bluuebunny/crossref_metadata_2025_split huggingface dataset:
New Silicon (111) Surface (7×7) Reconstruction Benzene-Like Ring Model
Authors: Hao You Gong , Roth Laura M. 
Published: March 2011

ABSTRACTA new silicon (111) surface (7×7) reconstruction model is proposed. The model for the unit mesh consists of 13 benzene like rings formed by lowering triples of top layer atoms and 12 raised top layer atoms, with three of the rings combining to give depressed corners. The construction of the model is based on some considerations of the physical properties of the surface and on cluster calculations using the semi-empirical MNDOgeometrical optimization method. A cluster calculations with 9 silicon atoms gave the ring structure as an energy minimized result and also gives evidence that the two halves of the unit mesh should be at different height levels. A 13 silicon atom “flower” cluster calculation gave the height of the raised atoms to be .851A. The structure of the unit mesh resembles two types of (2×2) structure which can be formed from the rings and raised atoms (“flowers”), one of which is readily transformed into pi-bonded chain or molecular structures. The model gives a good qualititative account of the various hills and valleys seen in scanning tunneling microscopy, and since lateral displacement is small, it agrees with low energy channeling experiments. In addition good agreement with Cardillo s helium atom scattering experiment and Yang et al. s LEED experiment is found as the model has different heights for the two halves of the unit mesh and characteristics of a mixture of two types of structure.
Classical Two and Three-Body Interatomic Potentials for Silicon Simulations
Authors: Biswas R. , Hamann D. R. 
Published: March 2011

ABSTRACTWe develop two and three-body classical interatomic potentials that model structural energies for silicon. These potentials provide a global fit to a database of firstprinciples calculations of the energy for bulk and surface silicon structures which spans a wide range of atomic coordinations and bonding geometries. This is accomplished using a new “separable” form for the 3-body potential that reduces the 3-body energy to a product of 2-body sums and leads to computations of the energy and atomic forces in n2 steps as opposed to n3 for a general 3-body potential. Simulated annealing is performed to find globally minimum energy states of Si-atom clusters with these potentials using a Langevin molecular dynamics approach.
Special Purpose Processors for Computing Materials Properties
Authors: Bakker A. F. 
Published: March 2011

ABSTRACTThe need for computational power in the modeling of physical systems is rapidly increasing. Realistic simulations of materials often require complex interactions and large numbers of particles. For most scientists, full-time access to supercomputers is not possible, and even this might not be sufficient to solve their problems. As most of the calculations involved are straightforward and repetitive in nature, a possible solution is to design and build a processor for a specific application with a low cost/performance ratio. This approach is to be contrasted with the use of a general purpose computer, which is designed to treat a large class of problems and includes many expensive features (e.g. software) that are not utilized in the simulations. The architecture of a special purpose computer can be tailored to the problem; e.g., parallel and pipelined operations can be incorporated to obtain efficient computational throughput, and memory organization and instruction sets can be optimized for this purpose. A few of such algorithm oriented processors have been built in the last decade and have been utilized for certain specific jobs; for example: molecular dynamics simulations of systems of Lennard-Jones particles and Monte Carlo calculations of Ising models. An overview of some existing algorithm oriented processors and expectations for the future will be presented.
The Study of Surface Phonons by Electron Energy Loss Spectroscopy: Theory of the Excitation Cross Section
Authors: Mills D. L. 
Published: March 2011

ABSTRACTIn the past few years, experimental developments in electron energy loss spectroscopy have allowed study of the dispersion relations of surface phonons, on clean and adsorbate covered surfaces. Also, theoretical analyses of the angle and energy variation of the excitation cross sections have been developed. These have guided the choice of scattering geometry, and also have assisted directly in the interpretation of the data. We review the recent work, with emphasis on the theoretical side.
Computer Simulation of Electron Microscope Images from Atomic Structure Models
Authors: Krakow William 
Published: March 2011

