bluuebunny/crossref_metadata_2025 huggingface dataset:
Performances Of Porous Silicon Optical Waveguides
Authors: Araki M. , Takahashi M. , Koyama H. , Koshida N.
Published: April 2011

To verify the availability of a porous silicon (PS) technology for applications to photonic devices, fundamental chracteristics of PS waveguides are investigated. An edge-emitting device with an optical waveguide is fabricated by current modulation technique during anodization. The net internal loss for prepared samples is measured by varying the length of excitation beam. The experimental results show that the internal loss increases with decreasing the operation wavelength, possibly due to self-absorption and Rayleigh scattering. Besides the step-index PS waveguides, buried-type waveguides can be fabricated based on a doping modulation technique. A theoretical analysis about a bending loss is also carried out. The results of experimental and theoretical analyses suggest that a curveture radius capable of guiding lightwave can be significantly minimized, owing to a difinitely different refractive indedices between the core and cladding region.
Controllability And Homogeneity Of Optical Properties Of Thin Porous Silicon Films
Authors: Uehara S. , Kubo T. , Ogata S. , Sato T. , Hosono J. , Matsubara T.
Published: April 2011

Reflection spectroscopy was applied to evaluate the optical homogeneity and the refractive index of thin porous silicon (PS) layer. Variation in PS layer optical thickness was evaluated by measuring and mapping the reflectance over the surface area. For the circular anodization area of 17 mm diameter, the measured variation was less than 5 % on an area of 11 mm diameter. The anodization electrode position was found to have little influence on the homogeneity. A method to derive refractive index from a reflection spectrum is studied. The wavelength dependence of PS index measured by the method showed fair coincidence with the calculation based on effective medium approximation. In the two layer PS formation, optical thickness was found to change whether high porosity or low porosity layer is formed first.
Blue, Yellow-Green and Red Simultaneous Emission from SiO2 Matrices Co-Implanted with Si and C
Authors: López M. , Garrido B. , Bonafos C. , González-Varona O. , Pérez-Rodríguez A. , Rodríguez R. , Ruterana P. , Morante J. .R.
Published: April 2011

Dual Si+C implantations at high doses (3×1017 cm−2) were performed in thermal SiO2 layers. The C implantation energy was fixed at 60 keV while the Si ones were performed at different energies (150 keV and 60 keV) with the aim of synthetizing carbon rich luminescent centers. Annealings up to 1100 °C were performed in an inert N2 atmosphere either for short times in a rapid thermal processing furnace and up to two hours in a conventional furnace. Evidence of the formation of silicon and carbon-rich clusters was provided by XPS, SIMS, Raman and TEM experiments. Photoluminescence experiments under blue and ultraviolet excitations showed intense emission bands peaking at 1.5–1.7 eV (red), 2.20 eV (yellowgreen) and 2.75 eV (blue). The relative intensity and spectral shape of the emission bands was found to depend strongly upon implantation and annealing conditions and offered the possibility of tuning the desired dominant emission wavelength. The possible formation of SiC clusters and even SiC crystallites at the SiO2/Si interface could explain the origin of the highly intense blue emission band found for these samples.
Control of Sizes and Optical Emission of Sige Quantum Dots Prepared on Ordered Mesoporous Silica Coated Si Wafer
Authors: Tang Y. S. , Cai S. , Wang D. , Jin G. , Duan J. , Wang K. L. , Soyez H. M. , Dunn B. S.
Published: April 2011

A new way of preparing wafer sized SiGe and Ge quantum dots at extremely low cost is presented. The results show that two different controlled nanometer sizes of the quantum dots can be formed simultaneously into two layers on the same wafer with good dot size uniformity. Our initial experiments on SiGe and Ge system suggest that it is possible to squeeze the SiGe dots for much improved optical emission. The advantages of this dot preparation method are its fully compatibility with the Sitechnology, its simplicity in dot preparation and extremely low cost.
Luminescent Amorphous Silicon Layers
Authors: Allan G. , Delerue C. , Lannoo M.
Published: April 2011

