bluuebunny/crossref_metadata_2025_split huggingface dataset:
Self-Heating Effects in High-Power AlGaN/GaN HFETs
Authors: Kuball M. , Uren M.J. , Hayes J.M. , Martin T. , Birbeck J.C.H. , Balmer R.S. , Hughes B.T. 
Published: April 2011

AbstractWe report on the non-invasive measurement of temperature, i.e., self-heating effects, in active AlGaN/GaN heterostructure field effect transistors (HFETs). Micro-Raman spectroscopy was used to produce temperature maps with ≈1 μm spatial resolution and a temperature accuracy of better than 10°C. Significant self-heating effects in the source-drain opening of AlGaN/GaN HFETs were measured. Devices grown on sapphire and SiC substrates were compared. Three-dimensional finite-difference heat dissipation calculations were performed as function of device geometry.
A High-Power AlGaN/GaN Hetero Field-Effect Transistor
Authors: Yoshida Seikoh , Ishii Hirotatsu 
Published: April 2011

AbstractAn AlGaN/GaN hetero field effect transistor (HFET) was operated at 20 A. Its on-state resistance was lower than that of a Si-based FET. GaN and related materials were grown by gas-source molecular beam epitaxy (GSMBE). Sapphire substrates were used for GaN growth. An undoped GaN/Al0.2Ga0.8N heterostructure wasgrown on the substrate. The sheet carrier density of two dimensional electron gas was 1x1013 cm2 and the mobility was 1200 cm2/Vs at room temperature. The breakdown field of undoped high resistive GaN layer was about 2.0 MV/cm. Al0.2Ga0.8N/GaN HFET for a large current operation was fabricated. The gate width was 20 cm and thegate length was 2000 nm. The electrode material of the source and the drain was Al/Ti/Au and the Schottky electrodes were Pt/Au. The distance between the source and the drain was 6000 nm. The maximum breakdown voltage of gate and source was 600 V. The on-state resistance of the HFET was about 2 mohm•cm2 at 100 V. The transconductance (gm) of this HFET was about 120 mS/mm. It was confirmed that the HFET with a gate width of20 cm could be operated at the condition of a large current operation.
Back-Doping Design in AlGaN/GaN Heterostructure Field-Effect Transistors for High-Power Applications
Authors: Maeda Narihiko , Tsubaki Kotaro , Saitoh Tadashi , Kobayashi Naoki 
Published: April 2011

AbstractA novel doping design has been proposed that yields high two-dimensional electron gas (2DEG) densities in the AlGaN/GaN heterostructure field-effect transistors (HFETs) even when the AlGaN barrier layers are designed to be very thin. In the novel doping design, an asymmetric double-heterostructure is employed, and donor atoms are doped not only in the surface-side AlGaN layer but also in the underlying AlGaN layer. In this structure, electrons are efficiently supplied also from the back-doped AlGaN barrier layer to the GaN channel, with the help of the negative polarization charges at the heterointerface between the GaN channel and the underlying AlGaN barrier layer. High 2DEG densities can thus be obtained. Moreover, relatively high 2DEG mobilities can be obtained for high 2DEG densities, because back-doped donor atoms are sufficiently remote from the position of the 2DEG so that the 2DEG is less subjected to the ionized impurity scattering due to the relevant donor atoms. By using this back-doping design, a very high 2DEG density of 2.8x1013 cm-2 (2DEG mobility is 850 cm2/Vs) has been obtained at 300 K in the Al0.3Ga0.7N/GaN HFET whose barrier layer (Al0.3Ga0.7N) is as thin as 120 Å. Thus, the back-doping design is effective to obtain high 2DEG densities in the HFETs with thin barrier layers, and promising for high-power applications.
High Quantum Efficiency AlGaN/GaN Solar-Blind Photodetectors Grown by Metalorganic Chemical Vapor Deposition
Authors: Chowdhury U. , Wong M. M. , Collins C. J. , Yang B. , Zhu T. G. , Beck A. L. , Campbell J. C. , Dupuis R. D. 
Published: April 2011

AbstractWe report the growth, fabrication and characterization of back-illuminated AlGaN p-i-n photodetectors operating in the solar-blind wavelength range. A peak external quantum efficiency (EQE) of 45.5% (0.1A/W) was obtained at a wavelength of 270 nm. This EQE was obtained at zero volt bias and without the use of any anti-reflection coating on the back of the substrate. To our knowledge, this is the highest EQE reported to date for p-i-n photodetectors operating in the solar-blind wavelength range.
Novel conguration of Mg-H complexes in GaN
Authors: Limpijumnong Sukit , Northrup John E. , Walle Chris G. Van de 
Published: April 2011

