bluuebunny/crossref_metadata_2025_split huggingface dataset:
Structural and Optical Properties of Homoepitaxial GaN Layers
Authors: Baranowski J. M. , Liliental-Weber Z. , Korona K. , Pakuła K. , Stępniewski R. , Wysmołek A. , Grzegory I. , Nowak G. , Porowski S. , Monemar B. , Bergman P. 
Published: April 2011

ABSTRACTThe review of structural and optical properties of homoepitaxial layers grown by MOVCD on single crystals GaN substrates is presented. The TEM technique is used to characterise the structural properties of epi-layers. It is found that the structural properties of GaN homoepitaxial layers are determined by the polarity of the substrate surface on which the growth takes place. It is shown that threading dislocations are present only in the layers grown on the [0001] “smooth” surface. On the other hand the layers grown on the [0001] “rough” surface are free from vertical defects. The characteristic feature of the growth on the “rough” surface are pinholes. The optical properties of homoepitaxial layers are predominantly determined by the growth polarity as well. It is shown also that the reflectivity measurement is the most precise way to determine the exciton energies and that emissions due to free excitons are strongly affected by polariton effects.
Determination of the Percentage of the Cubic and Hexagonal Phases in Gan with Nexafs
Authors: Katsikini M. , Paloura E. C. , Moustakas T. D. , Holub-Krappe E. , Antonopoulos J. 
Published: April 2011

ABSTRACTAngle resolved near edge X-ray absorption measurements (NEXAFS) are used to access the existence and concentration of the two allotropic phases, cubic (β-GaN) and hexagonal (α-GaN), that coexist in a mixed-phase GaN sample grown by ECR-MBE. The resonance intensities in the NEXAFS spectra from a pure cubic GaN sample are independent of the angle of incidence ϑ, while they have a characteristic angular dependence on cos2ϑ for the hexagonal material, i.e. I=A±Bcos2ϑ. From the values of A and B the bond orientations with respect to the surface normal are calculated. Furthermore, the NEXAFS resonances in a pure α- or β-GaN sample appear at characteristic energies which are independent of the angle of incidence ϑ. Contrary to that the NEXAFS resonances in the spectra from a mixed-phase GaN show a characteristic ϑ-dependent shift. On the basis of this finding, a method is proposed and applied for the determination of the fractions of the co-existing polytypes in a mixed-phase sample.
Synthesis of Bulk, Polycrystalline Gallium Nitride at Low Pressures
Authors: Argoitia Alberto , Angus John C. , Hayman Cliff C. , Wang Long , Dyck Jeffrey S. , Kash Kathleen 
Published: April 2011

ABSTRACTBulk, polycrystalline gallium nitride was crystallized from gallium saturated with nitrogen obtained from a microwave electron cyclotron resonance source. The polycrystalline samples are wurtzitic and n-type. Well-faceted crystals give near-band-edge and yellow band photoluminescence at both 10K and 300K. The results show that atomic nitrogen is an attractive alternative to high pressure molecular nitrogen for saturation of gallium with nitrogen for synthesis of bulk gallium nitride.
Effect of Sapphire Nitridation on GaN by MOCVD
Authors: Byun Dongjin , Jeong Jaesik , Lee Jae-Inn , Kim Byongho , Yoo Ji-Beom , Kum Dong-Wha 
Published: April 2011

ABSTRACTEfficiency and lifetime of light emitting diodes and laser diodes inversely depend on defect density of the crystal. Reduction of defect density is accomplished by proper choice of the substrate or deliberate modification of substrate surface. Roughness of substrate surface for GaN deposition can be controlled by buffer growth and/or nitridation. Buffer layers or nitrided layers promote lateral growth of films due to decrease in interfacial free energy between the film and substrate. Optimum conditions for nitridation and GaN-buffer growth on Al2O3(0001) were determined by means of atomic force microscopy (AFM). AFM analysis of nitridated sapphire surfaces was also carried out to find the optimum condition for nitridation of sapphire substrate before GaN-buffer layer deposition. Nitridation of sapphires was performed only with nitrogen. Based on the fact that GaN deposited on modified surface exhibited the better crystal quality and optical property, use of AFM roughness as a reliable criterion is suggested for process optimization of GaN film growth by metallorganic chemical vapor deposition.
Electrical Characterization of Al-AlN(PSMBE Grown)-Si MIS Structures
Authors: Krupitskaya Regina Y , Auner Gregory W , Daley Tom E 
Published: April 2011