The electronic structure of amorphous silicon layers has been calculated within the empirical tight binding approximation using the Wooten-Winer-Weaire atomic structure model. We predict an important blue shift due to the confinement for layer thickness below 3 nm and we compare with crystalline silicon layers. The radiative recombination rate is enhanced by the disorder and the confinement but remains quite small. The comparison of our results with experimental results shows that the density of defects and localized states in the studied samples must be quite small.
Effect Of High Concentration Of Defect States At PS/c-Si Heterointerface On Transport Properties Of Al/PS/c-Si Photodiode Structures
Authors: Balagurov L. A. , Orlov A. F. , Petrova E. A. , Yarkin D. G. , Andryushin S. Ya.
Published: April 2011

Current-voltage characteristics, photosensitivity, frequency dependence of impedance, and electron beam induced current were measured for Al/porous silicon/c-Si structures with porous silicon (PS) layers of 60 and 80% porosity. Al/PS/c-Si structures have photosensitivity spectra typical for Al/c-Si structures. The sign of open circuit voltage changes in these structures when the wavelength becomes less than approximately 0.4 μm. It was shown that photosensitivity of Al/PS/c-Si structures and impedance dependence on the frequency, reverse bias and thickness of PS layer are completely determined by the PS layer of high resistivity and by the space charge region in c-Si substrate on PS/c-Si heterojunction. The annealing at above 100° C leads to a decrease of the forward current at a small bias. The photosensitivity of annealed structures reaches 10 A/W for some of the structures. The experimental results lead to inevitable conclusion about opposite band bending from the two sides of the heterojunction which is caused by high concentration of charged defects at PS/c-Si heterointerface. PS photodetectors with improved device parameter were obtained.
The Properties of Silicon Clusters in Zeolite
Authors: Muramatsu Shinya , Hirao Masahiko
Published: April 2011

Nanometer-sized silicon clusters show photoluminescence, but some technique to embed them in bulk materials is needed to enable practical device applications. Thus we calculated the properties of silicon clusters in zeolite, whose framework constitutes cages as large as the clusters. We chose the hydrogenated silicon cluster Si10H16 embedded in dehydrated all-silica zeolite-A. We first performed molecular mechanics simulation to obtain a thermodynamically stable structure. The cluster was located near the center of the cage. Then we calculated the electronic structure and optical properties for this stable structure by density functional method. The absorption spectrum of this material was nearly equal to the superposition of the spectra of Si10H16 and zeolite-A, which indicates that the properties of silicon clusters are retained even in zeolite cages. These results suggest that silicon clusters in zeolite may have the potential to be used as optoelectronic materials.
Porous Silicon as a Sacrificial Material for Micromachining of Silicon Optical Platforms
Authors: Guendouz M. , Joubert P. , Pedrono N. , Rouzic J. Le
Published: April 2011

Porous silicon obtained by the anodization of heavily doped n+-type silicon wafers was used as a sacrificial layer to micromachine silicon platforms. The effect of experimental parameters, such as the nature of the masking layer, current density and anodization time, on the geometry of the porous Si formed in patterned substrate is shown. Advantages of this method on the orientation-dependent chemical etching (ODCE), which is classically used for optical fiber alignment, are discussed.
Strain Evaluation of GaAs Layers Grown on Ultrahigh-Pressure-Annealed Strain-Free GaAs-On-Si Structures
Authors: Jimbo Takehito , Ishiwara Hiroshi
Published: April 2011

In order to form strain-free GaAs films on Si substrates, GaAs-on-Si structures have been formed by molecular beam epitaxy (MBE) or metalorganic chemical vapor deposition (MOCVD) and subsequently annealed under ultrahigh pressure at 2 GPa. The samples were then reannealed at atmospheric pressure in order to investigate the regeneration effect of strain in the GaAs films. It was found from X-ray diffraction (XRD) analysis and photoluminescence (PL) measurement that in the reannealed samples the strain near the surface of GaAs film was smaller than that near the interface with Si. Finally, additional GaAs layers were grown using MBE on both as-grown and strain-free GaAs-on-Si structures and it was found that the residual strain in the GaAs layer on strain-free GaAs-on-Si structure was smaller than in the as-grown case.
Progress on 850 nm Flip Chip Bondable VCSEL for Optical Interconnects
Authors: Cunningham J. E. , Jan W. Y. , Goossen K. W. , Krishnamoorthy A. , Walker J. A. , Tseng B. , Giaritta G. , Lothian J. , Kropf R. , Brenner I. , Sciortino P. , Chu G. , Leibenguth R.
Published: April 2011