AbstractWe describe a novel conguration of the Mg-H complex in GaN, which is not the lowest-energy conguration at T =0 but is stabilized at elevated temperatures by the large entropy associated with a set of low-energy rotational excitations. We focus on a comparison with two types of experimental results: (1) vibrational spectroscopy using polarized light [B. Clerjaud et al., Phys. Rev. B 61, 8328 (2000)]; and (2) ion-channeling [W. Wampler et al., J. Appl. Phys. 90, 108 (2001)]. New results on the predicted quantum-mechanical delocalization of the hydrogen atom and on the ect of replacing hydrogen with deuterium are reported.
GaNxAs1-xGrowth by Molecular Beam Epitaxy with Dispersive Nitrogen
Authors: Wang S.Z. , Yoon S.F. , Ng T.K. , Loke W.K. , Fan W.J. 
Published: April 2011

AbstractThe effect of energetic nitrogen ion bombardment during growth may have a deleterious effect on the material quality. To avoid the bombardment effect of energetic nitrogen ions, a modified mode for GaAsN growth using dispersive nitrogen is reported. High quality GaAsN epilayers and good GaAsN/GaAs interface were achieved using this growth mode. The results suggest that the surface of samples grown using dispersive nitrogen has fewer defects than those grown using direct nitrogen beam. The optical property of GaAsN samples grown using the dispersive nitrogen technique was found to improve, due to the lower ion bombardment effect. This growth technique is expected to be advantageous for growing high quality GaAsN materials for optoelectronic applications.
Surface Instability and Associated Roughness of Pendeo-epitaxy GaN (0001) Films Grown via Metalorganic Vapor Phase Epitaxy
Authors: Roskowski Amy M. , Miraglia Peter Q. , Preble Edward A. , Einfeldt Sven , Davis Robert F. 
Published: April 2011

AbstractA growth process route that results in thin film GaN templates with a smooth surface morphology at the optimum temperature of 1020°C has been developed. Atomic force microscopy (AFM) reveals hillocks on films grown above 1020°C. Hillocks resulted from the rotation of heterogeneous steps formed at pure screw or mixed dislocations which terminated on the (0001) surface. Growth of the latter feature was controlled kinetically by temperature through adatom diffusion. The 106 cm-2 density of the hillocks was reduced through growth on thick GaN templates and regions of pendeo-epitaxy (PE) overgrowth with lower pure screw or mixed dislocations. Smooth PE surfaces were obtained at temperatures that reduced the lateral to vertical growth rate but also retarded hillock growth that originated in the stripe regions. The (1120 ) PE sidewall surface was atomically smooth, with a root mean square roughness value of 0.17 nm which was the noise limited resolution of the AFM measurements.
White Beam Synchrotron X-ray Topography and X-ray Diffraction Measurements of Epitaxial Lateral Overgrowth of GaN
Authors: Chen W.M. , McNally P.J. , Jacobs K. , Tuomi T. , Danilewsky A.N. , Lowney D. , Kanatharana J. , Knuuttila L. , Riikonen J. 
Published: April 2011

AbstractThe Epitaxial Lateral Overgrowth (ELO) of GaN on Al2O3 using a SiO2 mask with different fill factors (ratio of stripe opening width to stripe period) is examined with White Beam Synchrotron X-ray Topography (WBSXT) and X-ray rocking curve analysis. The sapphire substrate was identified with a dislocation density of the order of ~106cm-2. WBSXT in both transmission and back reflection mode is used to image the ELO GaN and confirms that crystal planes in the lateral overgrown part (wing) are tilted, and that the wing tilt increases as the fill factor increases. X-ray rocking curve and WBSXT measurements confirm the same wing tilt tendency as the fill factor changes. The WBSXT method provides a measure of the maximum wing tilt, while the X-ray rocking curve method gives the average wing tilt. The average wing tilt reaches about 1602 arcsec at a fill factor of 0.625, but the maximum wing tilts can reach values as large as 2372 arcsec when the fill factor is only 0.571. This study shows that WBSXT is an effective method in dislocation and wing tilt determination for the GaN on Al2O3 ELO epilayer system or indeed for similar systems. The tilted wings induce a slightly lower compressive stress in the coalesced region of the GaN epilayer.
Removal of 6H-SiC substrate influence when evaluating GaN thin film properties via x-ray
Authors: Preble Edward A. , Miraglia Peter Q. , Roskowski Amy M. , Einfeldt Sven , Davis Robert F. 
Published: April 2011

