvement afforded by the new buffer, MESFETs were fabricated using a number of different buffer layers and structures. A schematic cross section of the MESFET structure used in this study is shown. The measured gate length, gate width, and source-drain spacing of this device are 2,98, and 5.5 microns, respectively. An ohmic contact, isolated from the MESFET by mesa etching, served as the sidegate. The MESFETs were fabricated in MBE n-GaAs layers grown on the new buffer and also in MBE n-GaAs layers grown on buffer layers of undoped GaAs, AlGaAs, and GaAs/AlGaAs superlattices. All the buffer layers were grown by MBE and are 2 microns thick. The active layer is doped to approximately 2 x 10 to the 17th/cu cm with silicon and is 0.3 microns thick.

Source record↗
DOE OSTI2014

Origins of interlayer formation and misfit dislocation displacement in the vicinity of InAs/GaAs quantum dots

We have examined the origins of interlayer formation and misfit dislocation (MD) displacement in the vicinity of InAs/GaAs quantum dots (QDs). For QDs formed by the Stranski-Krastanov mode, regularly spaced MDs nucleate at the interface between the QD and the GaAs buffer layer. In the droplet epitaxy case, both In island formation and In-induced “nano-drilling” of the GaAs buffer layer are observed during In deposition. Upon annealing under As flux, the In islands are converted to InAs QDs, with an InGaAs interlayer at the QD/buffer interface. Meanwhile, MDs nucleate at the QD/interlayer interface.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Interplay between strain, quantum confinement, and ferromagnetism in strained ferromagnetic semiconductor (In,Fe)As thin films

We systematically investigated the influence of strain on the electronic structure and ferromagnetism of (In,Fe)As thin films. It is found that while the shift of the critical point energies of compressive-strained (In,Fe)As layers grown on (In{sub 1−y},Ga{sub y})As (y = 0.05, 0.1) buffer layers can be explained by the hydrostatic deformation effect (HDE) alone, those of tensile-strained (In,Fe)As layers grown on (Ga{sub 1−z},Al{sub z})Sb (z = 0, 0.5, 1) buffer layers can be explained by the combination of HDE and the quantum confinement effect (QCE). The Curie temperature T{sub C} of the (In,Fe)As layers strongly depends on the strain, and shows a maximum for the (In,Fe)As layer grown on a GaSb buffer layer. The strain dependence of T{sub C} can be explained by the s-d exchange mechanism taking into account HDE and QCE.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Over 11% Efficient Eco-Friendly Kesterite Solar Cell: Effects of S-Enriched Surface of Cu2ZnSn(S,Se)4 Absorber and Band Gap Controlled (Zn,Sn)O Buffer

For high efficiency kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cell, CdS thin film was usually used as a buffer layer. However, due to the toxicity of Cd and pollution problems involved from the solution-based chemical bath deposition, eco-friendly high efficiency CZTSSe solar cell with Cd-free buffer is necessary. As an Cd-free buffer layer, we investigated (Zn,Sn)O (ZTO) film deposited by sputtering method. In order to achieve high power conversion efficiency, we controlled energy band gaps of CZTSSe absorber as well as ZTO buffer, which was required to optimize conduction band offset (CBO) between the absorber and the buffer and to increase open circuit voltage (Voc) and fill factor (FF). The CBO was optimized by controlling the band gap of ZTO. By varying the Sn/(Zn + Sn) ratio and its deposition temperature, band gap of ZTO was successfully adjusted. Experimental and computational calculation results showed that solar cell performance was strongly affected by the CBO between absorber and buffer. Besides CBO matching, larger band gap of ZTO improved short circuit current density (Jsc) with enhanced external quantum efficiency value in blue photon spectrum range. As an additional way to improve power conversion efficiency of solar cell, band graded CZTSSe absorber was developed by using spray-based two-step process. The sprayed CZTSSe film was engineered to have S-enriched surface, which makes surface band gap widened and surface passivated, and resultantly increases Voc, Jsc and fill factor (FF). By controlling band gaps of both CZTSSe absorber and ZTO buffer, we obtained 11.22% environment-friendly CZTSSe solar cell without MgF2 anti-reflection coating.

