bluuebunny/crossref_metadata_2025_split huggingface dataset:
Deep Level Characterization of Interface-Engineered ZnSe Layers Grown by Molecular Beam Epitaxy on GaAs
Authors: Hierro A. , Kwon D. , Ringel S. A. , Brillson L. J. , Young A. P. , Franciosi A. 
Published: April 2011

AbstractDeep level defects have been detected and analyzed in epitaxial ZnSe layers grown by molecular beam epitaxy (MBE) on GaAs and on In0.04Ga0.96As using deep level optical spectroscopy (DLOS). A series of samples, which differ only in the initial Zn:Se beam pressure ratio (BPR = 1:1, 1:10, 10:1) during the growth nucleation step, were characterized by DLOS in order to assess the dependence of bulk deep level formation on interface nucleation conditions. The transient and steady state photocapacitance measurements were performed using 100 W Quartz Halogen and 450 W Xe lamps as light sources, in the spectral range of 0.9 to 2.9 eV with a resolution better than 0.02 eV. The capacitance transients were recorded for time windows of 10 msec to 5 sec after light excitation of the sample, which was kept at a temperature of 100 K. Using semi-transparent Au Schottky contacts, several deep levels in the ZnSe layer were detected for all BPR's, with optical threshold energies of 1.1, 1.46 and 1.9 eV below the conduction band. These energies were obtained from the slope of the capacitance transient at different time intervals and were confirmed by steady state photocapacitance. The concentration of the levels was in the range 1012 to 1014 cm−3. Both the 1.1 and 1.46 eV trap concentrations were found to depend strongly on lattice mismatch conditions, whereas the latter was shown to largely depend on BPR. The optical threshold of the 1.9 eV trap correlates well with a ˜1.9 eV cathodoluminescence (CL) peak, which has been previously associated with either Zn vacancies or Gazn substitutional defects in Zn-deficient material.
Growth and Characterization of GaN and ALxGA1−xN Thin Films Achieved Via Lateral- and/or Pendeo-Epitaxial Overgrowth on 6H-SIC(0001) Substrates
Authors: Davis Robert F. , Nam O-H. , Zheleva T. S. , Bremser M. D. , Linthicum K. J. , Gehrke T. , Rajagopal P. , Thomson D. B. , Carlson E. P. 
Published: April 2011

AbstractDiscrete and coalesced monocrystalline layers of lateral- and pendeo-epitaxially grown GaN and AlxGal−xN layers originating from GaN stripes deposited within windows contained in SiO2 masks or from side walls of GaN seed structures containing SiNx top masks have been grown via organometallic vapor phase deposition on GaN/AlN/6H-SiC(0001) substrates. Multilayer heterostructures of GaN and AlxGal−N were also achieved. Scanning and transmission electron microscopies and atomic force microscopy were used to evaluate the microstructures, the type and distribution of dislocations and the surface roughness of the resulting films. The extent and microstructural characteristics of the laterally overgrown GaN regions were a strong function of stripe orientation and temperature. These regions contained a low density of dislocations. The RMS roughness of the (1120) sidewall plane of the pendeoepitaxial structures was approximately 0.100 nm.
Evolution of Fractal Particles in Systems with Conserved Order Parameter
Authors: Kalinin S.V. , Gorbachev D.L. , Tomashevitch K.V. , Borisevich A.Yu. , Vertegel A.A. , Markworth A.J. , Tretyakov Yu. D. 
Published: April 2011

AbstractIn the present research, the evolution of fractal aggregates in systems with conserved order parameter is studied. The aggregates are generated by diffusion limited aggregation (DLA). This model describes such important processes as annealing of dendrite inclusions in solids, healing of cracks in ceramics, temperature-induced transformations in composites, relaxation of rough surfaces, aging of colloid particles, etc. It is shown that the evolution in fractal media differs significantly from the evolution from the initially homogeneous state and leads to the different values of the scaling exponent.
Multiscale Modeling of Recrystallization
Authors: Miodownik M.A. , Holm E.A. , Godfrey A.W. , Hughes D.A. , Lesar R. 
Published: April 2011

