bluuebunny/crossref_metadata_2025_split huggingface dataset:
Deposition of Gallium Nitride Films using Ammonia and Triethylgallium Seeded Helium Beams
Authors: Sumakeris J. J. , Chilukuri R. K. , Davis R. F. , Lamb H. H. 
Published: April 2011

ABSTRACTGallium nitride films have been deposited on Si(100) and Al2O3(0001) substrates using triethylgallium and ammonia seeded into highly expanded helium gas streams. A two step deposition process that reproducibly results in continuous crystalline GaN films has been developed. The microstructure and composition of the resultant films were characterized by scanning electron microscopy, reflection high energy electron diffraction and Auger electron spectroscopy and film character was correlated to deposition conditions.
Structural Defects in Heteroepitaxial and Homoepitaxial GaN
Authors: Liliental-Weber Zuzanna , Ruvimov S. , Kisielowski CH. , Chen Y. , Swider W. , Washburn J. , Newman N. , Gassmann A. , Liu X. , Schloss L. , Weber E.R. , Grzegory I. , Bockowski M. , Jun J. , Suski T. , Pakula K. , Baranowski J. , Porowski S. , Amano H. , Akasaki I. 
Published: April 2011

ABSTRACTThe microstructure and characteristic defects of heteroepitaxial GaN films grown on sapphire using molecular beam epitaxy (MBE) and metal-organic-chemical-vapor-deposition (MOCVD) methods and of homoepitaxial GaN grown on bulk substrates are described based on transmission electron microscopy (TEM), x-ray diffraction, and cathodoluminescence (CL) studies. The difference in arrangement of dislocations along grain boundaries and die influence of buffer layers on the quality of epitaxial films is described. The structural quality of GaN epilayers is compared to diat of bulk GaN crystals grown from dilute solution of atomic nitrogen in liquid gallium. The full width at half maximum (FWHM) of the x-ray rocking curves for these crystals was in the range of 20–30 arc sec, whereas for the heteroepitaxially grown GaN the FWHM was in the range of 5–20 arc min. Homoepitaxial MBE grown films had FWHMs of about 40 arc sec. The best film quality was obtained for homoepitaxial films grown using MOCVD; these samples were almost free from extended defects. For the bulk GaN crystals a substantial difference in crystal perfection was observed for the opposite sides of the plates shaped normal to the c direction. On one side the surface was almost atomically flat, and the underlying material was free of any extended structural defects, while the other side was rough, with a high density of planar defects. This difference was related to the polarity of the crystal. A large difference in crystal stoichiometry was also observed within different sublayers of the crystals. Based on convergent beam electron diffraction and cathodoluminescence, it is proposed that GaN antisite defects are related to the yellow luminescence observed in these crystals.
Simulated Thermal Effects on Structural and Electronic Properties of GaN
Authors: Serra S. , Miglio L. , Fiorentini Vincenzo 
Published: April 2011

ABSTRACTWe present preliminary results of tight binding molecular dynamics (TBMD) simulations concerning the thermal effects on the structural and electronic properties of GaN. We derived a semiempirical tight binding (TB) potential which is able to reproduce the band structure and the phase diagram of GaN for zincblende, wurtzite and rock-salt phases. We have found that even at few hundreds K above the experimental melting temperature the local ordering is fairly well conserved, with the fraction of wrong bonds quite low. Defects states appear in the gap at approximately 2.3 eV in agreement to the experimental indication for annealed films.
Excitation Wavelength and Saturation Effects on Gallium Nitride Photoluminescence
Authors: Banas Mike , Liu Guangtian , Ramer Jeff , Zheng Kang , Hersee Steve , Malloy Kevin 
Published: April 2011

