bluuebunny/crossref_metadata_2025 huggingface dataset:
Dielectrics for GaN Based MIS-Diodes
Authors: Ren F. , Abernathy C. R. , MacKenzie J. D. , Gila B. P. , Pearton S. J. , Hong M. , Macos M. , Schurman M. J. , Baca A. G , Shul R. J.
Published: April 2011

GaN MIS diodes were demonstrated utilizing AIN and Ga2O3(Gd2O3) as insulators. A 345 Åof AIN was grown on the MOCVD grown n-GaN in a MOMBE system using trimethylamine alane as Al precursor and nitrogen generated from a wavemat ECR N2 plasma. For the Ga2O3(Gd2O3) growth, a multi-MBE chamber was used and a 195 Å oxide is E-beam evaporated from a single crystal source of Ga5Gd3O12. The forward breakdown voltage of AlN and Ga2O3( Gd2O3) diodes are 5V and 6V, respectively, which are significantly improved from ˜1.2 V of schottky contact. From the C-V measurements, both kinds of diodes showed good charge modulation from ccumulation to depletion at different frequencies. The insulator/GaN interface roughness and the thickness of the insulator were measured with x-ray reflectivity.
Heat Spreader Characteristics Of Multilayer Diamond Films For High Frequency Power Devices
Authors: Ramaswami B. , Jagannadham K.
Published: April 2011

Single layer diamond and multilayer diamond films consisting of diamond and aluminum nitride are deposited on molybdenum and silicon nitride substrates. Silicon or GaAs device wafers and the diamond substrates are prepared by metallization and bonded using gold-tin eutectic solder. Results of characterization of the bond by thermal cycling, scanning electron microscopy of the cross-section samples and X-ray mapping of the distribution of different elements are presented. Advantages of use of multilayer diamond composite heat spreaders in power semiconductor devices are discussed.
Photovoltaic Properties of Fullerene (C60) Thin films
Authors: Katz E. A. , Faiman D. , Goren S. , Shtutina S. , Shames A. , Mishori B. , Shapira Yoram
Published: April 2011

The crystalline structure, Electron Paramagnetic Resonance and Surface Photovoltage (SPV) spectra of C60 thin films and the photovoltaic properties of C60/Ag and C60/Si interfaces are reported. The SPV spectra of C60 films, C60/Ag and C60/Si interfaces are presented and analyzed on the basis of a model of C60 film electron structure including mobility gap, band tails extending into the gap and two deep level states in the gap. I-V characteristics of the C60/Ag and the C60/P-Si interfaces were measured. Both device structures are shown to exhibit rectifying behavior in the dark and photovoltaic properties. The solar cell parameters are presented.
Surface Modification of Cu(In,Ga)Se2 Thin Films During Aqueous Oxidation Etch
Authors: Canava B. , Guillemoles J. F. , Lincot D. , Vedel J. , Ardelean H. , Malengreau F.
Published: April 2011

The control of the barrier height at the Cu(In,Ga)Se2(CIGS)/CdS interface, via the chemical state of the CIGS surface, is a key to improve solar cell performances. In this paper chemical modifications have been achieved by the electrochemical method, by varying both the pH of the solution and the applied potential. It is shown that the potential for electrochemical oxidation depends on the pH, in good agreement with the thermodynamic predictions. The resulting surface composition of CIGS, analyzed by XPS, also depends on the pH. Contact Potential Differences (CPD) and Surface Photovoltage (SPV) measurements show that the electronic properties of the CIGS surface are modified. The important result is that the oxidation leads to the formation of permanent positive surface charges, which increase the band bending up to 0.20 V in a direction favorable for the solar cell performances.
Characterization of Cu(In,Ga)Se2/Mo Interface in CIGS Solar Cells
Authors: Nishiwaki S. , Kohara N. , Negami T. , Nishitani M. , Wada T.
Published: April 2011

The interface between a Cu(In,Ga)Se2 (CIGS) and an underlying Mo layer was studied by X-ray diffraction and high resolution transmission electron microscopy. The CIGS layer was deposited onto Mo coated soda-lime glass using the “3-stage” process. A MoSe2 layer found to form at the CIGS/Mo interface during the 2nd stage of the “3-stage” process. The thickness of the MoSe2 layer depended on the substrate temperature used for CIGS film deposition as well as the Na content of the CIGS and/or Mo layers. For higher substrate temperatures, thicker MoSe2 layers were observed. The Na in the CIGS and/or Mo layer is felt to assist in the formation of MoSe2. Current-Voltage measurements of the heterojunction formed by the CIGS/Mo interface were ohmic even at low temperature. The role of the MoSe2 layer in high efficiency CIGS solar cells is discussed.
Preparation of High-Quality CuInSe2 Thin Films by Molecular Beam Epitaxy
Authors: Tseng B. H. , Lin S. B. , Yang D. J.
Published: April 2011

CuInSe2 epitaxial films having superior luminescence properties were prepared by using (001) GaAs substrates and then by thermal annealing in the presence of a Se-beam flux. We also found that exciton emission became dominant when the film composition was very close to the stoichiometry.
In-Situ Investigation of Cu-In-Se Reactions by Thin Film Calorimetry
Authors: Wolf D. , Müller G.
Published: April 2011

Studies of the reaction path during annealing of Cu-In-Se thin films for solar cell absorbers have been limited up to now to ex-situ analyses of the phase composition by X-Ray Diffraction (XRD) after processing by a specific temperature-time program. As an indirect method, the application of ex-situ XRD is not sufficient for the determination of reaction temperatures and reaction times for setting up a general model of CIS-formation.

