bluuebunny/crossref_metadata_2025_split huggingface dataset:
Computer Investigations on Phase Decomposition in Real Alloy Systems Based on a Discrete Type Phase Field Method
Authors: Miyazaki Toni , Koyama Toshiyuki , Kozakai Takao 
Published: April 2011

ABSTRACTOn the basis of the discrete phase field method where the difference expression of non-linear kinetic equation, the composition dependencies of the atomic interchange energy and the inhomogeneous elasticity are taken into consideration so as to be applicable for the real alloy system. The phase decomposition and the microstructure formation are computer simulated for the Fe-Al- Co ternary ordering alloy by utilizing the thermodynamic data.
Mobility of Ag Atoms in the Binary System Ag2Se-Ag2S: A Molecular Dynamics Study
Authors: Smith A. , Ravelo R. , Pingitore N. 
Published: April 2011

ABSTRACTThe compounds in the binary system Ag2Se-Ag2S are known to undergo a phase transition to a superionic phase at a temperature which varies with concentration. The diffusion mechanism of Ag ions have been studied in a Molecular Dynamics formalism. We have calculated the diffusion coefficient as function of temperature and concentration and have calculated the activation energy for diffusion from the Arrhenius plot of DAg vs temperature. Results obtained for the end points Ag2Se and Ag2S agree with experimental measurements.
Orientational Order Phase Transition in Solid C60
Authors: Soto J. R. , Valladares R. M. , Calles A. 
Published: April 2011

ABSTRACTThe phonon spectrum and thermal parameters such as specific heat, effective temperature and amplitude of vibration of solid C60 are studied from ab-initio calculations. The relative orientations between the C60 balls are taken into account in a sc phase below 261.4K. The same properties are studied above the transition temperature (261.4K) where the balls are randomly oriented in a fcc structure. A difference is found in some of the thermal parameters due to the relative inter-molecular orientation. In particular, the low energy vibrational spectrum (which is separated by a ∼20meV gap from the intra-molecular spectrum) is changed.
Surface Energy Driven Crystallization of Amorphous Pd81si19
Authors: Schumacher Gerhard , Wahi Rajeshwar P. 
Published: April 2011

ABSTRACTIn-situ TEM investigations during thermal treatment of amorphous Pd81Si19 have been performed. It was found that crystalline nuclei are formed near the perforation edge of the hole produced by electrochemical polishing. After impinging with neighboring crystals, a crystallization front formed which was aligned parallel to the perforation edge. The crystallization front moved in the direction perpendicular to the perforation edge. Crystal growth was found to proceed faster in thinner parts of the specimen than in thicker parts. The results are described qualitatively within a thermodynamic model taking into account volume-, surface- and interfacial free energies and an appropriate specimen geometry.
Magnetic Softening and Hardening in Metglas® 2605SC at the Onset of Nanocrystallization
Authors: Aburto S. , Jiménez M. , Marquina V. , Gómez R. , Marquina M. L. , Ridaura R. , Valenzuela R. 
Published: April 2011

ABSTRACTMössbauer spectroscopy, coercive field and permeability were used to study the transformations produced by annealing at 375°C as-quenched amorphous ribbons of Metglas® 2605 SC of nominal composition Fe81B13.5Si3.5C2. The coercive field showed a decrease (softening) in the 0-30 min range, followed by an increase (hardening) for annealing times above 30 min. Permeability exhibited the opposite behavior, with a maximum for about 30 min. The Mössbauer magnetic hyperfine spectra showed variations in the intensities ratios, which were related with macroscopic measurements in good agreement with the softening-hardening processes.
Structural Stability Diagrams for Thin-Film Multilayers
Authors: Dregia S. A. , Banerjee R. , Fraser H. L. 
Published: April 2011

ABSTRACTStructural stability in thin-film multilayers is described in terms of classical thermodynamics, involving the competition between bulk and interfacial energies. A new type of phase diagram is introduced, thebiphase diagram,in which concurrent phase stabilities are mapped as a function of two degrees of freedom, corresponding to two independent layer thicknesses in a periodic multilayer. The model is used to explain our observations of phase stabilities in Al/Ti multilayers, as a function of varying layer thicknesses. The model is also applied to explain the experimental observations made by other investigators on phase stability in Co/Cr thin-film multilayers.
The Selective Synthesis of Molybdenum Silicides from Modulated Elemental Reactants
Authors: Johnson Christopher D. , Johnson David C. 
Published: April 2011