ABSTRACTWe report on the excitation wavelength and power dependence of LPMOCVD grown GaN photoluminescence (PL). A mode-locked Ti:Sapphire laser that is doubled to provide a tunable wavelength range of 350-500 nm is used to pump the GaN samples. Special attention has been paid to the “yellow” (∼567 nm) luminescence. Different power dependence characteristics of the yellow emission are observed when pumping above and below the bandgap. A measurement of the photoluminescence excitation spectrum of the yellow peak (∼567 nm) is also performed and this spectrum shows a peak at room temperature at 3.36 eV (369 nm), an additional broad peak at 77 K and only the broad peak at 7 K. An excitation model is used to explain the behavior of the yellow emission, suggesting the neutral donor serves as a key species for the yellow emission.
Hydrazoic Acid: A Potent ‘Active Nitrogen’ Precursor for Group III Nitride Growth
Authors: Freedman Andrew , Robinson Gary N. 
Published: April 2011

ABSTRACTHydrazoic acid, HN3, was shown to be an extremely reactive molecule and an effective precursor species for 'active nitrogen'. At room temperature, hydrazoic acid dissociatively chemisorbed on GaAs (110) surfaces to form the NH radical and N2. At 473 K, the NH fragment reacted with the substrate, broke Ga-As bonds and formed GaN and AsN species. At 673 K, an anionic exchange mechanism commenced; the surface became completely depleted of As and a thin film of GaN formed. When the surface was dosed at 773 K, a 20 Å thick GaN film was grown. This evidence demonstrates that hydrazoic acid is highly reactive at comparatively low surface temperatures and offers a viable alternative to ammonia as a nitrogen precursor gas.
InGaN Light-Emitting Diodes with Quantum-Well Structures
Authors: Nakamura Shuji 
Published: April 2011

ABSTRACTInGaN single-quantum-well-structure (SQW) light-emitting diodes (LEDs) with an emission wavelength between violet and orange were fabricated. The maximum on-axis luminous intensity of green LEDs was 12 cd and the external quantum efficiency was as high as 6.3 % at a forward current of 20 mA, while those of blue LEDs were 2 cd and 9.1 %, respectively. The peak wavelength and the full width at half-maximum of the green LEDs were 520 nm and 30 nm, respectively, while those of blue LEDs were 450 nm and 20 nm, respectively. The color of green InGaN SQW LEDs was greener than those of conventional GaP and AlInGaP LEDs.
Electric Breakdown in Nitride PN Junctions
Authors: Dmitriev V.A. , Kuznetsov N.I. , Irvine K.G. , Carter C.H. 
Published: April 2011

ABSTRACTElectric breakdown of mesa terminated GaN and AlGaN pn diodes was investigated. The nitride pn structures were grown on 6H-SiC (0001) wafers by metalorganic chemical vapor deposition (MOCVD). Mesa structures were fabricated by reactive ion etching (RIE). Capacitance-voltage (C-V) and current-voltage (I-V) characteristics of the pn junctions were measured. It was found that the junctions were linearly graded. Microplasmic breakdown was observed. The breakdown voltage for GaN and AlGaN diodes ranged from 40 to 150 V and from 40 to 100 V, respectively. The electric breakdown field and temperature coefficient of the breakdown voltage were measured.
Deposition of Nbtex Thin Films Using Laser Ablation: Crystallographic Structure And Spatial Composition of Deposits
Authors: Sassoli H. , Grangeon F. , Mathey Y. , Autric M. , Pailharey D. , Marine W. 
Published: April 2011

ABSTRACTPulsed laser deposition (PLD ) is known for its capacity to reproduce a target composition on a substrate. We have used this deposition technique to produce thin films of transition metal chalcogenides. However, the deposits were always deficient in Te relative to the starting material ( composed by a refractory metal ( niobium ) and a chalcogene ( tellurium ) ). Variations of the interreticular distances have been observed with respect to fluence and substrate temperature. We show that spatial composition of the films is determined by a degree of crystallinity of deposit and by the reaction of formation of Te2 molecule within laser induced plume Two kinds of deposits have been obtained : Nb5Te4-type thin films which have a one-dimensional structure and NbTe2-type thin films which have a two-dimensional structure. While NbTe2 films have been realized by sputtering, it is the first time that Nb5Te4 films have been deposited.
Strained Linbo3/Sapphire Heterostructures Grown by Pulsed Laser Deposition
Authors: Shibata Yoshihiko , Kuze Naohiro , Matsui Masahiro , Kanai Masaki , Kawai Tomoji 
Published: April 2011