(Zn↗
NASA NTRS1977

GaAs solar cell development

The motivation for developing GaAs solar cells is based on their superior efficiency when compared to silicon cells, their lower degradation with increasing temperature, and the expectation for better resistance to space radiation damage. The AMO efficiency of GaAs solar cells was calculated. A key consideration in the HRL technology is the production of GaAs cells of large area (greater than 4 sg cm) at a reasonable cost without sacrificing efficiency. An essential requirement for the successful fabrication of such cells is the ability to grow epitaxially a uniform layer of high quality GaAs (buffer layer) on state-of-the-art GaAs substrates, and to grow on this buffer layer the required than layer of (AlGa)As. A modified infinite melt liquid phase epitaxy (LPE) growth technique is detailed.

Knechtli, R. C.↗
DOE OSTI2014

The growth of GaSb/Al{sub 0.33}Ga{sub 0.67}Sb MQW on n-Silicon (1 0 0) with Al{sub 0.66}Ga{sub 0.34}Sb/AlSb SPS layers

Highlights: • GaSb/Al{sub 0.33}GaSb MQW layer was grown on Si (1 0 0) by MBE. • The effect of miscut angle of Si substrate was studied. • A lot of twins were removed by Al{sub 0.66}Ga{sub 0.34}Sb/AlSb SPS layers. • Good quality of GaSb/Al{sub 0.33}Ga{sub 0.67}Sb MQW layers were proved by PL spectra. • Optimum growth temperature of the AlSb buffer layer was studied. - Abstract: GaSb/Al{sub 0.33}Ga{sub 0.67}Sb multi-quantum well (MQW) film on n-Si (1 0 0) substrates is grown by molecular beam epitaxy. The effects of a miscut angle of the Si substrate (0°, 5°, and 7°) on the properties of an AlSb layer were also studied. The suppression of the anti-phase domains (APD) was observed at a miscut angle of 5° on Si (1 0 0). It was found that the growth temperature in the range of 510–670 °C affects the quality of AlSb layers on Si. Low root-mean-square surface (RMS) roughness values of 3–5 nm were measured by atomic force microscopy at growth temperatures ranging from 550 °C to 630 °C. In addition, Al{sub 0.66}Ga{sub 0.34}Sb/AlSb short period superlattice (SPS) layers were used to overcome problems associated with a large lattice mismatch. The RMS values of samples with a SPS were partially measured at approximately ∼1 nm, showing a larger APD surface area than samples without a SPS layer. Bright-field cross-sectional transmission electron microscopy images of the GaSb/Al{sub 0.33}Ga{sub 0.67}Sb MQW, the AlSb buffer layer and the Al{sub 0.66}Ga{sub 0.34}Sb/AlSb SPS layers show that numerous twins from the AlSb/Si interface were removed by the AlSb buffer layer and the Al{sub 0.66}Ga{sub 0.34}Sb/AlSb SPS. The GaSb/Al{sub 0.33}Ga{sub 0.67}Sb MQW PL spectra were obtained at 300 K and 10 K with a fixed excitation power of 103 mW. Emission peaks appeared at 1758 nm and 1620 nm, respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Electron Microscopic Examination of an Irradiated TRISO Coated Particle of AGR-2 Experiment: AGR2-222-RS019

The transmission electron microscopic (TEM) examination of an irradiated tristructural isotropic (TRISO) coated particle from advanced gas reactor (AGR) 2 fuel Compact 2-2-2 is presented in this report. Compact 2-2-2 refers to the compact in Capsule 2 at Level 2 of Stack 2. The examination on AGR2 222 RS019 is focused on the interface between the silicon carbide (SiC) to inner pyrolytic carbon (IPyC) layer, central part of SiC, and the interface between SiC and outer pyrolytic carbon layer. The SiC grain boundary distribution and associated fission product precipitates were characterized by scanning transmission electron microscopy (STEM) energy dispersive spectroscopy (EDS), and precession electron diffraction (PED) in the transmission electron microscope. Two separate locations on SiC layer were selected for on the basis of the intact of the buffer layer in the irradiated TRISO particle. The key observation in this particle is that the compositional nature of grain boundary precipitates is more complex near IPyC/SiC layer where Pd, Pd-Ag, Pd-U containing precipitates can be observed. However, the compositions of precipitate towards outer most region of SiC mainly contain Pd. Ag has been observed in the IPyC/SiC layer in one of the two locations. Comparison of the results to previously irradiated and analyzed, Variant 3, safety tested AGR-1 TRISO particles (AGR1-433-001 and AGR1 222-RS004) showed the grain boundary distributions have some noticeable differences. The differences in coincident site lattice grain boundary and misorientation angles are possibly inherent to the fabrication process. Neutron irradiation induced void distribution has been found to be nonuniform. A high concentration of smaller voids is found at stacking faults compared to those in the SiC matrix. Larger void sizes are observed in the SiC location that is near to the broken buffer layer. This void size variation with integrity of buffer layer can potentially affect the fission product retention. However, the average void size continuously decreases along the radius of SiC in both locations.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2017

