bluuebunny/crossref_metadata_2025_split huggingface dataset:
Delta-doped AlGaN/GaN Heterostructure Field-Effect Transistors with Incorporation of AlN Epilayers
Authors: Fan Z. Y. , Nakarmi M. L. , Lin J. Y. , Jiang H. X. 
Published: April 2011

ABSTRATCThe simulation and experiment results of delta-doped AlGaN/GaN heterostructure field-effect transistors (HFETs) with the incorporation of highly-resistive AlN epilayer are reported. The high quality AlN epilayer is used as the dislocation filter for the HFET structure growth, and the high resistivity of AlN also removes the parasitic conduction related with the GaN bulk buffer. Delta doping can reduce gate leakage, further more, our simulation and growth results demonstrate that delta-doping in the barrier is more effective than uniform doping scheme to increase the sheet electron density. The influence of spacer layer thickness on the electron mobility and sheet electron density is also presented. The DC characterization of the fabricated devices shows our structure has a very high performance with a maximum current ∼ 1 A/mm.
A Study of Elemental Interdiffusion in GaN/Si Wafer Grown by Metalorganic Vapor Phase Epitaxy
Authors: Chen X. , Ishiko M. , Kuroiwa Y. , Sawaki N. 
Published: April 2011

ABSTRACTSecondary ion mass spectrometry (SIMS) study shows that a short-time interdiffusion process occurs at the interface of buffer/Si in the GaN/Si wafers grown by metalorganic vapor phase epitaxy (MOVPE). Increasing the growth temperature of the buffer layer from 1150°C to 1210°C reduces the dropping rate of Al concentration in the silicon substrate from ∼20nm/decade to ∼30nm/decade near the interface. The impact of the interdiffusion on the band structure is investigated by photocurrent spectroscopy measurement. The experimental results indicate that different build-in electric fields are formed in the GaN/n-Si(111) and GaN/p-Si(111) devices as the consequence of the MOVPE growth.
Real-time optical monitoring of gas phase dynamics for the growth of InN at elevated pressures
Authors: Dietz N. , Born H. , Strassburg M. , Woods V. 
Published: April 2011

ABSTRACTThe request for increased performance in high-power / high-frequency optoelectronic devices requires new methods for the fabrication of high quality III nitride alloys that exhibit large thermal decomposition pressure such as InN and related materials. To extend the process and growth window towards elevated pressures, a high-pressure CVD system with integrated real time optical characterization techniques has been constructed. The built-in real-time monitoring techniques allow the characterization of gas flow dynamics, precursor decomposition kinetics, as well as the monitoring of the crucial steps of nucleation and film formation. The gas flow dynamics has been characterized and the process parameter are obtained under which the thin film growth process can be maintained under laminar flow condition. Laser light scattering (LLS) has been proven as the most robust optical tool to characterize the onset of turbulence.
Emission Mechanisms in UV Emitting GaN/AlN Multiple Quantum Well Structures
Authors: Furis Madalina , Cartwright Alexander N. , Wu Hong , Schaff William J. 
Published: April 2011

ABSTRACTThe need for efficient UV emitting semiconductor sources has prompted the study of a number of heterostructures of III-N materials. In this work, the temperature dependence of the photoluminescence (PL) properties of UV-emitting GaN/AlN multiple quantum well (MQW) heterostructures were investigated in detail. In all samples studied, the structure consisted of 20 GaN quantum wells, with well widths varying between 7 and 15 Å, clad by 6nm AlN barriers, grown on top of a thick AlN buffer that was deposited on sapphire by molecular beam epitaxy. The observed energy corresponding to the peak of the emission spectrum is in agreement with a model that includes the strong confinement present in these structures and the existence of the large built-in piezoelectric field and spontaneous polarization present inside the wells. The observed emission varies from 3.5 eV (15 Å well) to 4.4 eV (7 Å well). Two activation energies associated with the photoluminescence quenching are extracted from the temperature dependence of the time-integrated PL intensity. These activation energies are consistent with donor and acceptor binding energies and the PL is dominated by recombination involving carriers localized on donor and/or acceptor states.Moreover, the temperature dependence of the full width at half-maximum (FWHM) of the PL feature indicates that inhomogeneous broadening dominates the spectrum at all temperatures. For the 15 and 13 Å wells, we estimate that the electron-phonon interaction is responsible for less than 30% of the broadening at room temperature. This broadening is negligible in the 9 Å wells over the entire temperature range studied. Well width fluctuations are primarily responsible for the inhomogeneous broadening, estimated to be of the order of 250meV for one monolayer fluctuation in well width.
Surface Control of ZrB2 (0001) Substrate for Molecular-Beam Epitaxy of GaN
Authors: Suda Jun , Yamashita Hiroyasu , Armitage Robert , Kimoto Tsunenobu , Matsunami Hiroyuki 
Published: April 2011