We show in this paper that the use of a calorimetric method (Thin Film Calorimetry, TFC) offers the advantage of a direct (in-situ) observation of thin film reactions. Special care is taken to use film thicknesses of practical interest for industrial application (1.5 – 3 μm). In a first step we show results of binary reactions in the Cu-In, In-Se and Cu-Se systems. Their knowledge is necessary for understanding the processes involved in the ternary CIS-layers. It turned out that thin Cu-In and Cu-Se films react already at room temperature and behave as predicted by the bulk equilibrium phase diagrams during heating. In-Se thin films show prominent exothermic reactions starting with the melting of In. The first phase to be formed is generally In2Se which is then converted to more Se-rich compounds. In ternary Cu-In-Se films (Cu/In = 1.00) we observe transitions of the Cu-Se-system which can be attributed to the decomposition of CuSe2 and CuSe. Consequences for the model of improved CIS-growth by a Cu-Se flux agent are discussed.
Several Efficiency Influencing Factors In CdTe/CdS Solar Cells
Authors: Li K. , Feng Z. C. , Wee A. T. S. , Chou H. C. , Lin J. Y. , Kamra S. , Tan K. L. , Rohatgi A. , Zhou L. , Li S. F. Y.
Published: April 2011

Several efficiency influencing factors in MOCVD-grown CdTe/CdS solar cells, including preferential crystal orientation of CdTe layers, CdTe grain size and surface roughness, interfacial mixing, and surface and interface geometrical morphology, are studied. X-ray diffraction (XRD) shows that polycrystalline CdTe/CdS solar cells with higher efficiencies tend to have more (111) planes of CdTe parallel to the macro-surface. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) analysis reveal the relationship between the grain size/surface roughness and cell efficiency. Secondary ion mass spectroscopy (SIMS) and Auger electron spectroscopy (AES) depth profiling show that the interfacial geometrical morphology has a significant influence on the efficiency of CdTe/CdS solar cells. Finally it is shown that interfacial mixing reduces the number of interfacial states and recombination centers and the energy loss due to internal reflectance, enhancing the performance of the solar cells.
The Perspectives Of High-Rate Low Frequency a-Si:H Films Deposition: Solar Cell Application And Stability Control
Authors: Budaguan B. G. , Aivazov A. A. , Meytin M. N. , Radosel'Sky A. G.
Published: April 2011

The perspectives for solar cell application of structural inhomogeneous a-Si:H films deposited at high growth rates (∼10–20 Å/s) from 100% SiH4 in low frequency (LF) 55kHz glow discharge plasma have been investigated. In this case the influence of structural inhomogeneity on dark dc and photoconductivities and light-induced defect generation kinetics (Staebler-Wronski effect, SWE) in a-Si:H films have been studied. The microstructure of films was investigated by IR spectroscopy analysis. Microstructural parameter R=[SiH2]/([;SiH]+[SiH2]), was used for the quantitative characterization of structural inhomogeneity in the material bulk.

It was found that Fermi level position is fixed by deep defect states and does not depend on microstructure parameter R. The comparative analysis of photoconductivity modeling and ESR measurements have shown that recombination in a-Si:H films is controlled by neutral dangling bonds and doesn't depend on parameter R. Meanwhile it was found that the kinetics of light-induced defect generation was controlled by SiH2 or clustered SiH groups content. Thus, the above results allow to perform an independent control of stability and electronic properties of a-Si:H films deposited in LF glow discharge plasma.
Grain Enhancement of Polycrystalline Silicon Films Aided by Optical Excitation
Authors: Sopori B. L. , Chen W. , Alleman J. , Matson R. , Ravindra N. M. , Tan T. Y.
Published: April 2011

A new technique for making large-grain thin Si films is described in which optical excitation is used to enhance the grain growth. Grain sizes much larger than the film thickness can be obtained at low temperatures and in short process times. This method is well suited for making thin-film Si solar cells on low-temperature substrates.
Index of Refraction and Strain Induced Birefringence of Pseudomorphic Si1−xGex
Authors: Mailhot S. , Baribeau J. M. , Bruce D. M. , Delâge A. , Janz S. , Jessop P. E. , Lafontaine H. , Robillard M. , Williams R. L. , Xu D. X.
Published: April 2011

The index of refraction of pseudomorphic layers grown on Si has been determined at wavelengths λ=1330 nm and λ= 1550 nm, for Ge concentrations between x=0.01 and x=0.1. The refractive index values were obtained from mode profile measurements on a series of Si1−xGex waveguides. The index of refraction is significantly larger for light polarized parallel to the growth direction than for light polarized in the plane of the epilayer. This birefringence is consistent with the anisotropic index change predicted using photoelelastic theory, given the biaxial strain present in the pseudomorphic Si1−xGex layers.
Strong Blue and Violet Light Emission from Silicon- and Germanium-Implanted Silicon-Dioxide Films
Authors: Rebohle L. , Tyschenko I. E. , Von Borany J. , Schmidt B. , Grötzschel R. , Markwitz A. , Yankov R. A. , Fröb H. , Skorupa W.
Published: April 2011