ABSTRACTAIN films were grown by plasma source Molecular Beam Epitaxy (PSMBE) on n-type Si(111) substrates under various growth parameters. I-V and C-V-f dependencies of AI-AIN-Si(111) MIS structures were measured. Electrical characterization of MIS structures with PSMBE grown textured AIN thin films as insulator shows that low current leakage can be achieved, although some samples have rectifying I-V dependencies. C-V-f characteristics reveal evidence of the presence of different types of traps. Additional investigations are needed in order to determine the nature of these traps.
Defect Transitions in GaN Between 3.0 and 3.4 eV
Authors: Rieger W. , Ambacher O. , Rohrer E. , Angerer H. , Stutzmann M. 
Published: April 2011

ABSTRACTWe have studied optical transitions (absorption and luminescence) in nominally undoped and Mg-doped GaN deposited by MOCVD and MBE. In the range between 3.0 and 3.4 eV, a variety of well known low-intensity luminescence lines are observed, whose origin is discussed. In particular, by comparing excitation with subgap versus above-gap laser lines as well as by combining optical subgap absorption with spectrally resolved photoconductivity, we identify localized optical transitions occuring in isolated cubic inclusions in the otherwise hexagonal GaN epitaxial layers. Implications of these strucural defects for photocurrent transients are also presented.
Characterization of Near-Edge-Optical Transitions in Undoped and Doped GaN/Sapphire Grown by MOVPE, HVPE, and GSMBE
Authors: Leroux M. , Beaumont B. , Grandjean N. , Massies J. , Gibart P. 
Published: April 2011

ABSTRACTWe report a comparative study by luminescence and reflectivity of GaN grown on sapphire by MOVPE, HVPE and GSMBE. Whatever the growth technique, undoped GaN shows well resolved reflectivity spectra, allowing hetero-epitaxial strain to be taken into account in the interpretation of luminescence spectra. MOVPE provides the highest purity samples so far. From the luminescence study of n- and p-type doped samples, activation energies of 265 ± 10 meV and 34 ± 2 meV are deduced for Mg acceptors and Si donors respectively. From the luminescence spectra of HVPE grown samples, the presence of a perturbed interfacial n+ layer is evidenced.
Effects of Strain Fields on Excitons and Phonons in Wurtzite GaN Epilayers
Authors: Gil B. , Briot O. , Aulombard R. L. , Demangeot J. F. , Frandon J. , Renucci M. 
Published: April 2011

ABSTRACTThe electronic structure of a set of several tens of a few micrometer-thick GaN epilayers grown by MOVPE on (000l)-oriented sapphire is investigated by means of photoluminescence and reflectance spectroscopy. The strain-fields effects are quantitatively interpreted using a group theory analysis which predicts seven pertinent parameters: the crystal field splitting, two spin-orbit interaction parameters and four deformation potentials. From a theoretical point of view, we also examine the exciton problem. We establish the selection rules which control the strength of optical transitions for Г5 and Г1 excitons, and estimate the electron hole exchange interaction and the longitudinal transverse splitting of A, B, C excitons. Raman spectroscopy measurements are performed at various temperatures to observe the strain-induced shift of Raman frequencies. We determine four phonon deformation potentials in w-GaN.
Theory of Point Defects and Interfaces
Authors: Van de Walle Chris G , Neugebauer Jörg 
Published: April 2011

ABSTRACTFirst-principles theoretical results can predict and explain a variety of materials properties of the nitride semiconductors. For n-type GaN, we summarize the current understanding about incorporation of unintentional donor impurities, as opposed to nitrogen vacancies. For p-type GaN, we discuss the cause of the limited doping levels, and the role of hydrogen. We describe the role of gallium vacancies in the yellow luminescence, and the interaction between these vacancies and donor impurities. Finally, we discuss our first-principles investigations of the atomic and electronic structure of heterojunction interfaces between the III-nitrides, and provide values for natural band lineups.
Incomplete Solubility in Nitride Alloys
Authors: Ho I. H. , Stringfellow G.B. 
Published: April 2011