AbstractNon-uniformity in GaN thin films deposited on 6H-SiC can make determining the effects of growth variables difficult. Results presented in this work show the effects of the SiC substrates on the GaN films, and how to correct for these effects to obtain meaningful data about the properties of the thin film rather than the substrate underneath. Rocking curve values of GaN thin films are found to track almost 1:1 with the values of the underlying SiC. Plotting rocking curves with respect to the substrate, as well as a variable of importance can therefore yield more meaningful and reliable comparisons instead of plotting the data for the variable alone. This procedure is used to demonstrate the effects of thickness and AlN and AlGaN buffer layers on GaN thin films.
Influence of substrate orientation on photoluminescence in InGaN/GaN multiple quantum wells
Authors: Chen P. , Chua S.J. , Wang W. 
Published: April 2011

AbstractIn this study, the photoluminescence (PL) spectra of InGaN/GaN multiple quantum wells (MQWs) grown on sapphire substrates with different orientation at 720°C were investigated at room temperature. Four different substrates were used, which have different surface step structures. Their orientations are: c-plane, a-plane, c-plane with off-set of 2 degree and c-plane with off-set of 6 degree towards a-plane. PL spectra from InGaN/GaN MQWs grown on c-plane substrates showed double-peak emission, while those from the MQWs grown on the others substrates showed single-peak emission. The strongest emission was found on the substrate with off-set of 2 degree towards a-plane. A Shift of PL peak position to long wavelength was observed on the substrates with an off-set angle. Meantime, all emissions were also investigated at different excitation power density. Experimental results indicate that the regular step structure on the substrates can seriously affect the growth of InGaN/GaN MQWs. Segregation effect in InGaN will lead to the high In composition regions (quantum-wire like structure) in the substrates with an off-set angle due to the regular steps on them, as observed by atomic force microscopy. It can be concluded that the surface steps on substrate play an important role in the formation of the In-rich InGaN quantum-wire like structure.
Nanobumps in InxAl1-xN/AlN/Sapphire System:A New Kind of Quantum Dots?
Authors: Danylyuk Yuri , Romanov Dmitri , Auner Gregory 
Published: April 2011

AbstractWe have investigated InxAl1-xN layers grown by plasma source molecular beam epitaxy (PSMBE) on c-plane sapphire (0001) substrates with thin (about 100 Å) buffer layers of AlN. The value of x varied from 0 to 1. For all these layers, high resolution X-ray diffraction scans (XRD) show no indication of face segregation, while the atomic force microscope (AFM) images of the structures demonstrate a large number of nanobumps. Our spectroscopic measurements (UV/VIS optical absorption and reflection spectroscopy) of these bumpy structures indicate additional peaks that can be only associated with additional energy levels. We ascribe these levels to electrons, which are confined and quantized near the nanobump tips by strong piezoelectric field caused mainly by biaxial strain of the AlN layer. The calculated energies of these quantized states are in a good agreement with the spectroscopic data.
Optical Absorption of Nitrogen Vacancy in Proton Irradiated AlxGa1-xN thin Films
Authors: Zhou Qiaoying , Manasreh M. O. , Pophristic M. , Ferguson Ian T. , Weaver B. D. 
Published: April 2011

AbstractOptical absorption measurements were used to investigate deep defects in proton irradiated doped and undoped AlGaN thin films grown on sapphire substrates. Several samples were proton irradiated with energies ranging between 10 keV and 1 MeV. In certain samples, multiple-energy ion implantation was found necessary to produce a defect, which is responsible for the absorption band observed at 4.61 eV with a shoulder at around 4.10 eV in Al0.6Ga0.4N. Furnace thermal annealing of the irradiated samples show that this absorption band starts to anneal out at temperature as low as 200 oC. A combined isochronal and isothermal annealing in the temperature range of 200- 350°C shows that the activation energy (enthalpy associated with the migration process) of this defect is approximately 0.41 eV. This leads us to conclude that this absorption band is due to a N-vacancy related defect. It is observed that the peak position energy of the absorption band due to this defect is shifted depending on the Al mole fraction in good agreement with the theoretical predictions.
Application of Amorphous GaN for Electroluminescence Device
Authors: Honda Tohru , Kawanishi Hideo 
Published: April 2011