Photoluminescence (PL) and electroluminescence (EL) from Si+- and Ge+-implanted SiO2 layers thermally-grown on a Si substrate were studied. The PL spectra were recorded after annealing at temperatures in the range of 400° to 1200°C. Single-peak PL at 460 nm and doublepeak PL at 420 and 385 nm due to Si+-implanted and Ge+-implanted oxide layers, respectively, reached a maximum following heat treatment at 500°C for 30 min. The EL spectra from the Gerich oxides after annealing at 1000°C correlated very well with the PL signal, and showed a linear dependence on the injection current. The EL emission was strong enough to be readily seen with the naked eye, and the EL efficiency was estimated to be about 5×10−4. High-resolution transmission electron microscopy (HRTEM) analysis showed that the formation of nanocrystals in the implanted oxide layers occurred at annealing temperatures in excess of 800°C. The observed light emission is attributable to the presence of-≡Si-Si≡, ≡Si-Ge≡ and ≡Ge-Ge≡ centres in SiO2.
High Quality Germanium Photodiodes on Silicon Substrates Using an Intermediate Chemical Mechanical Polishing Step
Authors: Samavedam Srikanth B. , Currie Matthew T. , Langdo Thomas A. , Ting Steve M. , Fitzgerald Eugene A.
Published: April 2011

Germanium (Ge) photodiodes are capable of high quantum yields and can operate at gigahertz frequencies in the 1–1.6 μm wavelength regime. The compatibility of SiGe alloys with Si substrates makes Ge a natural choice for photodetectors in Si-based optoelectronics applications. The large lattice mismatch (≈4%) between Si and Ge, however, leads to the formation of a high density of misfit and associated threading dislocations when uniform Ge layers are grown on Si substrates. High quality Ge layers were grown on relaxed graded SiGe/Si layers by ultra-high vacuum chemical vapor deposition (UHVCVD). Typically, as the Ge concentration in the graded layers increases, strain fields from underlying misfit dislocations result in increased surface roughness and the formation of dislocation pile-ups. The generation of pile-ups increases the threading dislocation density in the relaxed layers. In this study the pileup formation was minimized by growing on miscut (001) substrates employing a chemical mechanical polishing (CMP) step within the epitaxial structure. Other problems such as the thermal mismatch between Si and Ge, results in unwanted residual tensile stresses and surface microcracks when the substrates are cooled from the growth temperature. Compressive strain has been incorporated into the graded layers to overcome the thermal mismatch problem, resulting in crack-free relaxed cubic Ge on Si at room temperature. The overall result of the CMP step and the growth modifications have eliminated dislocation pile-ups, decreased gas-phase nucleation of particles, and eliminated the increase in threading dislocation density that occurs when grading to Ge concentrations greater than 70% Ge. The threading dislocation density in the Ge layers determined through plan view transmission electron microscopy (TEM) and etch pit density (EPD) was found to be in the range of 2 × 106/cm2. Ge p-n diodes were fabricated to assess the electronic quality and prove the feasibility of high quality photodetectors on Si substrates.
Near Infrared Light Detectors Based on Uhvcvd Epitaxial Ge on Si (100)
Authors: Colace L. , Masini G. , Galluzzi F. , Assanto G. , Capellini G. , Di Gaspare L. , Palange E. , Evangelisti F.
Published: April 2011

In the present work we investigate photo-detectors based on thick relaxed Ge layers, epitaxially grown on silicon after insertion of a low-temperature-grown Ge buffer layer. By using this procedure it was possible to grow films having thicknesses comparable with light penetration depth in the 1.3–1.6μm spectral range. The films exhibited flatness on the atomic scale. Two kinds of detectors were investigated: vertical heterojunction diodes and a planar Metal-Semiconductor-Metal structure.
The detectors show a good responsivity at normal incidence at both 1.3 and 1.55μm. The photocurrent increases with the voltage applied, reaching a maximum responsivity of 0.24 A/W at 1.3μm under a bias of IV. A complete optoelectronic characterization of the fabricated devices is performed. The results confirm the feasibility of the proposed approach for the fabrication of 1.3–1.55μm near infrared photodetectors integrated on silicon chips.
Practical Nanoscale Silicon Light Emitters
Authors: Fauchet Philippe M.
Published: April 2011

Impressive advances have been made over the last few years in teaching silicon how to emit light. Recently, light-emitting devices made of porous silicon and other forms of nanoscale silicon have been demonstrated with specifications that start to make them attractive for commercial applications. This paper reviews the state-of-the-art in the materials science and device properties of nanoscale silicon-based LEDs, including their integration with microelectronic circuits.
Structural Band Gap Engineering
Authors: Demkov Alexander A. , Sankey Otto F. , Fuentes M.
Published: April 2011

In this paper we discuss a novel approach to the band gap engineering of semiconductors Si, Ge, GaAs and AIN. We suggest that nanoporous polymorphs of these materials may exist which would offer a significant variation of the electronic band gap. Structurally, nanoporous semiconductors are related to the zeolitic nets, and a systematic procedure of generating these structures from the (4;2) nets is described. We use the ab initio total energy quantum molecular dynamics method Fireball96 to investigate the energetics and the electronic properties of these nanoporous or “expanded” semiconductor phases.
Study of the Si/ZnS Multilayer System for Optoelectronic Applications
Authors: Voon L. C. Lew Yan
Published: April 2011

The suitability of the (001) Si/ZnS system for optoelectronic applications is theoretically investigated and compared to the Si/Ge system. The band-structure calculations show that Si/ZnS is superior to Si/Ge in a number of respects: directness of band gap can be easily obtained; and optical intersubband transitions fall within the important 1.3–1.6 μm window.
Membrane Curvature Induced by Sugar and Polymer Solutions
Authors: Döbereiner H.-G. , Lehmann A. , Goedel W. , Selchow O. , Lipowsky R.
Published: April 2011