ABSTRACTA model based on the valence-force-field (VFF) model has been developed specifically for the calculation of the irascibility gaps in III-V nitride alloys. In the dilute limit, this model allows the relaxation of the atoms on both sublattices. It was found that the energy due to bond stretching and bond bending was lowered and the solubility limit was increased substantially when both sublattices were allowed to relax to distances as large as the sixth nearest neighbor positions. Using this model, the equilibrium mole fraction of N in GaP was calculated to be 6×l0−7 at 700°C. This is slightly higher than the calculated results from the semi-empirical delta lattice parameter (DLP) model. Both the temperature dependence and the absolute values of the calculated solubility agree closely with the experimental data. The solubility is more than three orders of magnitude larger than the result obtained using the VFF model with the group V atom positions given by the virtual crystal approximation, i.e., with relaxation of only the first neighbor bonds. Other nitride systems, such as GaAsN, AlPN, AlAsN, InPN, and InAsN were investigated as well. The equilibrium mole fractions of nitrogen in InP and InAs are the highest, which agrees well with recent experimental data where high N concentrations have been produced in InAsN alloys. Calculations were also performed for the alloy systems with mixing on the group III sublattice that are so important for device applications. Allowing relaxation to the 3rd nearest neighbor gives an In solubility in GaN at 800°C of less than 6%. Again, this is in agreement with the results of the DLP model calculation. This result may partially explain the difficulties experienced with the growth of these alloys. Indeed, evidence of solid immiscibility has recently been reported. A significant miscibility gap was also calculated for the AlInN system, but the AlGaN system is completely miscible.
Energetics of AlN Epitaxial Wetting Layers on SiC (0001)
Authors: Di Felice R. , Northrup J. E. , Neugebauer J. 
Published: April 2011

ABSTRACTWe present a first-principles characterization of the initial stages of formation of AlN films on c-plane SiC substrates. Studying the competition between two-dimensional films and three-dimensional islands as a function of Al and N abundances, we find that a two-dimensional film can wet the surface in N-rich conditions. Ordered layer-by-layer growth can proceed to some extent on this wetting layer, and is improved by the formation of an atomically mixed interface which eliminates interface charge accumulation. Our results indicate that the stable AlN films grow in the (0001) orientation on the Si-terminated SiC(0001) substrate.
Band Structure and Cation Ordering in LiGaO2
Authors: Limpijumnong Sukit , Lambrecht Walter R.L. , Segall Benjamin , Kim Kwiseon 
Published: April 2011

ABSTRACTFull-potential linear muffin-tin orbital calculations were performed for LiGaO2 in different crystal structures in order to investigate the nature and origin of the cation ordering, structural relaxation and their effects on the band structure. It is found that the most important factor for the bonding is the exclusive occurrence of Li2Ga2 tetrahedra surrounding oxygen. Structures including LiGa3 and Li3Ga tetrahedra have significantly higher total energies and smaller bandgaps. The band-offset between GaN and LiGaO2 is estimated using the dielectric midgap approach.
A First Model of Amorphous GaN from Ab Initio Molecular Dynamics
Authors: Drabold D. A. , Stumm Petra 
Published: April 2011

ABSTRACTA probable byproduct of growth of crystalline GaN is an amorphous phase of the material. In this paper, we propose a structural model of amorphous GaN obtained from ah initio molecular dynamics. The radial distribution function, local bonding, electronic density of states and vibrational spectra are described. The network we obtain is highly disordered but exhibits a large state-free optical gap, and has no wrong (homopolar) bonds. These predictions are intended to elucidate the experimental signatures of amorphous GaN.
First Microscopic Observation of Cadmium-Hydrogen Pairs in GaN
Authors: Burchard A. , Deicher M. , Forkel-Wirth D. , Haller E. E. , Magerle R. , Prospero A. , Stötzler R. ,   The Isolde-Collaboration
Published: April 2011

ABSTRACTThe formation and properties of acceptor-hydrogen pairs in GaN have been studied using radioactive 111mCd acceptors and the perturbed γγ angular correlation spectroscopy (PAC). After H-loading by low energy implantation (100 eV) at temperatures between 295 K and 473 K, the formation of two Cd-H complexes involving about 30% of the Cd-acceptors is observed. The complexes have been identified as single hydrogen atoms bound to the Cd acceptor in two different configurations. The dissociation enthalpies of these configurations have been determined as 1.1(1) eV and 1.8(1) eV, respectively.
Characterization and Metrology of Low Pressure Chemical Vapor Deposited (LPCVD) Polysilicon
Authors: Asinovsky Leo , Schroth Michael , Shen Fei , Sweeney John 
Published: April 2011