ABSTRACTIt is generally the case that simple direct interpretation of high resolution electron microscope images is not possible due to the phase contrast imaging modes necessary to achieve atomic level spatial resolution. Therefore, an extensive number of computer programs have been developed to perform electron diffraction and image computations. Both single scattering or dynamical scattering processes can be simulated as well as any form of imaging mode currently available on most modern high performance transmission electron microscopes. Since one is interested in imperfections rather than perfect crystal structures, a large number of sampling points in real and reciprocal space are required. Often, large atom position arrays must be sampled requiring large mainframe computer memories and fast CPU's. High quality displays are also required for realistic image representations and even faster computational methods via television rate digital frame store devices. This paper will be centered about a number of materials areas requiring high resolution electron microscopy computer simulation from atomic structure models. These areas include: organometallic molecules, point defects, surface structure and reconstructions, amorphous thin films, quasi-crystals, semiconductor interfaces and grain boundary structure in metals.
Rate-Sensitivity of Mode I and Mode II Fracture of Concrete
Authors: Shah Surendra P. , John Reji 
Published: March 2011

ABSTRACTFor rational and accurate analysis of concrete structures subjected to impact or impulsive loads the knowledge of crack propagation under such dynamic loads is essential. Cracks frequently propagate under mixed-mode (opening and sliding: mode I and mode II) conditions. The rate of loading effects on the mechanical properties have been attributed to the rate sensitivity of crack propagation. The strain-rate effects may be different in mode I and mode II fracture of concrete. Single edge notched beams were subjected to varying rates of loading, to establish the mode I Stress Intensity Factor, KIvs. Crack Velocity, V, relationship for mortar and concrete. Impact tests were conducted using a Modified Instrumented Charpy Impact Test System. The rate of crack growth was obtained using brittle ‘Krak Gages'.Mode I test results lead to the following conclusions. (1) Slow (prepeak) crack growth for concrete is larger than that of mortar, at a given strain rate. (2) Pre-peak crack growth decreases with increase in strain-rate. This could be the reason for decrease in pre-peak non-linearity at higher strain rates. Hence LEFM approach may be valid at high rates of loading. (3) log KI- log V relationship is non-linear especially at the higher rates of loading.
Influence of the Crack Opening Rate on the Fracture Toughness of Plain Concrete
Authors: Rossi P. , Boulay C. , Acker P. , Malier Y. 
Published: March 2011

ABSTRACTThree D.C.B. tests (Double Cantilever Beam) under different loading histories have shown that the applied crack opening rates can have an effect on the KIC fracture toughness values computed on the basis of linear elastic behavior of concrete. This assumption can be well justified for a certain range of crack opening rates, outside of which viscous phenomena must be taken in account. The concepts of ductile and brittle fracture appear then as related to the crack opening rate.
Characterisation of Fly Ash Hydration Reactions from Microstructure
Authors: Mills R. H. 
Published: March 2011

ABSTRACTReaction products of high lime fly ash (FA), mixed with distilled water and hydrated in contact with Portland cement (PC), and in hardened PC/FA pastes of various mix proportions, were examined by SEM and EDX. Structures contributing to early strength originated mainly through solution depositions of ettringite, and portlandite. Some evidence of ettringite instability suggested gypsum deficiency in PC/FA mixtures.
Modeling of Fly Ash Concrete Mixtures
Authors: White Elizabeth L. , Roy Della M. 
Published: March 2011

ABSTRACTA Factor Analysis Model was applied to existing literature data on fly ash and cement properties to relate them to the strength and durability of the cured pozzolanic concrete products. A normal curve was fitted to each independent fly ash and cement property and to each dependent strength property. Functions of the moments, such as indicators of skewness and kurtosis of each property, were calculated and the significance of their departures from the expected values of a normal population was examined. About one-half of the properties were log-normally distributed; the remainder were normally distributed as given. A predictive model was proposed for compressive strength, using the percent sulfate as representative of Factor I, the cement quantity as representative of Factor II, and the percent Fe2O3 as representative of Factor III.
Short and Long-Term Leaching Behavior of a Low-Calcium Fly Ash and Cement-Stabilized Fly Ash
Authors: Malek R. I. A. , Licastro P. H. , Roy D. M. 
Published: March 2011