We monitor the effect of transversal membrane asymmetry on the morphology of giant uni-lamellar vesicles in sugar and polymer solutions. The shapes of fluid lipid vesicles are governed by the bending elasticity of their membrane which is characterized by the bending modulus and the spontaneous curvature of the bilayer. We present a recently developed technique for the measurement of the spontaneous curvature using quantitative phase contrast microscopy. Different mechanisms for elastic membrane asymmetry and the role of the bending energy concept for the morphology of cellular organelles are discussed.
Synthesis and Characterization of Supported Bioactive Lipid Membranes
Authors: Winger Theodore M. , Chaikof Elliot L.
Published: April 2011

Single and multicomponent membrane-mimetic surfaces of DPPC and synthetic lipid-peptide conjugates were formed on an alkylated glass surface by a process of vesicle fusion. Correlative atomic force microscopy and radiochemical titration techniques confirmed the generation of a single substrate supported monolayer and predictable deposition of defined concentrations of lipopeptide. Mixed systems were stable for periods exceeding 1 month if stored at room temperature in phosphate buffered saline.
Fine-Scale Oscillatory Banding in Otoliths from Arctic Charr (Salvelinus Alpinus) and Pike (Esox Lucius)
Authors: Meldrum A. , Haldent N. M.
Published: April 2011

Transmission electron microscopy of otoliths from the inner ear of arctic charr and pike has revealed the presence of fine banding on the scale of several nanometers. The thickness of the bands was observed to vary in different portions of the sample, and some areas were not banded. EDS analysis could not detect chemical differences within the bands, but electron diffraction showed that the crystallographic orientation of the bands is related by a lattice mismatch indicative of a twinning relationship. Previously, banding on the scale of 50 to 100 microns was observed by SEM in otoliths from arctic charr and was attributed to seasonal variations in growth. The fine-scale twinning observed in this study, however, is unlikely to be nucleated through daily variations. Electron diffraction from the pike samples shows that the material is composed of CaCO3 having the both the vaterite and aragonite structure, and hydrous CaCO3 was also observed.

The interaction of the electron beam with the sample material was investigated by conducting several electron-irradiation experiments. The electron beam was observed to interact strongly with the sample and caused the precipitation of cubic CaO from the calcium carbonate matrix. Bright-field imaging showed the development of fine grained (∼ 5 nm) randomly-oriented crystallites which accumulated with increasing electron dose. These initial results suggest that the precipitation of GaO is not driven by electron-beam heating. Previously, a similar phase-change phenomenon has been observed in hydroxyapatite from dental enamel. Other Ca-bearing biominerals may therefore also be expected to be sensitive to electron irradiation.
A Model For Diffusion and Competition in Cancer Growth and Metastasis
Authors: Pescarmona G. P. , Scalerandi M. , Delsanto P. P. , Condat C. A.
Published: April 2011

A master equation formalism is used to model the growth and metastasis of a tumor as a function of the diffusion and absorption of a nutrient. Healthy and cancerous (C-) cells compete to bind the nutrient, which is allowed to diffuse starting from a prescribed region. Two thresholds are defined for the quantity of nutrient bound by the C-cells. If this quantity falls below the lower threshold, the cell dies, while if it increases above the upper threshold, the cell divides according to a predefined stochastic mechanism. C-cells migrate when they record a low concentration of free nutrient in the local environment. The model is formulated in terms of a coupled system of equations for the cell populations and the free and bound nutrient. This system can be solved by using the Local Interaction Simulation Approach (LISA), a numerical procedure that permits an efficient and detailed solution and is easily adaptable to parallel processing. With suitable parameter variation, the model can describe multiple tumor configurations, ranging from the classical spheroid with a necrotic core favored by mathematicians to very anisotropic shapes with inhomogeneous concentrations of the various populations. This is important because the nature of the anisotropy may be crucial in determining whether and how the cancer metastasizes. The effects of stochasticity and the presence of additional nutrients or inhibitors can be easily incorporated.
Tactoidal Granules in Concentrated Actin Gels: A Solidlike State of Protein Filaments
Authors: Tang Jay. X. , Oldenbourg Rudolf , Allen Philip G. , Janmey Paul A.
Published: April 2011

We report here the first observation and microscopic characterization of tactoidal granules of actin in concentrated gels of actin filaments (F-actin). Phase contrast microscopy shows these stable tactoids of densely packed F-actin to be of various sizes on the order of 10 μm. The background gel of F-actin is optically birefringent, indicating orientational order of the filaments consistent with theoretical predictions. In contrast, no birefringence is detected through the tactoids, suggesting very distinct yet undetermined packing of the filaments inside. The tactoids demonstrate elastic response upon micromanipulation. The microscopic segregation of F-actin into two states of different protein concentrations is consistent with a first order phase transition between a nematic gel of lower concentration on the order of ten mg/nl and a highly packed, possibly columnar state of protein filaments. In addition to the excluded volume effects which are known to drive thermodynamic phase transitions as a function of particle concentration, additional molecular forces must also play important roles since the formation was found to be irreversible upon dilution.
Computer Simulation of Thermomigration Process
Authors: Gershanov V. Yu. , Garmashov S. I. , Minyaev A. R. , Beletskaya A. V.
Published: April 2011

Mathematical model of thermomigration of liquid inclusions through a crystal under stationary and non-stationary thermal conditions is presented. It is assumed that the mass-transfer is provided by diffusion only, the crystallization and dissolution processes are carried out in accordance with the diffuse interface mechanism for atomic-rough (non-singular) interfaces and screw-dislocation or two-dimension nucleation mechanisms for singular interfaces.