ABSTRACTElementally modulated reactants with repeat layer thicknesses of less than 30Å were found to crystallize various molybdenum silicides depending on their composition. Reactants with compositions near 1:2 Mo:Si were found to nucleate h-MoSi2, at 400°C with an activation energy of 1.9eV, even though h-MoSi2 is metastable with respect to t-MoSi2 below 1900°C. Reactants with compositions near 5:3 Mo:Si were found to nucleate Mo3Si3 at 650°C with an activation energy of 3.0eV. Reactants with compositions near 3:1 Mo:Si were found to nucleate Mo3Si at 720°C with an activation energy of 2.2eV. The ability to control the crystalline product via the initial composition is distinctly different from the behavior reported for multilayers with larger repeat spacings, which were observed to form h-MoSi2 regardless of overall composition. The measured nucleation energy as a function of composition suggests that the phase selectivity is nucleation controlled. The selectivity depends on the elimination of large composition gradients at the reacting interfaces before nucleation of the crystalline product.
Formation of Metallic Systems Far From Equilibrium By Pulsed Laser Deposition
Authors: Krebs Hans-Ulrich , Hamp Marco , Fähler Sebastian , Störmer Michael , Sturm Kai , Weisheit Martin 
Published: April 2011

ABSTRACTThe properties of metallic thin films prepared by pulsed laser deposition (PLD) strongly differ from samples grown by conventional thin film techniques like sputtering and evaporation. In many systems (Fe-Nb, Fe-Zr, Cu-Co, Fe-Ag and Ni-Ag), strongly supersaturated solid solutions are formed, a broader formation range occurring for amorphous samples, and nanocrystalline phases are produced. Additionally, metastable phases of larger thickness at the interfaces of multilayers occur. Reasons for these behaviors are discussed with respect to atomic mixing and implantation effects induced by the high instantaneous deposition rate during PLD and the high kinetic energy (>100 eV) of the deposited ions.
Phase Stability in the Nanocrystalline Tio2 System
Authors: Zhang Hengzhong , Banfield Jillian F. 
Published: April 2011

ABSTRACTInitial thermodynamic analysis predicts surface tension of fine solid particles decreases with decrease in particle size. Free energy of fine solid particles increases with decrease in particle size. In the nanocrystalline TiO2 system, the surface tensions and the surface energies of both anatase and rutile were estimated by modeling available thermochemical data and kinetic data. The particle size versus temperature phase diagram of the nanocrystalline TiO2 system was calculated, which reveals 1–8 nm anatase is more stable than rutile of the same size at certain temperatures.
Low-Temperature Mechanical Alloying Of Cu-Fe and Cu-Ta Powders
Authors: He J.-H. , Ma E. 
Published: April 2011

ABSTRACTA model analysis is presented which explains ball-milling induced alloying in positive-heatof- mixing systems in terms of a dynamic balance between externally forced mixing and thermal phase decomposition mediated by deformation-enhanced population of defects. The possibility of eliminating the thermal decomposition to force single phase formation is examined by milling Cu- Fe and Cu-Ta powder mixtures at the liquid nitrogen temperature (LN2T). Over a range of compositions for Cu-Fe and almost the entire composition range for Cu-Ta, the two-phase region observed for room-temperature (RT) milling persisted after cryomilling. The moderate temperature dependence of milling-induced alloying is interpreted by analyzing the dynamics of the generation and annihilation of the nonequilibrium vacancies during deformation and impacts in a SPEX mill.
Phase Evolution During Ball Milling of Al In NH3 and Subsequent Annealing
Authors: Nikolov J. I , Williams J. S. , Llewellyn D. J. , Calka A. 
Published: April 2011

ABSTRACTPhase evolution during ball milling of Al in both N2 and NH3 gas has been compared and the annealing behaviour studied in some detail. X-ray diffraction, differential thermal analysis, combustion analysis and scanning and transmission electron microscopy have been used as analytical techniques. Results have shown that a nitride is not formed in N2 but that Al forms into many small, hollow spheres during milling. In contrast, milling in NH3 results in an amorphous AlxNy(O) phase which transforms into crystalline AIN and A12O3 on annealing to 1000°C.
The Effect of Hydrogen Carrier Gas on the Morphological Evolution and Material Properties of GaN on Sapphire
Authors: Ng T. B. , Han J. , Biefeld R. M. , Zolper J. C. , Crawford M. H. , Follstaedt D. M. 
Published: April 2011