ABSTRACTThin LiNbO3 films are deposited on (001) sapphire substrates by Ar F pulsed laser ablation. The films are evaluated by X-ray diffraction analysis at various temperatures, as well as high-resolution transmission electron microscopy (TEM). The deposited films are highly epitaxial but that are strained, that is, the a-axis is longer and the c-axis is shorter than those of LiNbO3 single crystals. X-ray diffraction analysis at deposition temperature, as well as TEM show that the strain is caused by the difference in thermal expansion coefficients between LiNbO3 and sapphire substrates.
Low Resistivity-Highly Transparent Zno:Ga Tco's Grown by Laser Ablation
Authors: Siqueiros J. M. , Díaz J.A. , Contreras O. , Hirata G.A. , McKittrick J. 
Published: April 2011

ABSTRACTHighly transparent with low electrical resistivity gallium-doped zinc oxide (ZnO:Ga) thin films were prepared on glass by Pulsed Laser Deposition at different substrate temperatures. ZnO:Ga evaporation targets were prepared by pressing and sintering powder materials obtained with a novel combustion synthesis technique.An excellent transmittance of more than 85% in the visible range and a low sheet resistance value as low as 13 Ω/sq. were measured on a 200 nm thickness film grown at Ts=300°C. From optical measurements we observed an increase in the ZnO:Ga bandgap when the substrate temperature is raised from 150°C to 300°C. With this, a remarkable improvement in the blue-green region transmittance is obtained.X-Ray diffraction patterns indicate that the films grow highly oriented in the basal plane direction (c-axis) of the hexagonal ZnO grains.
Phase Transitions Induced By Femtosecond Pulses
Authors: Glezer E. N. , Huang L. , Siegal Y. , Callan J. P. , Mazur E. 
Published: April 2011

ABSTRACTOptical studies of semiconductors under intense femtosecond laser pulse excitation suggest that an ultrafast phase transition takes places before the electronic system has time to thermally equilibrate with the lattice. The excitation of a critical density of valence band electrons destabilizes the covalent bonding in the crystal, resulting in a structural phase transition. The deformation of the lattice leads to a decrease in the average bonding-antibonding splitting and a collapse of the band-gap. Direct optical measurements of the dielectric constant and second-order nonlinear susceptibility are used to determine the time evolution of the phase transition.
Reversible Photoredox Processing of Transition Metal Oxides
Authors: Villarica R. Martin , Nash Fazio , Chaiken J. , Osman Joe , Bussjager Rebecca 
Published: April 2011

ABSTRACTWe describe a novel laser chemical process having potential for optical data storage and processing applications. Reversible oxygen exchange involving WO3, using readily available laser sources offers improved durability and versatility over existing erasable optical data storage media. Being interconvertable using heat and blue-green laser sources, the well known yellow WO3and blue W2O5can function as erased and written states.
Investigation of Laser-Induced Etching of Ti in Phosphoric Acid
Authors: Nowak R. J. , Metev S. M. , Meteva K. B. , Sepold G. 
Published: April 2011

ABSTRACTLaser-induced chemical etching of Ti in phosphoric acid has been investigated using cw Nd:YAG (1.064 μιm) and Argon lasers (514 nm) operating in the fundamental Gaussian mode. Two different regions of etching were observed, which are separated by a characteristic threshold value of the laser power and ascribed to melting of the metal. Below the threshold an exponential dependence of etch rates on laser power suggest a thermally activated etching mechanism. Time-resolved measurements indicate in this region the dissolution of the passivation layer followed by surface etching of the metal grains. After laser illumination an immediate repassivation of the re-cooled surface stops the etch reaction.
Silicon Growth Rate Enhancement Using Trisilane in a Laser Direct-Writing Technique
Authors: Boughaba S. , Auvert G. 
Published: April 2011