The package of computer programs based on this model enables simulation of the evolution of the cross-sectional shape of cylindrical liquid inclusions. It is possible to simulate the cases of various inclusion sizes, various relationship between the interface and volume mass-transfer restrictions, various liquid phase composition, thermal gradient under stationary and non-stationary thermal conditions as well. The main results of the simulation are presented.
Modelling Analysis of Oxygen Transport During Czochralski Growth of Silicon Crystals
Authors: Egorov Yu. E. , Makarov Yu. N. , Rudinsky E. A. , Smirnov E. M. , Zhmakin A. I.
Published: April 2011

Global model of oxygen transport in the melt and inert gas is proposed which is based on coupled two-dimensional stationary modeling of global heat transfer, laminar flow of inert gas and turbulent flow in Si melt, and transport of dissolved oxygen in the melt and SiO vapor in the gas. The model allows to study effects of process parameters like flow rate and pressure of inert gas, and rotation of the crucible on incorporation of oxygen into Si crystal.
Demonstration of Improved Quantitative Mobility Spectrum Analysis (i-QMSA)
Authors: Vurgaftman I. , Meyer J. R. , Hoffman C. A. , Redfern D. , Antoszewski J. , Faraone L. , Lindemuth J. R.
Published: April 2011

We discuss an improved quantitative mobility spectrum analysis (i-QMSA) of magnetic-field-dependent Hall and resistivity data, which can determine multiple electron and hole densities and mobilities. A fully automated computer implementation of i-QMSA is applied to a variety of synthetic and real data sets. The results show that the new algorithm increases the information available from a given data set and is suitable for use as a standard tool in the characterization of semiconductor materials and devices.
Tight-binding calculations of electronic structure and resistivity of liquid and amorphous metals
Authors: Bose S. K.
Published: April 2011

We discuss various aspects of calculating the electronic structure of liquid and amorphous metals using the recursion method and the tight-binding linear muffin-tin orbitals (TB-LMTO) basis. Resistivity calculations for such systems based on the Kubo-Greenwood formula and the TB-LMTO-recursion method are presented and compared with similar calculations based on the linear combination of atomic and atomic-like orbitals (LCAO) and the chemical pseudopotential approach. Results for amorphous Fe and Co and liquid Hg, Pd, and some 3d transition metals are presented. Sources of error in the calculation and ways to improve upon the present calculations are discussed.
A Selfconsistent-Charge Density-Functional Tight-Binding Scheme
Authors: Elstner M. , Porezag D. , Jungnickel G. , Frauenheim Th. , Suhai S. , Seifert G.
Published: April 2011

We present an extension to the tight-binding (TB) approach to improve total energies, forces and transferability in the presence of considerable long-range Coulomb interactions. We derive an approximate energy expression in terms of charge density fluctuations δn at a reference (input) density n0, which is a second order approximation to the total energy expression in density functional theory (DFT). With the choice of n0 as a superposition of densities of neutral atomic fragments, we can define a repulsive potential as in standard TB theory, which is pairwise, short ranged and transferable. The zero order terms in the total energy expression are recoverd as the standard terms of our density-functional based tight-binding (DF-TB). For the second order terms, the charge density fluctuations δn are approximated by the total charge fluctuation Δqα at atom α, which is qualitatively estimated by employing the Mullikan charge analysis. Within this approximations the total energy expression contains new parameters, which are related to ab-intio DFT calculations. Finally, by introducing localized basis functions and applying the variational principle we arrive at the Hamilton matrix elements, wich themselves depend on the charge fluctuations and, therefore, the general eigenvalue problem has to be solved self-consistently. To obtain forces for efficient geometry relaxation and molecular-dyamics, we calculated analytical derivatives of the total energy with respect to the atomic sites. In order to demonstrate the strenghts of our self-consistent-charge tight-binding (SCC-TB), we calculated reaction energies, geometries and vibrational frequencies for a large set of molecules and compare the results to semi-empirical methods, density-functional calculations and experiment.
Tight-Binding Hamiltonians for Carbon and Silicon
Authors: Papaconstantopoulos D. A. , Mehl M. J. , Erwin S. C. , Pederson M. R.
Published: April 2011

We demonstrate that our tight-binding method - which is based on fitting the energy bands and the total energy of first-principles calculations as a function of volume - can be easily extended to accurately describe carbon and silicon. We present equations of state that give the correct energy ordering between structures. We also show that quantities that were not fitted, such as elastic constants and the band structure of C60, can be reliably obtained from our scheme.
Negative Cauchy Pressure within the Tight-Binding Approximation
Authors: Nguyen-Maxh D. , Pettifor D. G. , Znam S. , Vitek V.
Published: April 2011

It is well-known that the Embedded Atom Method (EAM) predicts positive Cauchy pressures for cubic metals if physically-motivated embedding functions are used. Supris-ingly, even if the angular character of the covalent bonding is included within an orthorgonal or non-orthorgonal Tight-Binding (TB) description, the Cauchy pressure for most elemental and binary metallic systems remains positive. We describe the results of a detailed breakdown of the different contributions to the Cauchy pressure within the Harris-Foulkes approximation (HFA) to density functional theory. We show that negative values of the Cauchy pressure for both elemental transition metals such as Ir and binary intermetallics such as Ti3Al, TiAl and TiAl3 are well reproduced by the HFA. We argue that the negative Cauchy pressure (NCP) arises namely from the environment dependence of the local TB orbitals which leads to both environment-dependent bonding integrals and overlap repulsion. We discuss a particular functional form for overlap repulsion which leads to NCP and compare it with different fitting schemes proposed recently in TB theory.
Large Scale Atomistic Simulations using the Tight Binding Approach
Authors: Celino M. , Cleri F. , Colombo L. , Rosati M. , Rosato V. , Tilson J.
Published: April 2011