AbstractIn-situ optical reflectance is used to monitor the morphological evolution of the two-step GaN growth on sapphire. The amount of H2 carrier gas used in the growth is observed to strongly influence the morphological evolution of the low temperature buffer layer and the subsequent high temperature nucleation behavior, which in turn affects the structural and electrical properties of the GaN epitaxial films. The optical reflectance transients correlate with the sizes and distributions of nuclei as observed by AFM.
Diluent Gas Effects on Properties of Ain and GaN Thin Films Grown by Metalorganic Vapor Phase Epitaxy on α(6H)-SiC Substrates
Authors: Hanser Andrew , Wolden Colin , Perry William , Zheleva Tsvetanka , Carlson Eric , Hartlieb Philip , Davis Robert F. 
Published: April 2011

AbstractThin films of AIN and GaN were deposited on α(6H)-SiC(0001) wafers using metalorganic vapor phase epitaxy (MOVPE) and H2 and N2 diluents. A computational fluid dynamic model of the deposition process was used to analyze the film growth conditions for both diluents. Low temperature (12 K) photoluminescence of the GaN films grown in N2 had peak intensities and full widths at half maximum of ∼7 meV which were equal to or better than those films grown in H2. Cross-sectional and plan-view transmission electron microscopy of films grown in both diluents showed similar microstructures with a typical dislocation density of 109/cm2. Hall measurements of n-type (Si doped) GaN grown in N2 revealed Hall mobilities equivalent to those films grown in H2. Acceptor-type behavior of Mg-doped GaN grown in N2 was repeatably obtained without post-growth annealing. Secondary ion mass spectrometry revealed equivalent levels of H in Mg-doped GaN films grown in both diluents.
AIN/GaN and AIGaN/GaN Heterostructures Grown by HVPE on SiC Substrates
Authors: Melnik Yu. V. , Nikolaev A. E. , Stepanov S. I. , Zubrilov A. S. , Nikitina I. P. , Vassilevski K. V. , Tsvetkov D. V. , Babanin A. I. , Musikhin Yu. G. , Tretyakov V. V. , Dmitriev V. A. 
Published: April 2011

AbstractGaN, AIN and AIGaN layers were grown by hydride vapor phase epitaxy. 6H-SiC wafers were used as substrates. Properties of AIN/GaN and AIGaN/GaN structures were investigated. AIGaN growth rate was about 1 μm/min. The thickness of the AIGaN layers ranged from 0.5 to 5 μm. The AIN concentration in AIGaN layers was varied from 9 to 67 mol. %. Samples were characterised by electron beam micro analysis, Auger electron spectroscopy, X-ray diffraction and cathodoluminescence.Electrical measurements performed on AIGaN/GaN/SiC samples indicated that undoped AIGaN layers are conducting at least up to 50 mol. % of AIN.
Sublimation Sandwich Growth of Free Standing GaN Crystals
Authors: Vodakov Yu. A. , Mokhov E. N. , Ramm M. G. , Ramm M. S. , Roenkov A. D. , Ostroumov A. G. , Wolfson A. A. , Karpov S. Yu. , Makarov Yu. N. , Jürgensen H. 
Published: April 2011

AbstractThick epitaxial layers of GaN on SiC and sapphire are grown by using the sublimation sandwich method. It is shown that growth of good quality GaN crystals with the growth rates up to 0.5 mm/hour is possible using this technique. The grown layers have been separated from the seed and free standing GaN crystals up to 15 mm size were obtained.
MBE Growth of GaN on NdGaO3(101)
Authors: Fechtmann C. , Kirchner V. , Einfeldt S. , Heinke H. , Hommel D. , Lukasiewicz T. , Luczynski Z. , Baranowski J. 
Published: April 2011

AbstractWe report on the growth of GaN on NdGaO3(101) by plasma assisted molecular beam epitaxy (MBE). NdGaO3(101) is an interesting alternative substrate compared to A12O3due to its smaller lattice mismatch of +1.2 % to hexagonal (0001) GaN. Using photoluminescence (PL), X-ray diffraction (XRD), reflection high energy electron diffraction (RHEED) and atomic force microscopy (AFM) the optical and structural properties of the GaN thin films grown on NdGaO3are investigated and compared to those grown on c-plane sapphire. The intended epitaxial relationship of [0001] GaN ║ [101] GaN is hard to realise as confirmed by several tilted or even polycrystalline films. However, the layer quality was found to improve considerably, when the NdGaO3surface showed a (l × 1) instead of other reconstructions.
Single Crystal Gallium Nitride on Silicon Using SiC as an Intermediate Layer
Authors: Ustin S. A. , Ho W. 
Published: April 2011