ABSTRACTAn argon-ion laser based direct-writing technique was used to deposit micron-size silicon lines from the decomposition of silane (SiH4) and trisilane (Si3H8) gases. The substrates used were 0.1 μrn polysilicon/1 μ.m silicon dioxide/<100> monosilicon multilayered structures. The vertical silicon deposition rate was investigated as a function of the laser-induced surface temperature and gas pressure. For temperatures ranging between 1000 and 1410 °C, the pressure was varied in the range 5-250 mbar and 0.1-30 mbar for SiH4 and Si3H8, respectively. For both gases, three growth regimes could be distinguished according to precursor pressure. The deposition rates achieved using trisilane are far higher than those obtained with silane in spite of the use of a reduced gas pressure range. For a laser-induced surface temperature of 1300 °C and a precursor pressure of 10 mbar, the deposition rates achieved using SiH4 and Si3H8 are, respectively, 0.42 and 20 μ.m/s, representing an enhancement factor of 50 with the later.
Interface Versus Diffusion Controlled Growth in Nanocrystallization Kinetic Studies
Authors: Clavaguera N. , Clavaguera-mora M.T. 
Published: April 2011

ABSTRACTNanocrystallization by heat treatment of initially amorphous alloys occurs for high nucleation frequency I and low crystal growth rate u values. Calorimetrie data extracted, for instance, from differential scanning calorimetry are normally presented by the dependence of the crystallized fraction x, on: i) x=x(T,t) when obtained under isothermal annealing at temperature T as a function of time t; ii) x=x(τ,β) when obtained under continuous heating at a scan rate β as a function of temperature. Theoretical analysis of the crystallization kinetics is presented which accounts for nuclei either pre-quenched or created by homogeneous nucleation whose initial steps of growth are controlled by the interface formation between the nanocrystals and the matrix and subsequent growth is limited by diffusion.
Temporal Evolution of Bimodal Distribution of γ’ Precipitates in Ni-Al-Mo Alloys under the Influence of Coherency Strains
Authors: Sequeira A.D. , Calderon H.A. , Kostorz G. 
Published: April 2011

ABSTRACTThe influence of coherency strains produced by the γ-γ’ lattice mismatch, δ, on the decomposition process of Ni-Al-Mo alloys with a bimodal size distribution is presented. Samples with δ ranging from positive to negative, were investigated in a double-step aging procedure. The evolution of the microstructure and the kinetics of coarsening were studied using transmission electron microscopy (TEM). The lattice mismatch between the matrix and the different classes of precipitates was determined by high-resolution high-temperature x-ray diffraction. It is shown that the strain fields produced by the lattice mismatch can influence dramatically the decomposition of metallic alloys. It is suggested that the reduction of the coarsening rate of the large precipitates, the fast coarsening rate of the small precipitates and the distortions detected in the matrix are all direct consequences of the elastic fields produced by the γ-γ’ lattice mismatch.
Wet Oxidation of Si1-x-yGexCy Layers on (100) Si
Authors: Bair A. E. , Atzmon Z. , Alford T. L. , Chandrasekhar D. , Smith David J. 
Published: April 2011