ABSTRACTLow-calcium fly ashes from eastern Pennsylvania power plants, stabilized with Portland cement, have a potential application as a pipeline bedding material. A typical fly ash, and a cement-stabilized material made with this fly ash, were subjected to extensive physical and chemical characterization. Two procedures were used to investigate whether leachates from the bedding materials were nonhazardous according to the criteria of the regulatory agencies. The first was the short-term EPA test, used to define a hazardous waste under the RCRA regulations, and the second was a long-term flow-through test developed at MRL/PSU. In the second test, driving pressure, flow rate and permeability were monitored during the experiment. Calculations showed that this flow-through technique simulates rain water percolation in a similar sized bed for about 25 years. The leaching fluids in both methods were deionized water (EPA procedure) and simulated rain water. Analyses for seventeen metals by DC plasma emission spectrometry and for seven anions by ion chromatography were performed. Concentrations of the leachates from the two procedures were well below the levels that define a hazardous waste. Considerable insight into the long term leaching mechanisms of various elements was obtained from the flow-through experiments.
Heteroepitaxy of Si, Ge, and GaAs Films on CaF2/Si Structures
Authors: Ishiwara H. , Asano T. , Lee H. C. , Kuriyama Y. , Seki K. , Furukawa S. 
Published: March 2011

ABSTRACTRecent progress in the research of heteroepitaxial growth of Si, Ge, and GaAs films on CaF2/Si structures is reviewed. Growth conditions and material properties of the Si/CaF2/Si structures are first discussed. It is shown that such growth techniques as the predeposition technique and the recrystallization method are useful to improve the crystalline quality of Si films on the CaF2/Si structures. Then, device application of the Si/CaF2/Si structure to field effect transistors with epitaxial MIS (metal-insulatorsemiconductor) gate electrodes is described. Finally, epitaxial growth of Ge and GaAs films on the CaF2/Si structure are discussed, in which such growth parameters as the substrate temperature and growth rate are optimized to obtain high-quality films with excellent crystallinity and smooth surface.
Electrical Properties and Structural Defects in Epitaxial CaF2 on Si
Authors: Schowalter L. J. , Fathauer R. W. , Ponce F. A. , Anderson G. , Hashimoto Shin 
Published: March 2011

ABSTRACTIn this paper, a study is made of the electrical properties and of the types of structual defects which can occur in epitaxial CaF2 grown on Si(111) substrates by molecular beam epitaxy (MBE). High-resolution transmission electron microscopy (HRTEM) and high-energy (≥2 MeV) He+ ion channeling techniques are used to characterize defects in the epitaxial layer and at the CaF2 /Si interface while current-voltage (I–V) and capacitance-voltage (C–V)3 measurements are used to characterize the electrical properties. The HRTEM images show an atomically abrupt interface between the CaF2 and Si. The most common defect we have been able to identify is associated with an atomic step at the interface and is similar to a Shockley partial dislocation at a (111) twin interface in an fcc crystal structure. The ion channeling measurements also indicate the presence of defects at the CaF2 /Si interface. The apparent defect density measured by ion channeling decreases in the CaF layer as one moves away from the interface at a rate which depends on ihe final thickness of the epitaxial film. Ion channeling has also been used to measure strain and it is found that, while thin layers of epitaxial CaF2 on Si(111) have large strain, the strain becomes vanishingly small as the layers exceed 200 nm in thickness. This result can be adequately explained using an equilibrium model for the introduction of strain relieving misfit dislocations and indicates that epitaxial fluoride layers may be useful as thermal mismatch buffers in heteroepitaxial structures. In C–V measurements of epitaxial CaF2 layers on Si(111) which have been fabricated into metal-insulator-semiconductor structures, the capacitance is observed to remain constant over a large variation in applied voltage. This constant capacitance region can be explained as due to a high density of interface states in the band gap. In addition, I–V measurements indicate that, at low fields, the CaF2 resistivity exceeds 1014 Ω-cm. At high fields, the CaF2 starts to conduct in a manner which we speculate is due to defects at the CaF2/Si interface. The field at which this conduction takes place has been ovserved to exceed 1 MV/cm for a 42-nm-thick CaF2 film with the device geometry used for this work.
MOCVD Growth of III-V Compounds on Si Using Strained Superlattices
Authors: Sakai Shiro , Soga Tetsuo , Takeyasu Masanari , Umeno Masayoshi 
Published: March 2011