ABSTRACTZirconium diboride (ZrB2) is a promising lattice-matched substrate for GaN-based materials. A key issue to realize high-quality heteroepitaxial growth is preparation of the substrate surface. The ZrB2 surface was studied by x-ray photoemission spectroscopy (XPS) and reflection high-energy electron diffraction (RHEED). XPS results indicated the presence of both ZrO2 and ZrB2 on the as-received substrate surface. Thermal cleaning at 1000°C in ultra-high vacuum, Ar+ ion sputtering, and wet chemical treatments were examined as surface preparation methods. After treatment with HF acid, the O peak intensity was much reduced. The combination of HF treatment and thermal cleaning resulted in sharp and intense RHEED from the ZrB2 surface. GaN grown on the surface by molecular-beam epitaxy exhibited intense photoluminescence, suggesting that this treatment is effective to obtain high-quality GaN on ZrB2 substrates.
The Influence of Substrate Polarity on the Blue Emission from As-doped GaN Layers Grown by Molecular Beam Epitaxy
Authors: Novikov S. V. , Zhao L. X. , Foxon C. T. , Harrison I. , Campion R. P. , Staddon C. R. , Kang S. W. , Kryliouk O. , Anderson T. 
Published: April 2011

ABSTRACTThe influence of sample orientation and polarity on the blue emission from As-doped GaN layers grown by plasma-assisted molecular beam epitaxy (PA-MBE) was investigated. Arsenic doped GaN layers were grown under identical PA-MBE conditions on several types of substrates including c-plane (0001) sapphire and polar and non-polar GaN templates grown by metal-organic vapour phase epitaxy (MOVPE). Non-polar GaN MOVPE templates were grown on aplane (11–20) sapphire and LiAlO2 (100). The orientation and polarity have a strong influence on the morphology and the optical properties of As-doped GaN layers. Strong blue emission from As-doped GaN was observed only in the case of (000–1) oriented N-polarity layers.
Characterization of Photovoltaic Cells Using n-InN/p-Si Grown by RF-MBE
Authors: Morioka Chiharu , Yamaguchi Tomohiro , Naoi Hiroyuki , Araki Tsutomu , Suzuki Akira , Nanishi Yasushi 
Published: April 2011

ABSTRACTA newly reported narrow bandgap for indium nitride means that the indium gallium nitride system of alloys can be a candidate for new high efficiency solar cells covering most of the solar spectrum. In this paper, n-InN films were grown on p-Si (100) substrates. We characterize, for the first time, photovoltaic properties using n-InN/p-Si hetero-junction grown by RF-MBE.
Laser Diode Facet Degradation Study
Authors: Schwarz Ulrich T. , Schoedl Thomas , Kümmler V. , Lell A. , Härle V. 
Published: April 2011

ABSTRACTWe study the degradation behaviour of GaN gain guided laser diodes (LDs) on SiC substrates with cleaved facets and reflective coatings on none, one, or both facets. This allows us to demonstrate that in addition to volume effects there is a contribution of the laser facets to laser degradation. We observe that for the uncoated LDs the threshold current density is increasing considerably faster compared to LDs with mirror coatings. Degradation is observed during operation but not during storage at ambient conditions and thus expected to be photon or current induced. Operation of the uncoated laser in a nitrogen atmosphere reduces the degradation rate with respect to operation in air.
Optical and Electrical Properties of Low to Highly-Degenerate InN Films
Authors: Haddad D. B. , Dai H. , Naik R. , Morgan C. , Naik V. M. , Thakur J. S. , Auner G. W. , Wenger L. E. , Lu H. , Schaff W. J. 
Published: April 2011

ABSTRACTThe optical and electrical properties of InN films with different levels of carrier concentrations have been investigated. Hall effect measurements at room temperature show that the InN films are n-type with carrier concentration, ne, ranging from ∼ 7 ×1017 cm-3 to ∼ 3 × 1020 cm-3 and corresponding mobility, //, of ∼ 1300 to 50 cm2V-1S-1. Optical absorption spectra of these films show a bandgap absorption edge ∼ 0.6 eV for the InN sample with the lowest ne, and 1.5 eV for the InN sample with the highest ne. However, after corrections for the degeneracy effects, all samples show an intrinsic Eg ∼ (0.60 ± 0.05) eV. Temperature dependent (5 – 600 K) electrical measurements show that ne is nearly independent of temperature below 300 K, perhaps due to the presence of donor energy levels resonating with the InN conduction band. However, all the samples show an exponential increase in ne above 300 K due to excitation of other shallow donor like sources. Mobility versus temperature graph shows a maximum ∼ 200 K for InN film with ne = 7 × 1017 cm-3 and moves towards lower temperature with increasing ne.
Pressure Dependence of Optical Transitions in In-rich Group III-Nitride Alloys
Authors: Li S. X. , Wu J. , Walukiewicz W. , Shan W. , Haller E. E. , Lu Hai , Schaff William J. 
Published: April 2011