AbstractGaN films have been grown atop SiC intermediate layers on Si(001) and Si(111) substrates using supersonic jet epitaxy (SJE). GaN growth temperatures ranged between 600 °C and 775 °C. Methylsilane (H3SiCH3) was used as the single source precursor for SiC growth and triethylgallium (TEG) and ammonia (NH 3) were the sources for GaN epitaxy. The GaN growth rate was found to depend strongly on substrate orientation, growth temperature, and flux. Structural characterization of the films was done by transmission electron diffraction (TED) and x-ray diffraction (XRD). Growth of GaN on SiC(002) produces a cubic or mixed phase of cubic and wurtzite depending on growth conditions. Growth on SiC(111) produces predominantly wurtzite GaN(0002). Minimum rocking curve widths for GaN(0002) on SiC/Si(111) and GaN(002) on SiC/Si(001) are 0.6° and 1.5°, respectively. Cross Sectional Transmission Electron Microscopy (XTEM) was also performed.
Structural and Optical Properties of Group III-Nitride Quantum Wells Studied by (S)Tem and CL
Authors: Lakner H. , Liu Q. , Brockt G. , Radefeld A. , Schulze-Kraasch F. , Scholz F. 
Published: April 2011

AbstractWurtzite InGaN/GaN and AlGaN/GaN heterostructures grown on sapphire by metal organic vapor phase epitaxy were studied using scanning transmission electron microscopy (STEM), cathodoluminescence (CL) combined with secondary electron (SE) imaging, high resolution x-ray diffractometry (HRXRD), and atomic force microscopy (AFM).SE imaging and AFM were used to study the surface morphology. The results indicate the presence of the following structural defects on the surface of InGaN/GaN heterostructures: hexagonal mesa-like structures, hexagonal pyramids and micropipes, while the surface of the AlGaN/GaN heterostructures are mirror-like smooth. The local optical properties of defects and defect free regions were studied using spatially resolved CL at low temperature. In addition, the dependence of the optical properties of both sorts of heterostructures on the quantum well width or chemical composition of ternary materials was investigated. The structural properties of the heterostructures were studied by STEM and HRXRD. Convergent beam electron diffraction (CBED) and corresponding simulations, convergent beam imaging (CBIM), and high resolution x-ray diffraction (HRXRD) were used to study the strained layers. Dislocations and interface properties were characterized using bright-field imaging, while the chemical compositions fluctuations were analyzed by Z-contrast imaging and energy dispersive x-ray microanalysis (EDX).
Organometallic Chemical Vapor Deposition of Group-III Nitride Thin Films using Single Source Precursors
Authors: Fischer Roland A. , Rogge Wolfram 
Published: April 2011

AbstractThe OMCVD of AlN, GaN and InN thin films using the novel single source precursors (N3)2Ga[(CH2)3NMe2] (1), (N3)In[(CH2)3NMe2]2 (2) and (N3)Al(CH2)3NMe2]2 (3) is reported. The compounds are non-pyrophoric. Compound 2 is air stable. No additional N-sources were used for the growth of the nitrides. We achieved epitaxial (AlN, GaN) or polycrystalline (InN) growth at least 200°C below the decomposition temperature of the respective nitride. Some aspects of the reactivity of the precursors with ammonia and the resulting influence on the deposition process were investigated.
In-Situ Rheed-Traxs Monitoring Alloy Composition of the Surface During RF-MBE Growth of GaInN and AIGaN
Authors: Ito A. , Sakai H. , Inagaki M. , Nomura G. , Nakamura Y , Yasuda T. , Amano H. , Akasaki I. 
Published: April 2011

AbstractIn-situ reflection high energy electron diffraction total reflection angle X-ray spectroscopy (RHEED-TRAXS) was performed to monitor alloy composition at the surface during growth of nitrides by RF-MBE for the first time. TRAXS signal of the GaLα line is found to be more sensitive to the composition at the surface than the GaKαline. A difference in the composition of layer adsorbed on the surface and the solid alloy layer has been identified.
Photoquenching Of Persistent Photoconductivity In N-Type GaN
Authors: Hirsch Michèle T. , Seiferta O. , Kirfel O. , Parisia J. , Wolk J. A. , Walukiewicz W. , Haller E. E. , Ambacher O. , Stutzmann M. 
Published: April 2011