ABSTRACTGaAs and GaAsP with the entire compositional range are grow on Si using an intermediate layer of GaAsP strained superlattices to relax the lattice mismatch. The orientation of the overgrown GaAs layer is found to be determined by the direction of the off-angle of the Si (100) surface. The grown layers are characterized by photoluminescence, x-ray diffraction, electro reflectance and DLTS. GaAs/GaAlAs double heterostructure lasers and GaAsP visible LED's are fabricated on Si substrates. The structural and electronic properties of the grown layers and the device performances are reported in this paper.
GaAs/AlGaAs Diode Lasers on Monolithic GaAs/Si Substrates
Authors: Windhorn T. H. , Turner G. W. , Metze G. M. 
Published: March 2011

ABSTRACTOne approach to the development of optical interconnects between Si systems utilizes diode lasers fabricated in III-V epitaxial layers grown on Si wafers. We have fabricated double-heterostructure lasers in GaAs/AlGaAs layers grown on a Ge-coated Si substrate, and both asymmetric largeoptical- cavity (LOC) lasers and graded-index, separate-confinement heterostructure (GRIN-SCH) lasers in such layers grown directly on a Si substrate. The GaAs/AlGaAs layers were grown by molecular beam epitaxy on (100) p-Si substrates. Si and Be were used as the n- and p-type dopants, respectively. Oxide-defined stripe-geometry devices, 300 μm long, were fabricated using standard AuSn and CrAu metallizations for the n- and ptype contacts, respectively. The laser facets were formed by ion-beamassisted etching. The double-heterostructure devices (8 μm stripe width), in which the active region contained about 10 mole percent AlAs, were evaluated using pulsed bias at 77 K. They produced power outputs up to 3.3 mW per facet and exhibited thresholds as low as 170 mA. The LOC devices (4 μm stripe width), which had a GaAs active region, were characterized using pulsed bias at 300 K. These devices produced power outputs up to 27 mW per facet. The lowest threshold was 775 mA. The GRIN-SCH devices (4 μm stripe width), which incorporated a 70 Å GaAs quantum well active layer, were also characterized using pulsed bias at 300 K. These devices were not operated at power outputs above −5 mW. Their lowest threshold was 220 mA.
Epitaxial Crystallization of Ge on Si Using Evaporation and Recrystallization Techniques
Authors: Lee El Hang , Awal M. Abdul , Celler G. K. , Pfeiffer L. , Sheng T. T. 
Published: March 2011

ABSTRACTGermanium films have been epitaxially crystallized on silicon substrates using the electron-beam evaporation technique and the laser! strip-heater recrystallization technique. Epitaxy was achieved either directly on Si surfaces or laterally on oxide/nitride-coated Si wafers. Evaporated Ge epi-layers were mirror-smooth and were found to be in excellent crystal quality. The recrystallized Ge on Si were also found to be good crystal, but showed somewhat roughened surface morphology. In both cases, the Ge near the Ge/Si interface is heavily replete with misfit dislocations arising from 4% lattice mismatch between Ge and Si. The recrystallized Ge on insulator (GOI) showed no mismatch dislocations at the Ge/insulator interface but developed voids arising from Ge dewetting from the insulator surface. Twins have been observed to be the most prominent defects in recrystallized Ge, whether seeded or unseeded. The objective of this study is to achieve quality Ge layers that can be used as intermediate template to accomodate GaAs on Si.
Initial Stages of Epitaxial Growth of GaAs on (100) Silicon
Authors: Biegelsen D. K. , Ponce F. A. , Smith A. J. , Tramontana J. C. 
Published: March 2011

ABSTRACTDirect observations of early stages of growth of GaAs on (100)Si are presented. Cross sectional TEM and plan view SEM images show three dimensional island growth, for growth above 300°C. Island size, island spacing, surface morphology and stacking fault defect spacing all decrease with substrate temperaturefor fixed Ga and As2 fluxes. Below 300C, 7nm thick films are uniform. Diffusion-controlled growth kinetics are inferred.
Observation of GaAs/Si Epitaxial Interfaces by Atomic Resolution Electron Microscopy
Authors: Otsuka N. , Choi C. , Nakamura Y. , Nagakura S. , Fischer R. , Peng C. K. , Morkoc H. 
Published: March 2011