Acceptance Test Data for the AGR-5/6/7 Irradiation Test Fuel Composite Defective IPyC Fraction and Pyrocarbon Anisotropy

Coated particle composite J52R-16-98005 was produced by Babcock and Wilcox Technologies (BWXT) as fuel for the Advanced Gas Reactor Fuel Development and Qualification (AGR) Program’s AGR-5/6/7 irradiation test in the Idaho National Laboratory (INL) Advanced Test Reactor (ATR). This composite was comprised of four coated particle fuel batches J52O-16-93165B (26%), 93168B (26%), 93169B (24%), and 93170B (24%), chosen based on the Quality Control (QC) data acquired for each individual candidate AGR-5/6/7 batch. Each batch was coated in a 150-mm-diameter production-scale fluidized-bed chemical vapor deposition (CVD) furnace. Tristructural isotropic (TRISO) coatings were deposited on 425-μm-nominal-diameter spherical kernels from BWXT Lot J52R-16-69317 containing a mixture of 15.5%-enriched uranium carbide and uranium oxide (UCO). The TRISO coatings consisted of four consecutive CVD layers: a ~50% dense carbon buffer layer with 100-μm-nominal thickness, a dense inner pyrolytic carbon (IPyC) layer with 40-μm-nominal thickness, a silicon carbide (SiC) layer with 35-μm-nominal thickness, and a dense outer pyrolytic carbon (OPyC) layer with 40-μm-nominal thickness. The TRISO-coated particle batches were sieved to upgrade the particles by removing over-sized and under-sized material, and the upgraded batches were designated by appending the letter A to the end of the batch number (e.g., 93165A). Secondary upgrading by sieving was performed on the A-designated batches to remove particles with missing or very-thin buffer layers that were identified during previous analysis of the individual batches for defective IPyC, as reported in the acceptance test data report for the AGR-5/6/7 production batches [Hunn et al. 2017]. The additionally-upgraded batches were designated by appending the letter B to the end of the batch number (e.g., 93165B).

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2017

Acceptance Test Data for BWXT Coated Particle Batches 93172B and 93173B—Defective IPyC and Pyrocarbon Anisotropy

Coated particle batches J52O-16-93172B and J52O-16-93173B were produced by Babcock and Wilcox Technologies (BWXT) as part of the production campaign for the Advanced Gas Reactor Fuel Development and Qualification (AGR) Program’s AGR-5/6/7 irradiation test in the Idaho National Laboratory (INL) Advanced Test Reactor (ATR), but were not used in the final fuel composite. However, these batches may be used as demonstration production-scale coated particle fuel for other experiments. Each batch was coated in a 150-mm-diameter production-scale fluidized-bed chemical vapor deposition (CVD) furnace. Tristructural isotropic (TRISO) coatings were deposited on 425-μm-nominal-diameter spherical kernels from BWXT lot J52R-16-69317 containing a mixture of 15.5%-enriched uranium carbide and uranium oxide (UCO). The TRISO coatings consisted of four consecutive CVD layers: a ~50% dense carbon buffer layer with 100-μm-nominal thickness, a dense inner pyrolytic carbon (IPyC) layer with 40-μm-nominal thickness, a silicon carbide (SiC) layer with 35-μm-nominal thickness, and a dense outer pyrolytic carbon (OPyC) layer with 40-μm-nominal thickness. The TRISO-coated particle batches were sieved to upgrade the particles by removing over-sized and under-sized material, and the upgraded batches were designated by appending the letter A to the end of the batch number (e.g., 93172A). Secondary upgrading by sieving was performed on the A-designated batches to remove particles with missing or very-thin buffer layers that were identified during previous analysis of the individual batches for defective IPyC, as reported in the acceptance test data report for the AGR-5/6/7 production batches [Hunn et al. 2017b]. The additionally-upgraded batches were designated by appending the letter B to the end of the batch number (e.g., 93172B).