ABSTRACTThe hydrostatic pressure dependence of the optical transitions in InN, In-rich In1-xGaxN (0 < x < 0.5) and In1-xAlxN (x = 0.25) alloys is studied using diamond anvil cells. The absorption edges and the photoluminescence peaks shift to higher energy with pressure. The pressure coefficient of InN is determined to be 3.0±0.1 meV/kbar. Together with previous experimental results, our data suggest that the pressure coefficients of group-III nitride alloys have only a weak dependence on the alloy composition. Photoluminescence gives much smaller pressure coefficients, which is attributed to emission involving highly localized states. This indicates that photoluminescence might not be an accurate method to study the pressure dependence of the fundamental bandgaps of group III-nitrides.
Either step-flow or layer-by-layer growth for AlN on SiC (0001) substrates
Authors: Suda Jun , Onojima Norio , Kimoto Tsunenobu , Matsunami Hiroyuki 
Published: April 2011

ABSTRACTAlN was grown on 4H- or 6H-SiC (0001) on-axis substrates by plasma-assisted molecular beam epitaxy. By utilizing optimized SiC surface pretreatment, RHEED oscillations just after the growth of AlN were obtained with high reproducibility. This study focused on the growth kinetics of AlN and the correlation between kinetics and the crystalline quality of the grown layers. It was found that the growth mode changed from layer-by-layer to step-flow for high growth temperatures, while for lower temperatures the layer-by-layer growth mode persisted. The mechanism responsible for the change in growth mode is discussed. Symmetrical (0002) and asymmetrical (01–14) x-ray rocking curve measurements were carried out to evaluate the crystalline quality. For the (0002) peak, both high-temperature and low-temperature grown layers showed almost the same FWHM values. On the other hand, for the (01–14) peak, the FWHM of low-temperature grown AlN was much smaller (180 arcsec) than that of the high-temperature grown AlN (450 arcsec).
ZnO/AlGaN ultraviolet light emitting diodes.
Authors: Bagnall D. M. , Alivov Ya. I. , Kalinina E. V. , Look D. C. , Ataev B. M. , Chukichev M. V. , Cherenkov A. E. , Omaev A. K. 
Published: April 2011

ABSTRACTGa doped n-type ZnO layers are grown using plasma assisted chemical vapor deposition on Mg doped p-type AlGaN epitaxial layers grown by hydride vapor phase epitaxy to form n-ZnO/p-AlGaN heterojunction light emitting diodes. I-V characteristics clearly show rectifying behavior with a threshold voltage of ∼3.2 V and intense ultraviolet electroluminescence with peak emission at 390 nm. The dominant emission mechanism is found to result from hole injection from the p-type AlGaN into the n-type ZnO. Significant emission up to 500 K is observed indicating possible applications in harsh environments.
Influence of AlN Overgrowth on GaN Nanostructures Grown by Molecular Beam Epitaxy
Authors: Gogneau N. , Monroy E. , Jalabert D. , Sarigiannidou E. , Rouvière J. L. , Daudin B. 
Published: April 2011

ABSTRACTThe effects of AlN overgrowth on the structural properties of GaN nanostructures grown at 750°C by plasma-assisted molecular beam epitaxy have been investigated using Rutherford backscattering spectroscopy and transmission electron microscopy. The capping process induces a remarkable change in the dimension of the nanostructures. We demonstrate that the thickness/size reduction occurs at the first stage of AlN overgrowth and affects only the top GaN/AlN interface. This phenomenon is attributed to an exchange mechanism between Al atoms from the cap layer and Ga atoms in the nanostructures and depends on the strain state of the nanostructures.
III-Nitride Photonic Crystals for Blue and UV Emitters
Authors: Shakya J. , Kim K. H. , Li J. , Lin J. Y. , Jiang H. X. , Oder T. N. 
Published: April 2011