AbstractPolycrystalline silicon (polySi) is widely used in the semiconductor industry as a gate electrode, interconnect material and for various other applications. Small variations in deposition conditions can significantly affect this material's properties, cause errors in metrology control of the film thicknesses and, ultimately, in device performance. In this work polySi was LPCVD deposited in a vertical reactor with multiple gas inlets and a flat temperature distribution. Deposition conditions were controllably changed to create a matrix of wafers. Characterization is done using production dual wavelength, multiple-angle-of-incidence and research grade spectroscopie ellipsometers. PolySi thickness and composition uniformity are analyzed.
Optical Properties of Nano Crystalline InP in Opal 3-Dimensional Gratings
Authors: Johnson N. P. , Romanov S. G. , Butko V. , Yates H. , Pemble M. E. , Sotomayor Torres C. M. 
Published: April 2011

AbstractOpal consists of a three dimensional array of silica balls with diameter in the range 150 -350 nm and acts as a 3-D grating. Within the microcrystals (a few hundred microns) of opal there is typically 5% variation in ball diameter arranged in a fee structure. Using a specially modified MOCVD process InP can be grown within the voids between the touching silica balls. These interconnecting voids are approximately 1/5 and 2/5 the silica sphere diameter, Raman spectroscopy confirms the crystallinity of the InP for two samples loaded with 2.5 volume % of InP. The LO phonon is shifted to the red by 10 cm−l from the bulk value of 345 cm−1, while the TO phonon is shifted 22 cm−l to the blue from the bulk value of 303.7 cm−l. This shift is attributed to strain in the nanocry stals.The half width of the reflectance is dependent upon InP loading with the 2.5 vol.% samples giving 0.19 eV and the lower loaded 0.4 vol. % giving 0.17 eV for samples with the same ball diameter. Photoluminescence emission near 1.7 eV is attributed to confined InP compared with a bulk value of 1.4 eV. The energy of the emission is determined by the combined contribution of the laterally confined InP and its interaction with the opal matrix.
Preparation, Characterization and Optical Properties of Zinc Oxide Nanoparticles
Authors: Li Shoutian , Silvers Stuart J. , Samy El-Shall M. 
Published: April 2011

AbstractZnO nanoparticles are produced by the laser vaporization-controlled condensation technique. These particles are connected in a web-like agglomeration. Their properties are compared to those of ZnO nanoparticles produced by solution sol-gel and reverse micelle techniques. All particles have the bulk crystal structure and show quantum size effects in absorption and emission. They show emissions that consist of a blue bandgap feature with a sub-nanosecond lifetime and a green feature with multiexponential lifetime decays. Emission from the stearate coated particles produced by the reversed micelle method is particularly intense.
Studies of Photoredox Reactions on Nanosize Semiconductors
Authors: Wilcoxon Jess P. , Parsapour F. , Kelley D. F. 
Published: April 2011

AbstractLight induced electron transfer (ET) from nanosize semiconductors of MoS2to organic electron acceptors such as 2,2′-bipyridine (bpy) and methyl substituted 4,4′,5,5′-tetramethyl-2,2′-bipyridine (tmb) was studied by static and time resolved photoluminescence spectroscopy. The kinetics of ET were varied by changing the nanocluster size (the band gap), the electron acceptor, and the polarity of the solvent. MoS2is an especially interesting semiconductor material as it is an indirect semiconductor in bulk form, and has a layered covalent bonding arrangement which is highly resistant to photocorrosion.
Theoretical Thermal Conductivity of Porous Silicon: Nonlinear Behavior
Authors: Lugo J. E. , de Río J. A. , Tagüeña-Martínez J. 
Published: April 2011

AbstractThe use of porous silicon (PS) in fabricating optoelectronic devices is in progress. However, the performance of such applications still needs to be improved. In particular, in solar cells heat must be dissipated to avoid a decay in their efficiency and one of the properties of PS that must be evaluated is the effective thermal conductivity. It is well known that the thermal conductivity of silicon is temperature dependent. Thus we cannot use a standard effective medium approach to obtain its effective thermal response. In this work, we extend the averaging volume and surface methods [1] to consider nonlinear effects in the effective transport coefficients. We model PS as composed of c-Si cylindrical columns covered by different overlayers (i.e. a-Si or SiO2) immersed in another medium. In our model the effective thermal conductivity has an explicit dependence on the temperature gradient. We present a parametric analysis of the model, compare it with the c-Si behavior and evaluate the importance of the nonlinear contribution.
Properties of Amorphous Silicon Thin Film Transistors with Phosphorous-Doped Hydrogenated Microcrystalline Silicon
Authors: Choi J. H. , Kim C. W. , Yang H. G. , Souk J. H. 
Published: April 2011