AbstractWe propose a multi length scale approach to modeling recrystallization which links a dislocation model, a cell growth model and a macroscopic model. Although this methodology and linking framework will be applied to recrystallization, it is also applicable to other types of phase transformations in bulk and layered materials. Critical processes such as the dislocation structure evolution, nucleation, the evolution of crystal orientations into a preferred texture, and grain size evolution all operate at different length scales. In this paper we focus on incorporating experimental measurements of dislocation substructures, misorientation measurements of dislocation boundaries, and dislocation simulations into a mesoscopic model of cell growth. In particular, we show how feeding information from the dislocation model into the cell growth model can create realistic initial microstructures.
Kinetic Monte Carlo Simulations of Fecu Alloys
Authors: Domain C. , Becquart C.S. , Duysen J.C. Van 
Published: April 2011

AbstractThe steel vessels of pressurized water reactors are embrittled by neutron irradiation. It is well known that copper atoms play an important role in the embrittlement and that different Cu-containing defects such as Cu-rich clusters (sometimes called atmospheres), Cu precipitates and Cu-vacancy complexes have been identified experimentally. It is still difficult to link the formation of these defects to the primary damage resulting from the neutron inducing displacement cascades. Therefore, we investigate the evolution of the primary damage in FeCu alloys using kinetic Monte Carlo simulations based on a vacancy diffusion mechanism. The calculations rely on adapted, phenomenological, n-body potentials that satisfactorily reproduce properties of FeCu. At room temperature, experimentally identified defects such as Cu-vacancy complexes (one Cu atom bound to three or four vacancies) form in the course of our simulations. Furthermore, it appears that complex defects such as a Cu atom linked to two vacancies are very mobile and are responsible for the formation of small Cu clusters.
Self-Consistent Kinetic Lattice Monte Carlo
Authors: Horsfield Andrew , Dunham Scott , Fujitani Hideaki 
Published: April 2011

AbstractWe present a brief description of a formalism for modeling point defect diffusion in crystalline systems using a Monte Carlo technique. The main approximations required to construct a practical scheme are briefly discussed, with special emphasis on the proper treatment of charged dopants and defects. This is followed by tight binding calculations of the diffusion barrier heights for charged vacancies. Finally, an application of the kinetic lattice Monte Carlo method to vacancy diffusion is presented.
AB-Initio Based Calculations of Vacancy Formation and Clustering Energies Including Lattice Relaxation in Fe3Al
Authors: Muratov Leonid S. , Cooper Bernard R. 
Published: April 2011

AbstractVacancy formation and clustering significantly affect structural properties of transition-metal aluminides. Ab-initio quantum mechanical total-energy calculations using a full-potential linear combination of muffin-tin orbitals (LMTO) technique provide a convenient method of studying relevant characteristics such as changes in density of states, and charge redistribution around defects. Augmented with Hellmann-Feymann forces, LMTO allows calculations of relaxation geometries and relaxation energies. We have performed such calculations for vacancies and antisite substitutional point defects in Fe3Al with DO3 crystallographic structure. There are two limiting factors complicating calculations of defect formation energies directly from ab-inito calculations. The first is that a single defect, due to the lattice periodicity necessitated by the use of ab-inito total energy techniques, cannot be considered as an isolated defect, even in the maximum computable simulation cell. Unlike previous calculations [ I ], which did not find a dependency on the size of the simulation cell, our calculations have shown a significant difference in results for 32- and 16- atom cells. This difference provides information about vacancy clustering since it can be explained by a relatively small attractive interaction energy ~0.2 eV between two vacancies located in adjacent simulation cells and separated by the lattice constant distance (5.52Å) [2]. By comparing the internal energies for two configurations of 30 atom cells (32 atom - 2 vacancies) we were able to estimate that the attractive interaction between two vacancies could reach 1.2 eV. The second complication is the fact that chemical potentials of elements cannot be directly extracted from the total energy calculations for the compound. To deal with this problem, we considered two possible approximations and compared results, which were found to be quite similar for iron vacancies.
An Effect of Hydrostatic Compression on Defects in Energetic Materials: AB Initio Modeling Maija
Authors: Kuklja M. , Kunz A. Barry 
Published: April 2011