ABSTRACTRecent studies have shown that high quality GaAs films can be grown by MBE on Si substrates whose surfaces are slightly tilted from the (100) plane. In order to investigate the effect of the tilting of substrate surfaces on the formation of threading dislocations, the GaAs/Si epitaxial interfaces have been observed with a 1 MB ultra-high vacuum, high voltage electron microscope. Two types of misfit dislocations, one with Burgers vectors parallel to the interface and the other with Burgers vectors inclined from the interface, were found in these epitaxial interfaces. The observation of crosssectional samples perpendicular to each other has shown that the tilting of the substrate surface directly influences the generation of these two types of misfit dislocations. The mechanism of the reduction of threading dislocations by the tilting of the substrate surface is discussed based on these observations.
Preferential Sputtering Effects in Elemental Depth Profiling of Tellurium-Based Alloy Thin Films
Authors: Ross R. C. , Jones G. L. , Chao S. S. , deNeufville J. P. , Bjornard E. J. , Tyler J. E. 
Published: March 2011

AbstractThe compositions of sputtered amorphous thin films of Se50Te50, Ge50Te50, and (Te90Ge5 In5)100-x 0 x=0-25 have been measured by electron microprobe x-ray analyzer (EMP) and Auger electron spectroscopy (AES) with sputter depth profiling. Preferential removal of certain elements from the surface region during sputtering is shown to significantly alter the AES-determined surface composition. The effects of sputtering ion mass (He1, Ar1, Xe1), Ar+ energy (1.5 and 4.0 keV), and target temperature (-100 and 25°C) were investigated. The degree of preferential sputtering is found to be controlled predominantly by differences in surface binding energy of the elements with secondary contributions in the case of 0 due to bombarding atom to target atom collisional energy transfer processes.
Strain Effects on Sensitization Development in 304 Stainless Steel as Measured by EPR and Stem/Eds Methods
Authors: Charlot L. A. , Arey B. W. , Bruemmer S. M. 
Published: March 2011

AbstractThe effect of uniaxial straining during isothermal heat treatment of a Type 304 stainless alloy has been examined using scanning transmission electron microscopy/energy dispersive x-ray spectroscopy (STEM/EDS) and electrochemical potentiokinetic reactivation (EPR) methods. Solution-annealed, flat tensile specimens were deformed at 600°C at various strain rates up to 6%/h. EPR and STEM/EDS results show an acceleration in sensitization development with increasing deformation. STEM/EDS measurement suggest that simultaneous straining affects both the kinetics of sensitization and the chromium concentrations established at grain boundaries.
Charge Collection Microscopy of Single Crystal and Polycrystalline GaAs
Authors: Paz O. , Borrego J. M. 
Published: March 2011

AbstractThis study demonstrates a new technique of measuring transport properties of minority carriers by varying the generation rate of the electron-beam induced carriers and measuring the amount of collected charge per incident electron. The technique is suitable. for measurements of very short diffusion length in bulk GaAs material and for measurements in polycrystalline, CVD grown GaAs layers containing grains that are smaller than the diffusion length.The diffusion length in single crystal GaAs measured at several values of electron-beam energy was.32μm. For l-2μm polycrystalline grains it was found that the diffusion length in the bulk of the. grain can remain high (8μm) but high recombination levels (up to 2 × 106 cm sec−1) prevail at the grain boundaries.
Fourier Transform Photoluminescence Analysis of Trace Impurities and Defects in Silicon
Authors: Duncan W. M. , Eastwood M. L. , Tsai H-L. 
Published: March 2011

AbstractIn this paper we describe Fourier transform Photoluminescence (FTPL) spectroscopy and report the first application of this technique to the analysis of semiconductor silicon. Two applications of FTPL spectroscopy to silicon are discussed. The first application is to quantitative trace analysis of the Ill-V impurities B,P, As and Al in silicon. Both high purity and compensated silicon have been studied and PL features correlated to the concentrations of the individual impurity species present. The capability of analysing B, P, As and Al at part per trillion atomic concentrations in silicon is demonstrated. In the second part of this paper, application of FTPL to the analysis of thermally induced microdefects in silicon is discussed. Defect band intensities are shown to be proportional to the concentration of interstitial oxygen precipitated in the case of plate precipitate morphology. In these two applications, FTPL is shown to uniquely provide both residual impurity and microdefect information of as grown and processed silicon.