Atomistic modelling of Materials Science problems often requires the simulation of systems with an irreducibly-large unit cell, such as amorphous materials, fullerites, or systems containing extended defects, such as dislocations, cracks or grain boundaries. Large-scale simulations with the Tight-Binding approach must face the computational obstacle represented by the O(N3)-scaling of the diagonalization of the Hamiltonian matrix. This bottleneck can be overcome by parallel computing techniques and/or the introduction of faster, O(N)-scaling algorithms. We report the activities performed in the frame of a collaboration among several research groups on the porting of TBMD codes on parallel computers. In particular, we describe the porting of a O(N3) TBMD code on different MIMD computers, with either distributed or shared memory, by using appropriate software tools. Furthermore, preliminary results obtained in the porting of an O(N) TBMD code on an experimental, hybrid MIMD-SIMD computer architecture are reported. The new perspective of using specialized platforms to deal with large-scale TBMD simulation is discussed.
Elasticity, Thermal Properties, and Molecular Dynamics Using Non-Empirical Tight-Binding
Authors: Cohen Ronald E. , Stixrude Lars , Wasserman Evgeny
Published: April 2011

We have further developed and applied a new non-empirical tight-binding total energy model to properties of Si, Xe, and Fe at high pressures. We have studied elasticity of various phases of each of these, demonstrating that the new model is applicable to a wide range of materials, including semiconductors, rare gases, and transition metals. We have used the particle-in-a-cell method to study the thermal equation of state of hep Fe and find excellent agreement with the shock equation of state. A molecular dynamics code has been developed based on this method, and we have studied the properties of Fe liquid at high pressures.
Structural Disorder and Localized Gap States in Silicon Grain Boundaries from a Tight-Binding Model
Authors: Cleri F. , Keblinski P. , Colombo L. , Phillpot S. R. , Wolf D.
Published: April 2011

Tight-binding molecular dynamics simulations of typical high-energy grain boundaries in silicon show that the atomic structure of the interface in thermodynamic equilibrium is similar to that of bulk amorphous silicon and contains coordination defects. The corresponding electronic structure is also amorphous-like, displaying extra states in the forbidden gap mainly localized around the coordination defects, where large changes in the bond-hybridization character are observed. It is proposed that such coordination defects in disordered high-energy grain boundaries are responsible for the experimentally observed gap states in polycrystalline Si.
Microstructure Characterization of Ceramics by Fractal Geometry
Authors: Xiong Z. X. , Baba-Kishi K. Z. , Shin F. G.
Published: April 2011

Following Mandelbrot's fractal theory, the irregular characteristics of the microstructural features of ferroelectric Pb(Sc0.5 Ta0.5)O3 ceramics, including grain boundaries and dislocation networks, were investigated. The microstructural features were imaged by electron microscopy. The fractal analyses were carried out manually and by image processing techniques, which show the value of the fractal dimension, D, varies according to the regularity of the microstructure. The value of, D, close to unity is an indication of an increasing degree of microstructural regularity, which is in good agreement with the simulated results.
A B.F.S. Method Survey of Surface and Interfacial Properties of Multicomponent Metallic Systems
Authors: Bozzolo G. , Ferrante J. , Good B.
Published: April 2011

Computer modeling of atomic processes on surfaces and bulk materials has seen noticeable growth due to the availability of new, powerful semiempirical techniques that provide both the numerical simplicity, physical foundation and computational efficiency necessary for the study of complex systems. In an attempt to partially summarize the current status of this work, we apply one of these techniques, the BFS method for alloys, to the study of several basic topics in surface structure and analysis: surface energies of alloy surfaces, multilayer relaxation of ordered alloy surfaces, temperature-dependent segregation profiles, surface alloying and thin film growth of multicompo-nent systems.
First-Principles Simulations and Z-Contrast Imaging of Impurities at <001> Tilt Grain Boundaries in Mgo
Authors: Yan Y. , Chisholm M. F. , Duscher G. , Pennycook S. J. , Maiti A. , Pantelides S. T.
Published: April 2011

First-principles density-functional calculations were used to study the effects of Ca impurities on the Σ=5 (310) <001> tilt grain boundaries in MgO. An equilibrium structure and two metastable structures of the grain boundaries in pure MgO have been established. The calculations further demonstrated that Ca impurities segregate at particular sites in the metastable grain boundary and induce a structural transformation. This result is consistent with atomic resolution Z-contrast imaging. The calculations also found that the impurities at the grain boundaries do not induce states in the band gap. The mechanism of the transformation is also discussed.
First-Principles Study of Interfacial Reaction Atomic Process at Silicon Oxidation
Authors: Kageshima H. , Shiraishi K.
Published: April 2011