AbstractFirst-principle theoretical investigation of the basic defects such as a molecular vacancy, a vacancy dimer, an edge dislocation, and a micro-crack in organic explosive molecular crystals is presented. As an example we considered solid RDX (C3H6N6O6) which is well studied unstable solid. It was established that external hydrostatic pressure changes optical properties of defect-free RDX as well as of the crystal with defects narrowing the band gap. The lattice defects (especially dislocations) are identified with the so-called “hot spots.” The nature of local electronic states introduced in the band gap by the edge dislocation and formed mainly by molecular orbitals of N-NO2 group is analyzed. Favorable conditions for molecular dissociation due to electronic excitation are shown.
Understanding Structure and Electronic Properties of Extended Self-Interstitial Defects in Silicon
Authors: Alippi P. , Colombo L. 
Published: April 2011

AbstractThe results of an atomistic investigation on the coalescence mechanisms of self-interstitial {311} defects are presented. Formation energies and equilibrium configurations of defect structures are determined by tight-binding molecular dynamics simulation. We focus on the characterization of the lattice strain field around the defect complex: By means of the determination of the atomic stress distribution, we discuss how it may influence the formation mechanisms of the planar {311} structures. We also attempt a correlation between structural features and electronic properties through the analysis of defect-related orbitals occupations and inverse participation ratios.
First-Principles Study of π-Bonded (100) Planar Defects in Diamond
Authors: Zapol Peter , Curtiss Larry A. , Gruen Dieter M. 
Published: April 2011

AbstractA periodic density functional study of the high-energy π-bonded (100) stacking fault in diamond that can serve as a prototype of a twist grain boundary has been carried out. Information on formation energies, geometries and the electronic structure has been obtained. A single point electronic structure calculation of a ∑5 twist grain boundary based on the geometry taken from a molecular dynamics simulation has also been performed.
Atomistic Studies of Generic Tilt Grain Boundary Structures
Authors: Pawaskar D. N. , Miller R. , Bai R. , Schwartzman A. , Phillips R. , Briant C. L. 
Published: April 2011

AbstractWe investigate the atomic-scale structures of high-sigma (∑) tilt grain boundaries in aluminum using lattice statics calculations. In particular, we examine the efficacy of the structural unit model (SUM) in the context of long-period boundaries. Our investigation of both the equilibrium and metastable structures for certain high ∑ boundaries may necessitate a revision of the SUM formalism. We also consider further departures from high symmetry boundaries by considering the structural rearrangements induced by steps on boundaries.
Point Defect Interactions with Extended Defects in Silicon
Authors: Justo J. F. , Antonelli A. , Fazzio A. 
Published: April 2011

AbstractWe investigated the interaction of point defects (vacancy and self-interstitials) with an intrinsic stacking fault in silicon. The calculations were carried out using ab initio total energy methods. The results show that point defects at an intrinsic stacking fault display a different behavior as compared to the same defect in the crystalline environment. This is evidenced by differences in formation energies and electronic structures. These results suggest that there is migration of point defects from the bulk to a stacking fault. This could affect the dislocation mobility in the crystal.
Bias Potentials for Ttyperdynamics Simulations
Authors: Rudd W. G. , Voter A. F. 
Published: April 2011

AbstractWe describe our studies of two methods for generating bias potentials for use in hyperdynamics simulations. In the first method, first reported by Steiner, et al, the bias potential is the additional energy needed to keep the total potential energy above a fixed level. This potential exerts a negligible computational load and is very easy to code. The second technique involves computing the iterative Hessian-based bias potential as usual, but including in those calculations only the atoms in a small “active region” that surrounds the area in which a state transition is expected to occur. This “subspace hyperdynamics” method is less costly than full HD. The extra computational effort required for HD scales as the size of the active region(s), instead of with the size of the entire simulation domain.We have carried out extensive tests of both methods on two problems, the diffusion of adatoms on silver surfaces and the migration of vacancies in bulk silver. Using the first method, to obtain large boosts the bias potential must be so high that many transitions are prevented from occurring. Our results for subspace hyperdynamics are promising; we obtain the same results as with full hyperdynamics, but with considerably less computational effort.
Thermal Conductivity of Solid Argon by Classical Molecular Dynamics
Authors: Kaburaki Hideo , Li Ju , Yip Sidney 
Published: April 2011