AbstractThe vacuum evaporation of GaN thin films using GaN powder (5N) for GaN-based electroluminescence devices (ELDs) is reported. The crystal structures of the evaporated GaN layers were amorphous, which was confirmed from reflection high-energy electron diffraction (RHEED) patterns. Auger electron spectra revealed that the layers have excess Ga metal. Bluish-white light emission was observed from the GaN-based ELD under AC operation at RT. Although the emission intensity was weak, the electroluminescence spectra started from the band edge of h-GaN.
High Mobility Nitrides
Authors: Scott K. , Butcher A. , Wintrebert-Fouquet Marie , Chen Patrick P.–T. , Tansley Trevor L. , Srikeaw Surapon 
Published: April 2011

AbstractThe highest mobility nitrides ever grown were indium nitride polycrystalline thin films. The original reactive ion sputtering unit used to produce those films is still in existence and has been substantially upgraded. In this paper we describe some of the parameters that are important for high purity indium nitride growth, while providing the most recent results for films grown with the upgraded system. A long lag time (greater than 100 hours of growth time) has been observed before obtaining stable material properties for a given set of growth conditions.
Electron and Hole Confinement in GaInN/GaN and AlGaN/GaN Quantum Wells
Authors: Hangleiter A. , Lahmann S. , Netzel C. , Rossow U. , Kent P. R. C. , Zunger A. 
Published: April 2011

AbstractWe show that the strong bowing of the bandgap of GaInN, which is primarily due to bowing of the valence band edge, translates into a strongly composition dependent ratio of the conduction band offset to the valence band offset with respect to GaN. For common In mole fractions of 0-20 % this leads to a reversal of the band offset ratio and to very weak electron con nement. This theoretical picture is veri ed by comparing results of time-resolved spectroscopy on asymmetric AlGaN/GaInN/GaN and AlGaN/GaN/AlGaN quantum wells. Since electron con nement is much stronger for GaN/AlGaN wells than for GaInN/GaN wells, the effect of asymmetry is very weak for the former and fairly strong for the latter.
Measurements of the Band Offset of SiO2 on Clean GaN
Authors: Hurt E.H. , Cook Ted E. , Tracy K.M. , Davis R.F. , Lucovsky G. , Nemanich R.J. 
Published: April 2011

AbstractThe band alignment of SiO2 and GaN is important for passivation of high voltage devices and for gate insulator applications. In this study XPS and UPS techniques are employed to determine the electronic states as SiO2 is deposited onto a clean GaN surface. The substrate was epitaxially grown n-type GaN on 6H-SiC (0001) substrates with an AlN (0001) buffer layer. The GaN surface was atomically cleaned via a 860°C anneal in an NH3 atmosphere. For the clean GaN surface, upward band bending of ~0.3 ±0.1 eV was measured, and the electron affinity was measured to be ~2.9 eV. Layers of Si were deposited on the GaN surface via Molecular Beam Epitaxy (MBE), and the Si was oxidized by a remote O2 plasma. The oxidation of the Si occurred without oxidizing the GaN. Densification of the created SiO2 film was achieved by annealing the substrate at 650°C. Surface analysis techniques were performed after each process, and yielded a valence band offset of ~2.0 eV, and a conduction band offset of ~3.6 eV for the GaN-SiO2 interface.
Electromagnetic Waves Through Disordered Systems: Comparison of Intensity, Transmission and Conductance
Authors: Zypman Fredy R , Cwilich Gabriel 
Published: April 2011

AbstractWe obtain the statistics of the intensity, transmission and conductance for scalar electromagnetic waves propagating through a disordered collection of scatterers. Our results show that the probability distribution for these quantities x, follow a universal form, YU(x) = xne−xμ. This family of functions includes the Rayleigh distribution (when α=0, μ=1) and the Dirac delta function (α →+ ∞), which are the expressions for intensity and transmission in the diffusive regime neglecting correlations. Finally, we find simple analytical expressions for the nth moment of the distributions and for to the ratio of the moments of the intensity and transmission, which generalizes the n! result valid in the previous case.
Photonic Crystals at Near-Infrared and Optical Wavelengths
Authors: Moroz Alexander 
Published: April 2011

AbstractAs demonstrated for the example of a diamond and zinc blende structure of dielectric spheres, small inclusions of a low absorbing metal with the volume fraction fm can have a dramatic effect on a complete photonic band gap (CPBG) between the 2nd-3rd bands. For example, in the case of silica coated silver spheres, the CPBG opens for fm ≍ 1.1% and exceeds 5% for fm ≍ 2.5%. Consequently, any dielectric material can be used to fabricate a photonic crystal with a sizeable and robust CPBG in three dimensions. Absorption in the CPBG of 5% remains very small (≤ 2.6% for λ ≥ 750 nm). The structure enjoys almost perfect scaling, enabling one to scale the CPBG from microwaves down to ultraviolet wavelengths.
Photonic Crystal Narrow Pass-band Filtersa
Authors: Ouyang Zhengbiao , Liu Haishan , Li Jingzhen 
Published: April 2011