AbstractWe report on the observation of optical quenching of persistent photoconductivity (PPC) in unintentionally doped n-type GaN films. The PPC is induced by subbandgap illumination between room temperature and 77K. The corresponding decay, which.is thermally activated, is substantially increased upon low energy illumination, e.g. illumination by wavelengths between 1O50nm and 700nm. We measure the saturation conductivity under simultaneous illumination with excitation and quenching light and find that some wavelengths can induce both excitation and quenching of photoconductivity. Additionally, we present a preliminary investigation of the spectral dependence of the quenching effect. A simulation indicates only a weak spectral dependence of the quenching cross-sections in the wavelength range from 470nm–1050nm.
Radiative And Nonradiative Relaxation Of Excitons In GaN
Authors: Göldner A. , Eckey L. , Hoffmann A. , Broser I. , Alemu A. , Gil B. , Ruffenach-Clur S. , Aulombard R. L. , Briot O. 
Published: April 2011

AbstractWe report on investigations of the excitonic quantum efficiency in GaN epilayers as function of buffer layer thickness, buffer layer material, sample thickness and residual oxygen content. These values are compared to that of GaN bulk material. The quantum efficiency of the free excitons rises with increasing buffer layer thickness, increasing sample thickness and decreasing residual oxygen content. The influence of oxygen on the quantum efficiency is stronger than that of the buffer layer thickness. Additionally, the homoepitaxial growth of GaN shows higher quantum efficiencies than the growth with an AIN buffer layer. In general, the observed quantum efficiencies in GaN epilayers are below 20% indicating the strong impact of nonradiative relaxation and recombination processes in the excitonic range. Only, GaN bulk material shows quantum efficiencies of 25 % for the free A-exciton XA and of 50 % for the donor-bound exciton complex D0,X).
Near Band Gap Photoluminescence Broadening In n-Gan Films
Authors: Iliopoulos E. , Doppalapudi D. , Ng H. M. , Moustakas T. D. 
Published: April 2011

AbstractThis paper addresses the broadening mechanism of the near band gap photoluminescence in GaN films doped n-type with silicon. The films were produced by plasma assisted MBE and their carrier concentration was varied systematically from 1015 to 1020 cm−3. The photoluminescence was excited with a 10 mW He-Cd laser at 77K. At low carrier conentration ( < 1017 cm−3 ) the photoluminescence peak has FWHM of about 18 meV, while at high carrier concentration (>1018 cm−3 ) the FWHM increase monotonically with the carrier concentration up to about 120 meV. The broadening of the line at high carrier concentration is attributed to impurity band broadening and tailing of the density of states. The data were quantitatively analyzed, as a function of carrier concentration and compensation ratio, using the impurity band broadening model of Morgan [1] and the agreement between model and experimental data supports the model's validity and suggest a potential method of determining the compensation in degenerate semiconductors.
Photoluminescence Characterization of p-type GaN:Mg
Authors: Corlatan Dorina , Krüger Joachim , Kisielowski Christian , Klockenbrink Ralf , Kim Yihwan , Sudhir G. S. , Peyrot Yann , Rubin Michael , Weber Eicke R. 
Published: April 2011

AbstractWe report on results of low-temperature photoluminescence measurements performed on GaN films, grown by molecular beam epitaxy (MBE) on sapphire substrates. The GaN films are either Mg doped (p-type) or consist of a Mg-doped layer on top of a Si doped GaN layer (n-type). In the p-doped samples, the sharpness of the donor-acceptor-pair transition is striking, three phonon replicas are clearly resolved. A transition band occurs around 3.4 eV, which becomes dominant for samples with an np-layer structure. The position and the composition of the near band edge transitions are influenced by the growth of the buffer layers. Depending on the growth conditions a transition at 3.51 eV can be observed.
The Effect of Low Temperature GaAs Nucleation on the Growth of GaN on Silicon (001) During MOVPE Process
Authors: Zheng L. X. , Liang J. W. , Yang H. , Li J. B. , Wang Y. T. , Xu D. P. , Li X. F. , Duan L. H. , Hu X. W. 
Published: April 2011

AbstractHigh quality cubic GaN was grown on Silicon (001) by metalorganic vapor phase epitaxy (MOVPE) using a GaAs nucleation layer grown at low temperature. The influence of various nucleation conditions on the GaN epilayers' quality was investigated. We found that the GaAs nucleation layer grown by atomic layer epitaxy (ALE) could improve the quality of GaN films by depressing the formation of mixed phase. Photoluminescence (PL) and X-ray diffraction were used to characterize the properties of GaN epilayers. High quality GaN epilayers with PL full width at half maximum (FWHM) of 130meV at room temperature and X-ray FWHM of 70 arc-min were obtained by using 10–20nm GaAs nucleation layer grown by ALE.