ABSTRACTSingle crystal Si0.63Ge0.36C0.01 and amorphous Si0.65Ge0.27C0.08 layers have been oxidized in a wet ambient at 700 °C and 900 °C. The oxide growth has been studied using Rutherford backscattering spectrometry and transmission electron microscopy. A reference sample of Si0.63Ge0.37 was also oxidized in order to determine the influence of C on the oxidation behavior. The lower C content alloy behaved similar to the SiGe alloy. Uniform Si1-xGexO2 was obtained at 700 °C whereas SiO2 was formed at 900 °C, and Ge piled up underneath the oxide. In both cases, C was not detected in the oxide layer. The amorphous Si0.65Ge0.27C0.08 alloy behaved significantly different at both oxidation temperatures in comparison with the crystalline Si0.63Ge0.36C0.01 and Si0.63Ge0.37. Negligible oxidation occurred at 700 °C whereas SiO2 was obtained at 900 °C and the rejected Ge distributed uniformly throughout the SiGeC alloy. It is proposed that fast Ge diffusion during oxidation at 900 °C resulted from diffusion at grain boundaries, since crystallization of the amorphous SiGeC layer occurred in conjunction with oxidation, leading to nucleation of ∼5 nm nanocrystals.
XRD Study of the Influence of Heat Treatment on the Microstructure of Sol-Gel Processed ZrO2-Y2O3 Ceramics
Authors: Barrera-Solano C. , PiÑero M. , Jiménez-Solís C. , Gago-Duport L. 
Published: April 2011

ABSTRACTYSZ samples containing 5 and 10 mol% of Y203 were prepared by controlled hydrolysis of metal alkoxides. The dried powders were calcined at 800°C and then they were uniaxially pressed and sintered at different temperatures and next heated at 1400°C (∼ 5 MPa) or annealed at 1600°C for 24 h. The quantitative analysis of the experimental X-ray diffraction (XRD) spectra was performed by Whole Pattern Fitting (WPF). A Pseudo-Voigt (Thompson-Cox-Hastings) was used as shape profile function. The respective phase fractions (wt %) were fitted for both solid state solutions using the scale factor. The heat treatment induced changes are discussed.
Solidification Front Velocity of Ternary Titanium-Aluminides
Authors: Wettlaufer M. , Laakmann J. 
Published: April 2011

ABSTRACTTernary titanium-aluminides with compositions of Ti51Al47Fe2, Ti51A147Cr2 and Ti51Al47Mn2 were investigated with respect to the correlation of their solidification front velocity v and bulk undercooling ΔT. The observation of the solidification front during the recalescence event has been realized using a high speed video system capable of recording up to 12,000 pictures per second. The temperature measurement was carried out by pyrometry, avoiding contact with the sample. The comparison of the experimental data with the LKT-theory (Lipton, Kurz, Trivedi; [1]) refers to a primary (hcp) β-Ti solidification for undercoolings below ΔT≈ 130 K and primary (bcc) α-Ti solidification for ΔT≥ 130 K. For undercoolings ≥ 150 K the theory differs greatly from the experimental results.The maximum undercoolings achieved were 268 K (Ti51Al47Fe2), 285 K (Ti51Al47Cr2) and 280 K (Ti51Al47Mn2), corresponding to a solidification front velocity v ≈ 9-10 m/s for all alloys.
Progression of the Surface Roughness of N+ Silicon Epitaxial Films as Analyzed by AFM
Authors: John S. , Quinones E. J. , Ferguson B. , Pacheco K. , Mullins C. B. , Banerjee S. K. 
Published: April 2011

ABSTRACTWe report on the morphology of heavily phosphorous doped silicon films grown by ultra high vacuum chemical vapor deposition at temperatures of ∼550° C. The effects of PH3 on epitaxial films have been examined for silicon deposited using SiH4 and Si2H6. It is found that films grown using silane experience an increase in surface roughness with increasing phosphine partial pressure. AFM and RHEED studies indicate 3-D growth. As epitaxy progresses, it is believed that phosphorus segregation on the growing film surface greatly diminishes the adsorption and surface mobility of the silicon bearing species. Initial Si deposition results in a pitted surface, but as growth advances and the phosphorus coverage increases, growth within the pits decreases the surface roughness. In contrast to SiH4, it is found that Si2H6 provides excellent quality, smooth films even at high PH3 partial pressures.
Molecular Dynamics Simulation of Energetic Deposition of AG/FE and AG/SI Bilayers and Multilayers
Authors: Mattei G. , Mazzone A.M. 
Published: April 2011