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2018

Pulsed Laser Deposition and Characterization of Heteroepitaxial LiMn 2 O 4 /La 0.5 Sr 0.5 CoO 3 Bilayer Thin Films as Model Lithium Ion Battery Cathodes

Epitaxial LiMn 2 O 4 (LMO)/La 0.5 Sr 0.5 CoO 3 (LSCO) bilayer thin films were grown on single crystalline SrTiO 3 (STO) (111) substrates as model lithium ion battery cathodes. The LSCO layer was used as an electrically conducting buffer layer for electrochemical testing. The LMO and LSCO layers were both epitaxial, with sub-nano flat LMO/LSCO interfaces as seen by X-ray diffraction, synchrotron X-ray scattering, and high-resolution transmission electron microscopy (HRTEM), but with a large LMO surface roughness due to the relatively large lattice mismatch with LSCO. Three-dimensional islands and depressions were formed on the strain-relaxed LMO layer, and misfit dislocations at the LMO/LSCO interface were discerned through HRTEM imaging, suggesting a Stranski–Krastanov (SK) mode thin-film growth. A crystalline structural change from cubic spinel at the LMO surface and interior to tetragonal oxygen-deficient LMO at the LMO/LSCO interface was examined. Furthermore, electrochemical tests along with in situ synchrotron X-ray scattering measurements on the epitaxial LMO/LSCO bilayers showed a significant loss of capacity after the first cycle, which was attributed to an electrical conductivity loss of the LSCO buffer layer due to irreversible lattice oxygen loss.

25 ENERGY STORAGE↗
DOE OSTI2019

Atomic Layer Deposition of Vanadium Oxide to Reduce Parasitic Absorption and Improve Stability in n-i-p Perovskite Solar Cells for Tandems

Two critical issues associated with semi-transparent, n-i-p perovskite solar cells for 2-terminal tandem devices are parasitic absorption and long-term instability associated with the widely used spiro-OMeTAD and MoOx hole transport and buffer layers, respectively. In this work, we present an alternative hole contact bilayer that consists of a 30 nm undoped layer of spiro-TTB in conjunction with 9 nm of air-stable vanadium oxide (VOx) deposited via atomic layer deposition. The low absorption of UV and visible light in this bilayer results in the fabrication of a semi-transparent perovskite cell with 18.9 mA cm-2 of photocurrent, a 14% increase compared to the 16.6 mA cm-2 generated in a control device with 150 nm of doped spiro-OMeTAD. The ALD VOx buffer layer shows promise as a stable alternative to MoOx; an unencapsulated Cs0.17FA0.83Pb(Br0.17I0.83)3 device with ALD VOx and ITO as the top contact maintains its efficiency following 1000 hours at 85 degrees C in a N2 environment. Lastly, we use transfer matrix modeling of the optimized perovskite stack to predict its optical performance in a monolithic tandem cell with heterojunction silicon.

14 SOLAR ENERGY↗
DOE OSTI2013

Effect of electron collecting metal oxide layer in normal and inverted structure polymer solar cells

We performed a systematic study of the effect of electron collecting metal oxide layer on the performance of P3HT: PCBM solar cells. Zinc oxide (ZnO) or titanium dioxide (TiO{sub 2}) buffer layers were prepared by either e-beam evaporation or solution processing method. We also compared the photovoltaic performance of inserting the buffer layer between indium tin oxide (ITO) and the polymer layer for the inverted structure (ITO/ ZnO or TiO{sub 2}/P3HT:PCBM/V{sub 2}O{sub 5}/Au) as well as inserting the buffers layers between the polymer and the aluminum electrode for the conventional structure (ITO/V{sub 2}O{sub 5}/P3HT:PCBM/ZnO or TiO{sub 2}/Al). The results are shown in detail.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Combinatorial Reactive Sputtering of In2S3 as an Alternative Contact Layer for Thin Film Solar Cells