AbstractPhosphorous (P) doped hydrogenated microcrystallme silicon (n+μ c-Si:H) films have been prepared by using the hydrogen-diluted plasma enhanced chemical vapor deposition (PECVD) method. The crystallinity of films deposited over the range of SiH4/H2 flow ratios and RF-power is studied by Raman spectroscopy. For a 900 Å thick film deposited at 250°C, a conductivity of 71Ω−1cm−1 and an average crystallinity of 49% is obtained. n+ μ c-Si:H films as well as n+ a-Si:H films are used for both etch stopper and back channel etch type TFTs and the I4-V8 characteristics are compared. For the etch stopper type TFT, the field effect mobility of 0.85 cm2/V.sec, threshold voltages of 2 – 3 V and Ion/Ioff ratio of ∼107 are obtained.
Influence of Nanocrystallinity on Properties of Photodiode and TFT Image Sensor Structure
Authors: Kolodziej A. , Nowak S. , Krewniak P. 
Published: April 2011

AbstractThis paper reports technological experiments which have strongly affected the properties of the multilayer image sensor structure. The high deposition rate and large hydrogen content during the reactive magnetron deposition process causes nanocrystallization of prepared silicon films. We also report experiments with laser crystallization. The effect of microstructure in a-Si:H films on TFT and pin diode characteristics has been investigated. Crystallinity was confirmed by Raman scattering, small angle X-ray diffraction (SAXD) and transmission electron microscopy (TEM). The parameters of the photodiodes such as quantum efficiency, dark and light currents, etc. have been tested using the measurements of samples in the configuration of a linear image sensor consisting of two rows, 30 to 160 pixels per row, with dimensions 0.1mm2-1mm2.
The Composition Effect on the Nanocrystallization of Finemet Amorphous Alloys
Authors: Zhu J. , Pradell T. , Clavaguera N. , Clavaguera-Mora M. T. 
Published: April 2011

ABSTRACTDifferential Scanning Calorimetry (DSC), X-ray Diffraction (XRD), Neutron Diffraction (ND) and Mössbauer Spectroscopy (MS) were used to study the nanocrystallization process of Fe73.5Cu1Nb3Si22.5–xBx (x=5, 7, 8, 9 and 12) amorphous alloys. Both the temperature range and the activation energy of Fe(Si) phase precipitation from the amorphous martrix increase with the initial B composition. The initial Si composition influences the mechanism of the nanocrystallization: for the Si rich samples, the beginning of nucleation and growth processes is interface controlled, for the B rich samples it is diffusion controlled. Secondary crystallization from the remaining amorphous is mainly Fe3B and Fe2B, the ratio of Fe3B/Fe2B being dependent on the initial composition too.
Microstructure, Mechanical Properties and Wear Resistance of WC/Co Nanocomposites
Authors: Jia Kang , Fischer Traugott E. 
Published: April 2011

ABSTRACTThe microstructure, mechanical properties, abrasion and wear resistance of WC-Co nanocomposites synthesized by the spray conversion technique by McCandlish, Kear and Kim have been investigated. The binder phase of WC-Co nanocomposites is enriched in W and C, compared to conventional cermets. Small amorphous regions exist in the binder despite the slow cooling after liquid phase sintering. Few dislocations are found in the WC grains. The increased WC content and the amorphous regions modify (i.e. strengthen) the binder phase of the composites. Vickers indentation measurements show a hardness of the nanocomposites reaching 2310 kg/mm2. While the toughness of conventional cermets decreases with increasing hardness, the toughness does not decrease further as the WC grain size decreases from 0.7 to 0.07 μm. but remains constant at 8 MPam1/2. Scratches caused by a diamond indenter are small, commensurate with their hardness. These scratches are ductile, devoid of the grain fracture that is observed with conventional materials. The abrasions resistance of nanocomposites is about double that of conventional materials, although their hardness is larger by 23% only. This is due to the lack of WC grain fragmentation and removal which takes place in conventional cermets. Sliding wear resistance of WC/Co is proportional to their hardness; no additional benefit of nanostructure is obtained. This results from the very small size of adhesive wear events in even large WC grains.