AbstractFollowing the Green-Kubo formalism in linear response theory, the lattice thermal conductivity of solid argon is determined by using classical molecular dynamics simulation to calculate the heat current correlation function. Comparing the absolute conductivities obtained using the Lennard-Jones potential with experiments, we find the predicted results to uniformly underestimate the measurements in magnitude, whereas the calculated temperature dependence corresponds well with the data. The temporal behavior of the heat current autocorrelation function shows that while a single exponential decay description is appropriate at elevated temperatures, below the half of the Debye temperature, the heat current relaxation clearly consists of two stages, an initial rapid decay associated with local dynamics followed by a slower component associated with the dynamics of lattice vibrations (phonons).
Bond-Order Potentials for Molybdenum and Niobium: An Assessment of Their Quality
Authors: Mrovec M. , Vitek V. , Nguyen-Manh D. , Pettifor D. G. , Wang L. G. , Sob M. 
Published: April 2011

AbstractThe bond-order potentials (BOP) have been constructed for Mo and Nb. These potentials are based on the real-space parametrized tight-binding method in which diagonalization of the Hamiltonian is avoided by direct calculation of the bond-order. In this scheme the energy consists of three parts: The bond part that comprises contributions of d electrons and introduces into the scheme the covalent character of bonding, the central-force many-body part that reflects the environmental dependence of sp overlap repulsion and a pair-wise contribution. The potentials were tested by calculation of energy differences between the bcc and several alternate structures and by investigating the trigonal deformation path. These calculations have been made in parallel using BOP and the full-potential linearized augmented plane-wave method. The central-force many-body Finnis-Sinclair type potentials have also been included into the study of the deformation path. This evaluation of BOP reveals that the potentials reproduce very closely the ab initio results and are, therefore, very suitable for atomistic studies of extended defects in the transition metals.
Hydrogenated Amorphous Silicon Nitride: Structural and Electronic Properties
Authors: Justo J. F. , Mota F. De Brito , Fazziom A. 
Published: April 2011

AbstractWe combined empirical and ab initio methods to study structural and electronic properties of amorphous silicon nitride. For such study, we developed an interatomic potential to describe the interactions between silicon, nitrogen, and hydrogen atoms. Using this potential, we performed Monte Carlo simulations in a simulated annealing scheme to study structural properties of amorphous silicon nitride. Then this potential was used to generate relevant structures of a-SiNx:Hy which were input configurations to ab initio calculations. We investigated the electronic and structural role played by hydrogen incorporation in amorphous silicon nitride.
Molecular Orbital Calculations on Atomic Structures of Si-Based Covalent Amorphous Ceramics
Authors: Matsunaga K. , Matsubara H. 
Published: April 2011

AbstractWe have performed ab-initio Hartree-Fock molecular orbital calculations of local atomic structures and chemical bonding states in Si-N covalent amorphous ceramics. Solute elements such as boron, carbon and oxygen were considered in the Si-N network, and the bonding characteristics around the solute elements were analyzed. When a nitrogen atom is substituted by a carbon atom, it was found that Si-C bonds reinforce the Si-N network due to strong covalency.
Nanocrystal Growth in Crystalline Insulators Irradiated with High-Current Cu IONs
Authors: Kishimoto N. , Umeda N. , Takeda Y. , Lee C.G. , Gritsyna V.T. 
Published: April 2011

AbstractNegative Cu ions of 60 keV have been applied to generate metal nanocrystals embedded in insulators. Crystalline (c-), amorphous (a-)SiO2 and a spinel oxide, MgO·2(Al2O3), were irradiated at various dose rates up to about 100 µA/cm2, at a total dose of 3.O×1016 ions/cm2. In a-Sio2, morphology of nanoparticles and the resultant optical property changed with dose rate and, at a critical condition, showed in-plane arrangement of nanocrystals. The optical property of c-Si0 2 (a-quartz) was qualitatively similar to that of a-SiO2, but the implanted region of c-SiO2 showed different irradiation responses. The c-SiO2 was susceptible to radiation-induced amorphization but the nanoparticle morphology was still different from a-SiO2, suggesting a stronger depth-oriented driving force. Unlike SiO2, the spinel oxide showed good structural stability against the implantation and no tendency of the long-range atomic rearrangement. The results indicate that the crystallinity and the relevant interactions may play an important role in the nanoparticle growth during the implantation.
Dose Rate Dependence of the Amorphization of Silicon Carbide
Authors: Snead L.L. , Zinkle S.J. , Eatherly W.S. , Hensley D.K. , Vaughn N.L. , Jones J. W. 
Published: April 2011