ABSTRACTThe growth of Fe/Ag and Ag/Si multilayers is simulated by using molecular dynamics with quantum forces. The results of these calculations indicate that from ballistic motions irregular structures and smeared interfaces may be generated. The temperature of the processing is critical and good epitaxial growth is obtained by forcing a fast heat dissipation.
Temperature-Dependent Strain Relaxation and Islanding of Ge/Si(111)
Authors: Deelman P.W. , Schowalter L.J. , Thundat T. 
Published: April 2011

ABSTRACTIn order to understand Ge island nucleation and evolution, we have studied strain relaxation and clustering of Ge grown on Si(111) by molecular beam epitaxy (MBE) with in situ reflection high energy electron diffraction (RHEED), atomic force microscopy (AFM), and Rutherford backscattering spectrometry (RBS). Our goal is to tailor the size and density of the nanocrystals by controlling thermodynamics and kinetics. At low temperature (∼ 450°C), we observe a sharp 2D-3D growth mode transition after 2.5ML ±0.1ML (we define a thickness of 1ML to be one-third the length of the body diagonal of the Ge conventional unit cell), when transmission diffraction features appear in RHEED and the surface lattice constant begins to relax. The mechanisms of island growth and strain relaxation change with growth temperature. At ∼ 700°C, transmission diffraction spots never appear in RHEED for Ge/Si(111) and strain relaxation occurs gradually. After 37ML of growth, the apparent in-plane lattice parameter increases only 1.5% over that of the Si substrate. This behavior is explained by the different manner in which islands initially nucleate and grow in the two temperature regimes. At low temperature, small islands nucleate and grow on a relatively rough wetting layer (which itself provides preferential sites for dislocation introduction). The areal density of the small islands is relatively high. At high temperature, a small number of islands grow very large from the outset. A general model indicates how, at low temperature, the relative difficulty of overcoming the barrier to dislocation formation actually results in an apparent larger degree of strain relief than at high temperature.
Growth Kinetics and Structure of Ultrathin Copper Films on the W(110) Surface Studied by Leem
Authors: Altman M.S. , Luo E.Z. , Chung W.F. , Orr B.G. 
Published: April 2011

ABSTRACTCompeting desorption during the epitaxial growth of Cu on the W(110) surface has been studied with low energy electron microscopy (LEEM). Direct imaging of a structural transformation from pseudomorphic (1×1) to relaxed (15×1) periodicity which occurs at a coverage of θCu = 2.13 monolayers is used as a very accurate, local probe of coverage during deposition. The desorption energy E = 3.67 eV and attempt frequency v = 2.15 × 1015 s−1 are determined by examining the balance condition when the incident and desorption fluxes are equal. A step-flow-like growth morphology occurs when the supersaturation is significantly reduced by competing desorption. An island nucleation and coalescence growth morphology results at higher supersaturation.
Atomistic Simulation of Nanocrystalline Materials
Authors: Wolf D. , Phillpot S. R. , Keblinski P. 
Published: April 2011

AbstractAtomistic simulations show that high-energy grain boundaries in nanocrystalline copper and nanocrystalline silicon are highly disordered. In the case of silicon the structures of the grain boundaries are essentially indistinguishable from that of bulk amorphous silicon. Based on a free-energy argument, we suggest that below a critical grain size nanocrystalline materials should be unstable with respect to the amorphous phase.
Influence of Pressure on the Stability of Metastable TiSi(B2) Phase
Authors: Davidov Genady , Fuks David , Dorfman Simon 
Published: April 2011

AbstractWe study the influence of pressure on the temperature of order-disorder phase transition in metastable TiSi(B2) phase as well as the pressure dependence of thermodynamic functions describing the stability of this phase. The suggested model outlines the technique to calculate the values of the energy parameters of ordering process from the first principles.