ABSTRACTWe report the nanofabrication and characterization of triangular lattice array of photonic crystals (PCs) with diameter/periodicity as small as 100/180 nm on III-nitride Light Emitting Diodes (LEDs) using electron beam lithography and inductively-coupled-plasma dry etching. Under optical pumping, a maximum enhancement factor of 20 was obtained from the PCs for emission light intensity at the wavelength of 475 nm at room temperature. Under current injection, the total power at 20 mA of 300 × 300 μm2 unpackaged LED chips revealed an increase by 63% and 95% for 460 nm blue and 340 nm UV LEDs, respectively, as a result of the PC formation. Our results show that the fabrication of PCs enhances the power output significantly on the III-nitride LEDs, which currently have very low external quantum efficiency especially in the UV range.
Effects of Si-doping on the Microstructure of AlGaN/GaN Multiple-quantum-well
Authors: Liu R. , Ponce F. A. , Sahonta S-L. , Cherns D. , Amano H. , Akasaki I. 
Published: April 2011

ABSTRACTThe effects of silicon-doping on the microstructure of Al0.07Ga0.93N/GaN multiple-quantum-well (MQW) have been studied by TEM. Significant changes of surface morphology and dislocation core structures have been observed due to Si-doping in the Al0.07Ga0.93N barriers. Threading dislocations create surface pits in the MQW as a result of Si doping. With an increasing doping level, the pits change the shape from small faceted pyramid to large cone. The formation mechanism of the surface pits has been discussed from both dynamics and kinetics points of view. We have also observed nanopipes constrict to form closed core screw dislocations in the MQW due to Si-doping.
Deep Defects in Fe-Doped GaN Layers Analysed by Electrical and Photoelectrical Spectroscopic Methods
Authors: Witte H. , Fluegge K. , Dadgar A. , Krtschil A. , Krost A. , Christen J. 
Published: April 2011

ABSTRACTThe electrical activity of iron in Fe- doped, and in Si and Mg co-doped GaN layers grown on sapphire substrates by metal organic vapor phase epitaxy was studied as shown by temperature dependent Hall Effect (TDH) measurements. In all samples iron doping generates an acceptor defect, which compensates donors in n-type GaN. Furthermore, iron doping causes strong potential inhomogeneities, which decrease the Hall mobility in the layers. To verify, if iron creates only hole traps, defects in n-type Si:Fe and Fe doped samples were investigated. The well known dominant electron traps in n-type GaN at 520 – 550 meV and 480 meV were found by deep level transient spectroscopy and thermal admittance spectroscopy, respectively. A high Fe-doped GaN layer shows a low p-type conductivity dominated by the iron acceptor. An activation energy of EV+ 460 meV was determined by TDH indicating, that the iron acceptor correlates with this defect level.
Optical evaluation of pretreated InGaN quantum well structures
Authors: Böttcher T. , Bertram F. , Bergman P. , Ueta A. , Christen J. , Hommel D. 
Published: April 2011

ABSTRACTIn order to optimize the quantum efficiency of InGaN quantum wells, different MOVPE growth sequences are compared using photo- and electroluminescence. In one study, the surface was pretreated with trimethylindium (TMIn) prior to the well deposition. In another study, growth interruptions were performed after the quantum well deposition to desorb segregated indium. In both cases, the room-temperature photoluminescence (PL) intensity is strongly enhanced. For the samples grown with TMIn preflow the wavelength distribution in low-temperature cathodoluminescence (CL) wavelength mappings is narrowed, which can be attributed to more homogeneous quantum wells. Furthermore, the decay times of the radiative recombination increase both at RT and 2K. A reason for this could be an improved indium profile along the growth direction or a more homogeneous In wetting layer due to the pre-wetted surface.
Modulation of Arsenic Incorporation in GaN Layers Grown by Molecular Beam Epitaxy
Authors: Novikov S. V. , Zhao L. X. , Foxon C. T. , Ja. Ber B. , Kovarsky A. P. , Harrison I. , Fay M. W. , Brown P. D. 
Published: April 2011

ABSTRACTGaN layers periodically-doped with arsenic were grown by plasma-assisted molecular beam epitaxy on sapphire substrates. Secondary ion mass spectroscopy studies using different secondary ions clearly confirmed the existence of As-modulation in the GaN/GaN:As periodically-doped structures, however, the degree of As-modulation is still under discussion. The use of modulation doping with As has a strong influence on the optical properties of GaN/GaN:As structures.
3C-SiC modulator for high-speed integrated photonics
Authors: Barrios Carlos Angulo , Thomas Christopher Ian , Spencer Michael , Lipson Michal 
Published: April 2011