AbstractWe present a photonic crystal narrow pass-band optical filter, which is made of two pieces of photonic crystals with different photonic band gaps. Our numerical investigations show that this kind of structure is feasible for narrow pass-band optical filtering. The pass-band may be less than 0.1nm in the infrared communication frequency band. In ideal cases, the optical transmission in the pass band is very close to 1. This means a very low insert energy loss.
The Correlation of Indentation Size Effect Experiments with Pyramidal and Spherical Indenters
Authors: Swadener J. G. , George E. P. , Pharr G. M. 
Published: April 2011

ABSTRACTExperiments were conducted in annealed iridium using pyramidal and spherical indenters over a wide range of load. For a Berkovich pyramidal indenter, the hardness increased with decreasing depth of penetration. However, for spherical indenters, hardness increased with decreasing sphere radius. Based on the number of geometrically necessary dislocations generated during indentation, a theory that takes into account the work hardening differences between pyramidal and spherical indenters is developed to correlate the indentation size effects measured with the two indenters. The experimental results verify the theoretical correlation.
Pressure Assisted Crystallization of MnAl Thin Films
Authors: Fischer Gregory A. , Rudee M. Lea , Nesterenko Vitali F. , Indrakanti Sastry 
Published: April 2011

ABSTRACTThe effect of hot isostatic pressure processing (HIP) on MnAl films has been compared to vacuum annealing for the purpose of obtaining substantial amounts of tau phase MnAl in films under 200 nm. Films were deposited by dc sputtering from both MnAlNiC an MnAl targets. As-deposited films were nearly amorphous. Post deposition annealing in vacuum produced only small amounts of the ferromagnetic tau-phase in films thinner than 200 nm.In all instances, regardless of substrate and sputtering target, the use of HIP in place of vacuum annealing increased the degree of crystallinity of the samples when compared to those annealed in vacuum. For the 100 nm samples deposited from the MnAlNiC target, these changes in crystallinity were accompanied by changes in the M-H loops of the samples. MnAlNiC HIP samples had improved magnetic properties compared to those of equal thickness annealed in vacuum. The 100 nm HIP sample sputtered from the MnAl target also showed an increase in moment, though the changes were not as dramatic as those seen in the samples sputtered from the MnAlNiC target.The 50 nm films from both targets also showed a change in crystallinity when compared to vacuum annealed samples. These films, unlike the 100 nm films, had ferromagnetic properties that were no better than those of the vacuum annealed samples. This suggests that while the 2 kbar of pressure used in this study assists in the formation of tau-phase in 100 nm films, the appropriate pressure for forming tau-phase in 50 nm films is yet to be determined.
MOCVD ZnS:Mn Films: Crystal Structure and Defect Microstructure as a Function of the Growth Parameters
Authors: Dunn Kathleen A. , Dovidenko Katharine , Topol Anna W. , Shekhawat Gajendra S. , Geer Robert E. , Kaloyeros Alain E. 
Published: April 2011

ABSTRACTThin film electroluminescent devices employing zinc sulfide doped with manganese are extensively used for applications in which the weight, brightness and mechanical robustness requirements preclude the use of other types of displays such as cathode ray tubes or liquid crystal displays. The physical, optical and electrical properties of phosphors such as ZnS:Mn can often depend strongly on microstructure, which in turn depends on the growth and processing of the film. For this study, ZnS:Mn layers were fabricated by metalorganic chemical vapor deposition (MOCVD) in the 250°-500°C range on an Al2TiO/ In2SnO5 /glass stack. Selected samples were then subjected to a post-deposition anneal in H2S/Ar at 700°C for up to 4 hours. The microstructure of the ZnS:Mn films was examined by Transmission Electron Microscopy (TEM). For all growth and annealing conditions, the films consisted of columnar grains whose column axis was parallel to the growth direction, and which widened laterally through the thickness of the films. For the as-deposited films, the crystal structure was found to be predominantly 2H structure, with the 8H polytype being identified in the low-temperature ZnS:Mn films. The 700°C post-deposition annealing was found to initiate a solid state transformation to the cubic (3C) ZnS crystal structure. All films contained high densities of stacking faults and microtwins, whose role in the 2H-3C transformation is discussed. Also discussed are initial Ultrasonic Force Microscopy (UFM) results which suggest a correlation between the defect microstructure and the elastic response of the material.