A theoretical atomic process model of the interfacial oxidation reaction on a silicon substrate is proposed based on first-principles calculation. In the calculation, H-terminated Si(100) surfaces are used for the first approximation of the silicon-oxide/silicon interfaces. The proposed atomic process model is based on the plausible assumption that the remaining stress in the oxidized region is kept at a minimum and does not break the grown Si-O-Si network when the oxidation proceeds. As a natural consequence, Si atoms are emitted from the interface as the oxidation proceeds to release the stress due to substitution of Si-Si bonds by Si-O-Si bonds. This emission is consistent with well-known experiments of oxidation-induced stacking faults and oxidation-enhanced diffusion. Our model presents a microscopic approach for understanding the silicon oxidation process, whereas the widely accepted Deal-Grove model presents a macroscopic one.
Etching Process of Al Oxide on Si Surface with HF Treatment
Authors: Hoshino T. , Nishioka Y.
Published: April 2011

ab initio theoretical calculations have been performed to reveal the etching mechanism of HF with Al metal oxide on Si surfaces. The following sequential process has been revealed to be the most probable reaction path. HF + AlO-Si surface → HO-Si surface + AlF (1), HF + HO-Si surface → F-Si surface + H2O (1'). Both of the reactions have potential energy barriers of 0.8∼1.0eV, and are exothermic by 0.3 and l. leV, respectively. Another reaction path that proceeds with one-step process, HF + AlO-Si surface → F-Si surface + AlOH (2), is also found to be probable for AlO etching.
Density Functional Theory for Studies of Multiple States of Inhomogeneous Fluids at Solid Surfaces and in Pores
Authors: Neimark A. V. , Ravikovitch P. I.
Published: April 2011

Two new versions of the density functional theory (DFT), which correspond to the canonical ensemble and the Gibbs ensemble respectively, are applied to study multiple equilibrium states and associated hysteresis on examples of capillary condensation of argon in nanopores of MCM-41 mesoporous molecular sieves and Kr adsorption on carbon. The canonical ensemble DFT (CEDFT) allows us to trace not only the stable and the metastable states along the hysteresis loop, but also the unstable states inside the hysteresis loop. The Gibbs ensemble DFT (GEDFT) allows us to determine precisely the points of equilibrium phase transitions in confined fluids.
Melting, Freezing, Diffusion, and Colescence of Gold Nanoclusters
Authors: Lewis Laurent J. , Deltour Pierre , Jensen Pablo , Barrat Jean-Louis
Published: April 2011

We present a detailed molecular-dynamics study of the coalescence of gold nanoclusters within the framework of the embedded-atom method. We find macroscopic sintering theories to be unable to describe the coalescing behavior of two small clusters, a failure we attribute to the fact that the nanocrystals are faceted; this has important consequences for the morphology of cluster-assembled materials. We also examine the static and dynamic properties of a 249-atom gold cluster on nickel surfaces.
Effect of Impurities on Σ3 (111) Grain-Boundary Fracture in Tungsten—Atomistic Simulation
Authors: Grujicic M. , Zhao H. , Krasko Genrich L.
Published: April 2011

The effect of various impurities and micro-alloying additions (B, N, C, O, Al, Si, S and P) on the intrinsic resistance of the Σ3 (111) GB in Tungsten has been investigated using Molecular Dynamics simulation. The atomic interactions have been accounted for through the use of the Finnis-Sinclair interatomic potentials. The fracture resistance of the GB has been characterized by computing, in each case, the ideal work of GB separation, the Mode I stress intensity factor and the Eshelby F - conservation integral at the onset of crack propagation. The results obtained suggest that pure Tungsten is relatively resistant toward GB decohesion and that this resistance is further enhanced by the presence of B, C and N. Elements such as O, Al and Si on the other hand, have a relatively minor effect on the cohesion strength of the Σ3 (111) GB. In sharp contrast, S and P greatly reduce this strength making Tungsten quite brittle. These findings have been correlated with the effect of the impurity atoms on material evolution at the crack tip.
Molecular Dynamics Studies of Thin-Films of Sn On Cu
Authors: Ravelo R. , Aguilar J. , Baskes M. I.
Published: April 2011

Using Molecular Dynamics, the evolution dynamics of Sn on the (111) and (100) surfaces of Cu have been investigated as a function of coverage and temperature. The interaction potentials are described by modified embedded atom method (MEAM) potentials. The calculated diffusion activation energies of Cu in Sn and Sn in Cu agree reasonably well with experimental values. We find that the structure of the overlayer depends on the morphology of the substrate and remains stable up to temperatures of the order of 70% of the melting temperature of the substrate at which diffusion of Sn into the substrate and Cu atoms onto the overlayer is observed.
Computer Simulation of Interfaces in Ceramics
Authors: Harding J. H. , Harris D. J. , Parker S. C.
Published: April 2011

The methods for computer simulation of grain boundaries and hetero-interfaces in ceramics have long been available. We review developments in the field in two areas; the study of general grain boundaries and the simulation of hetero-interfaces. In both cases we discuss extrapolation from the symmetric case. We will also consider the validity of the continued use of classical potential models in an age of large-scale quantum calculations.
Effect of Domain Structure on the Magnetoresistance of Epitaxial Thin Films of Ferromagnetic Metallic Oxide SrRuO3
Authors: Rao R. A. , Kacedon D. B. , Eom C. B.
Published: April 2011

We have grown epitaxial ferromagnetic metallic oxide SrRuO3 thin films with different domain structures on (001) LaAlO3 and miscut (001) SrTiO3 substrates. The effect of crystallographic domain structures on the magnetization and magnetoresistive behavior of epitaxial SrRuO3 thin films has been studied. Magnetization measurements on the single domain film on 2° miscut (001) SrTiO3 substrate showed that the in-plane [110] direction, which is aligned along the miscut direction, is the easier axis for magnetization compared to the [001] direction. This film also showed a strong anisotropie magnetoresistance (AMR) effect of ∼ 8% in magnitude. In contrast, the SrRuO3 thin film on (001) LaAlO3 substrate shows identical magnetization and magnetoresistance behavior in two orthogonal directions on the film due to the presence of 90 domains in the plane. For both the films, large negative magnetoresistance effects (-10%) were observed when the current and the applied magnetic field are parallel. The magnetoresistance behavior is explained in terms of suppression of spin fluctuations near Tc and the AMR effect.
Formation of Ferromagnetic/Ferroelectric Superlattices by a Laser MBE and Their Electric & Magnetic Properties
Authors: Tabata Hitoshi , Ueda Kenji , Kawai Tomoji
Published: April 2011