ABSTRACTWe propose a compact 3C-SiC electro-optic modulator for high-speed Si-based integrated optoelectronics operating at 1.55-μm-wavelength. The device is based on an optical microresonator using sub-micron size high-index-contrast SiC waveguides on a SiC-SiO2-Si [SiC-on-Insulator (SICOI)] platform. Three different electrode configurations are analyzed. Switching times on the order of tens of ps are predicted by using low bias voltages.
Effects of Very Small Particle Size on Processing and Safety Properties of Energetic Formulations
Authors: Wardle Robert , Lee Kenneth , Akester Jeff , Braithwaite Paul 
Published: April 2011

ABSTRACTDescribed in this paper are ongoing research efforts at ATK Thiokol on new formulations of gun propellants and explosives containing nano-sized materials. Funding for this work was provided both internally and by ARDEC. The nano-material properties as well as selected properties of intermediate compositions are reported.The explosives section of this paper focuses on the theoretical evaluation of nano-aluminum in explosives. The theoretical cylinder expansion energy of two common aluminized explosives, PAX-3 and PAX-28, are examined. The comparison examines the effect of aluminum content in small aluminum particles on metal pushing performance at V/V0 = 6.5.ATK Thiokol and TACOM-ARDEC have teamed together to study the effect of nano-sized materials on advanced ETPE gun propellants. The gun propellant section of this paper focuses on the impact of nano-sized additives, specifically TiO2 and aluminum, on selected ETPE propellants. The safety properties of the nano-aluminum and nano-TiO2 analogs of two previously developed gun propellants, TGD-031 and TGD-035, are presented.
Combustion Synthesis of NiTi – TiC Composites with Controlled Porosity for Biomedical Applications
Authors: Burkes Douglas E. , Gottoli Guglielmo , Moore John J. , Yi Hu Chun , Ayers Reed A. 
Published: April 2011

ABSTRACTCombustion synthesis, or Self-propagating High- temperature Synthesis (SHS), is currently being used by the Center for Commercial Applications of Combustion in Space (CCACS) at the Colorado School of Mines to produce advanced porous materials for several important applications. These materials include ceramic, inter- metallic, and metal- matrix composites that can be used for orthopedic implants, heat exchanger and damping systems and micro-and macro-filter applications. Functionally graded materials, both in porosity and composition, can be produced using a range of combustion synthesis reactions systems. There are multiple factors that contribute to the final SHS product, e.g. reactant stoichiometry, initial relative density and pre-heat. The synthesis of nickel-titanium (NiTi) intermetallic compounds and composites is of considerable interest due to the ability to create a porous, shape memory and super-elastic alloy with high corrosion resistance. The synthesis effects of adding a carbon reactant so as to modify the reaction products and reaction exothermicity have been studied through the use of two different reaction stoichiometries involving elemental nickel, titanium and carbon. This paper outlines the synthesis of NiTi intermetallic composites based on the following SHS chemical reaction:The effect of the carbon reactant and the initial sample green density on the apparent porosity, bulk density, pore size and pore distribution of the final materials has been studied and is presented within this paper. A NiTi- TiC intermetallic ceramic composite has been synthesized that is functionally graded in both composition and porosity: the latter grading being due to buoyancy and gas evolution effects. Proposed kinetic mechanisms that drive this synthesis process and control the graded structure are discussed in detail.
Modeling Laser Generated Shock Damage and Thermo-Mechanical Chaos in Nanoparticles
Authors: Gerstman Bernard S. 
Published: April 2011

ABSTRACTContinual advances in laser technology lead to shorter pulses and higher energies. As the duration of a laser pulse shortens, different physical mechanisms become important in determining the thermo-mechanical response of an absorbing particle. These thermo-mechanical responses fall into the general category of thermal heating (temperature rise), vaporization, and shock wave formation. Our theoretical work has produced a computational model that allows the quantitative calculation of all of these responses for a laser of any pulse duration or energy, absorbed by a particle of any size. We find that for relatively long pulses, particle damage occurs most easily, i.e. at the least pulse energy, due to thermal effects. As the pulse duration shortens, explosive vaporization can dominate as the primary damage mechanism. For short pulses, shock wave production becomes the dominant damage mechanism. We describe how the relative terms of “short” and “long” pulse duration can be determined from knowledge of the thermo-mechanical properties of the absorber. Conversely, when the thermo-mechanical properties are not known, we explain how our theoretical work leads suggests an experimental technique that allows measurement of these absorber properties. This technique is applicable to extremely small particles that present difficulties for thermo-mechanical measurements. Finally, we show computational evidence of chaotic behavioral response of the absorber. This results in some laser pulse durations and energies that cause anomalously small shock waves, whereas other durations and energies cause surprisingly large and damaging responses.