AbstractSingle crystal silicon carbide (SiC) has been 2 MeV silicon ion irradiated in various irradiation temperature and ion flux ranges to measure the effect of these parameters on the critical dose for amorphization. The temperature and flux range for which amorphization was observed ranged from 80 to 400 K and 0.066 to 3 × 104 dpa/s, respectively. The critical dose, Dcrit was found by locating the depth of the boundary between partially crystalline and fully amorphous material using dark field TEM from samples prepared in cross section. This depth was compared to the damage profile as calculated using the TRIM-96 code. The temperature dependence of Dcrit is found to agree well with previously reported values, though new evidence suggests a defect species becoming mobile in the 250-300 K range. Also of significance is that Dcrit was dependent on flux at 340 K, ranging from 0.79 displacements per atom at the lowest ion flux to ∼0.6 dpa at the highest flux level. The dose rate dependence of Dcrit, is compared with a chemical rate theory model previously described by the authors. It is seen that the dose rate dependence is substantially weaker than theorized. An extrapolation of the measured dose rate dependence is also compared with recent data on fast neutron amorphized SiC.
Formation of ZnAl2O4 and MgAl2O4 Spinel in Al2O3 by Ion Implantation
Authors: White C.W. , Meldrum A. , Sonder E. , Budai J.D. , Zuhr R.A. , Withrow S.P. , Henderson D.O. 
Published: April 2011

AbstractZnAl2O4 spinel has been formed in Al2O3 by ion implantation. Sequential implantation of Zn and 0 in overlapping profiles followed by annealing in Ar + H2 gives rise to a nearly continuous epitaxial layer of ZnAl2O4 oriented with (111) planes parallel to (0001) planes of Al2O3. If only Zn is implanted, then discrete bands of ZnAl2O4 oriented with (422) planes parallel to (0001) planes of Al2O3 are produced. By similar methods, oriented MgAl2O4 spinel also has been produced in Al2O3 by sequential Mg + O implantation.
Interphase Integrity of Neutron Irradiated SIC Composites
Authors: Snead L. L. , Lara-Curzio E. 
Published: April 2011

AbstractSiC/SiC composites were fabricated from Hi-Nicalon™ fibers with carbon, porous SiC and multilayer SiC interphases. These materials were then irradiated in the High Flux Beam Reactor with fast neutrons at 260 and 900–1060°C to a dose of l.1×1025 n/m2 corresponding to 1.1 displacements per atom (dpa). Results are presented for bend strength of both non-irradiated and irradiated materials. Within the interphases studied the multilayer SiC interphase material showed the least degradation (8-20%) in ultimate bend stress, while porous SiC underwent the greatest degradation (∼35%). The fiber matrix interphases are studied with TEM for both non- irradiated and irradiated materials. While no irradiation induced microstructural evolution of the interphase was observed, debonding of the interphase from the fiber was observed for all cases. This debonding is attributed to tensile stresses developed at the interface due to densification of the Hi-Nicalon™ fiber. Residual stress analysis of the fiber matrix interface indicates that the irradiation-induced densification of Hi-Nicalon™ and the volumetric expansion of the CVD SiC matrix cause tensile stresses well in excess of those which can be withstood by these, or any other viable SiC composite interphase.
Microstructure of Swift Heavyion Irradiated MgAl204 Spinel
Authors: Zinkle S.J. , Matzke Hj. , Skuratov V.A. 
Published: April 2011

AbstractPlan view and cross-section transmission electron microscopy was used to investigate the microstructure of magnesium aluminate spinel (MgAl2O4) following room temperature irradiation with either 430 MeV Kr, 614 MeV Xe, or 72 MeV I ions. The fluences ranged from 1×1016/m2 (single track regime) to 1×1020/m2. Destruction of the ordered spinel crystal structure on both the anion and cation sublattices was observed in the ion tracks at low fluences. At intermediate fluences, the overlapping ion tracks induced the formation of a new metastable crystalline phase. Amorphization with a volumetric expansion of ∼35% was observed in spinel irradiated with swift heavy ions (electronic stopping powers >7 keV/nm) at fluences above 1×1019/m2. These results demonstrate that swift heavy ion radiation can induce microstructural changes not achievable with conventional elastic collision irradiation at comparable temperatures.