We have constructed artificial superlattices with a combination of magnetic/magnetic and ferroelectric/ferromagnetic materials using a laser ablation technique. An ideal hetero-epitaxy can be obtained due to the similar crystal structure of the perovskite type di/ferroelectric BaTiO3, Pb(Zr,Ti)O3 (so-called PZT), SrTiO3 and ferro/antiferromagnetic LaFeO3, LaCrO3, (La,Sr)MnO3. First of all, we have controlled ferromagnetic order on LaFeO3/LaCrO3 superlattices formed on SrTiO3(l11) substrate. Such a spin structure(ferromagnetic order) can't be got in bulk condition. In the heterostructured ferromagnetic /ferroelectric devices, (La,Sr)MnO3/PZT there are remarkable and interesting phenomena. The electric properties of the ferromagnetic material can be controlled by the piezoelectric effect via distortion of the crystal structure.
Evaluation of Raman Scattering in La1-xMxMnO3 Single Crystals Due to Structural and Magnetic Transitions
Authors: Podobedovm V. B. , Weber A. , Romero D. B. , Rice J. P. , Drew H. D.
Published: April 2011

The study of doped and undoped La1-xMxMnO3 single crystals (x = 0 to x = 0.3) in the temperature range from 5 K to 423 K is reported. The activity of phonon vibrational modes in optical spectra is analyzed and a comparison to experimental Raman data is presented. Different contributions from optical phonons to the Raman scattering process are discussed. It is shown that both the value of doping and temperature have a significant influence on Raman features, and these are strongly affected by structural and magnetic phase transitions. Due to the symmetry present in distorted lanthanum manganese compounds their Raman spectra were found to contain comparable contributions from first- and second-order phonon scattering. The common features related to both the reduction of the Jahn-Teller (JT) distortion and the temperature-induced phase transition were found in the Raman spectra of La1-xMxMnO3 crystals. An anomalous behavior of the low-frequency Raman mode in doped systems was found and explained in terms of the temperature-dependent ionic radius of the La/Sr ion sites. It was found that the strong dependence of the Ag Raman mode on the value of doping may serve as a useful tool for optical characterization of La1-xMxMnO3 compounds, including the inhomogeneity of the surface.
Lattice deformation and magnetic properties in epitaxial thin films of Sr1-xBaxRuO3
Authors: Fukushima Noburu , Sano Kenya , Schimizu Tatsuo , Abe Kazuhide , Komatsu Shuichi
Published: April 2011

The crystal structure and magnetic properties in epitaxially grown Sr1-xBaxRuO3 on SrTiO3 substrates were determined. Epitaxial Sr1-xBaxRuO3 exhibits a simple perovskite structure in the whole region of Ba/Sr ratio, in contrast to a complex hexagonal layered perovskite in the case of Ba-rich bulk Sr1-xBaxRuO3 which has plane-sharing oxygen octahedra. Tetragonal deformation was enhanced from the pseudo cubic structure of SrRuO3 to a highly distorted tetragonal lattice in BaRu03. Electronic properties such as conductivity and magnetization were examined and compared to the results of band calculation in which a tetragonal distortion was taken into account. A metal-insulator transition was not observed in this system either in the experiment or in the simulation, and metallic conductivity was maintained in the whole region of Ba content. The ferromagnetic ordering which occurs at 160K in pseudo-cubic bulk SrRuO3 was suppressed in Sr1-xBaxRuO3 films with increasing tetragonal deformation and Curie temperature decreased to 50K in BaRuO3.
Resonant X-Ray Fluorescence Spectroscopy at The VL-Edges of Vanadium Oxides
Authors: Duda L.-C. , Stagarescu C. B. , Downes J. E. , Smith K. E. , Dräger G.
Published: April 2011

We have studied resonant VLα-fluorescence spectra of vanadium oxides with V in several different oxidation states. The spectra are dominated by the O 2p-contribution centered at about 6 eV below the top of the valence band (VB-top). The V 3d-contribution, found close to the VB-top, increases with decreasing valency of the vanadium atoms. Resonant inelastic (Raman) x-ray scattering is fairly weak in these compounds and overlaps with the ordinary fluorescence spectrum. Large spectral changes of VLα-fluorescence in the metal-insulator transition of V2O3have been observed.
Phase Stability of The Mixed-Conducting Sr-Fe-Co-O System
Authors: Ma B. , Balachandran U. , Hodges J. P. , Jorgensen J. D. , Miller D. J. , Richardson J. W.
Published: April 2011

Mixed-conducting ceramic oxides have potential uses in high-temperature electrochemical applications such as solid oxide fuel cells, batteries, sensors, and oxygen-permeable membranes. The Sr-Fe-Co-O system combines high electronic/ionic conductivity with appreciable oxygen permeability at elevated temperatures. Dense ceramic membranes made of this material can be used to separate high-purity oxygen from air without the need for external electrical circuitry, or to partially oxidize methane to produce syngas. Samples of Sr2Fe3-xCoxOy (with x = 0,0.6,1.0, and 1.4) were prepared by solid-state reaction method in atmospheres with various oxygen partial pressures (pO2) and were characterized by X-ray diffraction, scanning electron microscopy, and electrical conductivity testing. Phase components of the sample are dependent on cobalt concentration and pO2. Electrical conductivity increases with increasing temperature and cobalt content in the material.