High-throughput computational and experimental techniques have been used in the past to accelerate the discovery of new promising solar cell materials. An important part of the development of novel thin film solar cell technologies, that is still considered a bottleneck for both theory and experiment, is the search for alternative interfacial contact (buffer) layers. The research and development of contact materials is difficult due to the inherent complexity that arises from its interactions at the interface with the absorber. A promising alternative to the commonly used CdS buffer layer in thin film solar cells that contain absorbers with lower electron affinity can be found in ..beta..-In2S3. However, the synthesis conditions for the sputter deposition of this material are not well-established. Here, In2S3 is investigated as a solar cell contact material utilizing a high-throughput combinatorial screening of the temperature-flux parameter space, followed by a number of spatially resolved characterization techniques. It is demonstrated that, by tuning the sulfur partial pressure, phase pure ..beta..-In2S3 could be deposited using a broad range of substrate temperatures between 500 degrees C and ambient temperature. Combinatorial photovoltaic device libraries with Al/ZnO/In2S3/Cu2ZnSnS4/Mo/SiO2 structure were built at optimal processing conditions to investigate the feasibility of the sputtered In2S3 buffer layers and of an accelerated optimization of the device structure. The performance of the resulting In2S3/Cu2ZnSnS4 photovoltaic devices is on par with CdS/Cu2ZnSnS4 reference solar cells with similar values for short circuit currents and open circuit voltages, despite the overall quite low efficiency of the devices (-2%). Overall, these results demonstrate how a high-throughput experimental approach can be used to accelerate the development of contact materials and facilitate the optimization of thin film solar cell devices.

14 SOLAR ENERGY↗
DOE OSTI2018

A compositional gradient Ce1−xZrxO2 buffer architecture for producing high-performance YBCO film

Highlights: • The compositional gradient CZO buffer architecture can help improve the biaxial texture of YBCO film. • The high density stacking faults are directly observed via a spherical aberration corrected transmission electron microscope. • The enhanced flux pinning force and current-carrying capacity of YBCO films are achieved as a result of lattice strains. - Abstract: In this work, YBCO films were epitaxially grown on Ce{sub 1−x}Zr{sub x}O{sub 2} (CZO) buffered yttria stabilized zirconia (YSZ) single crystal substrates through a fluorine-free sol-gel process. The influence of two different CZO buffer architectures on the microstructure and superconducting properties of YBCO films was investigated. According to XRD results in combination with (S)TEM analysis, it was confirmed that YBCO film grown on the gradient CZO buffer layer (CZO-G) exhibits better in-plane and out-of-plane texture than YBCO film deposited on the uniform CZO buffer layer (CZO-20). And besides, the superconducting performances especially the current-carrying capacity under magnetic field of the former was much higher than that of the latter, which means that the enhanced flux pinning force will be presented in YBCO film on CZO-G buffer layer. The maximum pinning force of the YBCO film grown on CZO-G buffer architecture is up to 17.67 GN/m{sup 3} @ 65 K. HR-STEM analysis shows that the stacking faults within YBCO matrix gave rise to the considerable nanoscale lattice-strains which may suppress the formation of Copper pairs and produce nanoscale non-superconducting regions served as pinning centers to significantly improve the flux pinning performance of YBCO film. Given the above, it has been implied that high-performance YBCO coated conductor can be developed by using a compositional gradient Ce{sub 1−x}Zr{sub x}O{sub 2} films as buffer architecture.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Hysteresis phenomena of the two dimensional electron gas density in lattice-matched InAlN/GaN heterostructures

High-temperature transport properties in high-mobility lattice-matched InAlN/GaN heterostructures have been investigated. An interesting hysteresis phenomenon of the two dimensional electron gas (2DEG) density is observed in the temperature-dependent Hall measurements. After high-temperature thermal cycles treatment, the reduction of the 2DEG density is observed, which is more serious in thinner InAlN barrier samples. This reduction can then be recovered by light illumination. We attribute these behaviors to the shallow trap states with energy level above the Fermi level in the GaN buffer layer. The electrons in the 2DEG are thermal-excited when temperature is increased and then trapped by these shallow trap states in the buffer layer, resulting in the reduction and hysteresis phenomenon of their density. Three trap states are observed in the GaN buffer layer and C{sub Ga} may be one of the candidates responsible for the observed behaviors. Our results provide an alternative approach to assess the quality of InAlN/GaN heterostructures for applications in high-temperature electronic devices.

36 MATERIALS SCIENCE↗