We report here efforts to advance 850 nm VCSELs designed specifically for integration to Si- CMOS. Our VCSEL design is based on an oxidized current aperture and oxidized top mirror with the intent that the device operate as a bottom emitter after integration. Our design also employs intercavity contacts and the first non-alloyed buried contact have been realized. The devices produce resonantly enhanced light emission with record output of 70 μW/st.rad.
Electro-Optics of Unilamellar Vesicles
Authors: Asgharian N. , Meline R. L. , Schelly Z. A.
Published: April 2011

Electric field-induced transient birefringence and light scattering are reported for aqueous suspensions of synthetic unilamellar bilayer vesicles, prepared from the lipid dioleoylphosphatidylcholine (DOPC). The multiexponential birefringence relaxations observed on the microsecond and millisecond timescales are interpreted in terms of elongation and reorientation of induced dipolar vesicles, their linear chain formation, and electrofusion (and possibly electroporation) of the vesicles. Above certain threshold values of vesicle concentration, field-induced light scattering occurs concomitant with the birefringence. The corresponding transient signals corroborate the linear chain formation and subsequent fusion of the induced dipolar vesicles.
Structural Studies on Equimolar Suspensions of Palmitic Acid and 1-Lyso-Palmitoyl-Phosphatidylcholine
Authors: Lemmich J. , Richter F. , Callisen T. H.
Published: April 2011

An equimolar mixture of palmitic acid (PA) and 1-lyso-palmitoyl-phosphatidylcholine (Lyso-PPC) has been studied by time-resolved small angle and wide angle X-ray scattering during temperature cycles between room temperature and 53.5 °C. In addition to the X-ray experiments, differential scanning calorimetry (DSC) with a similar temperature vs. time course was performed. At room temperature we observe a coexistence of two different lamellar phases. Our results indicate that in one of these lamellar phases, Lyso-PPC and PA associate in lamellar structures resembling di-palmitoyl-phosphatidylcholine (DPPC) bilayers, whereas the other phase is an interdigitated lamellar phase, where the acyl chains of both Lyso-PPC and PA extend across the entire hydrocarbon width of the bilayer. Upon heating, the latter phase disappears at 42 °C, corresponding to the chain-melting temperature for DPPC bilayers. At a higher temperature the remaining lamellar phase undergoes a phase transition into an isotropic micellar phase. In the cooling scans a particular slow kinetics of the regeneration of the two lamellar phases is observed. Our experiments provide new information about this system where only the DPPC-like mesophase has been reported before.
A Phenomenological Model of the LS2 Ion Channel
Authors: Zhong Qingfeng , Newns Dennis M. , Klein Michael L.
Published: April 2011

A molecular dynamics simulation has been performed on a synthetic membrane-spanning ion channel, consisting of four α-helical peptides, each of which is composed of the sequence Ac-(LSLLLSL)3-CONH2. In the present simulation, the channel was initially assembled dynamically as a parallel bundle in the octane portion of a phase separated water/octane system, which provided a membrane-mimetic environment, without imposing any structural constraints. After more than one nanosecond, the four helices were found to adopt an associated dimer state with two-fold symmetry, which after a further 3 ns evolved to a coiled-coil tetrameric structure with a left-handed twist. Based on the simulation, we proposed a phenomenological model to describe the two-states of the channel.
Plasma Protein Adsorption and Platelet Adhesion on Poly[Bis(Trifluoroethoxy)Phosphazene]
Authors: Welle Alexander , Grunze Michael , Tur Dsidra
Published: April 2011

Poly[bis(trifluoroethoxy)phosphazene] (PTFEP) with a high purity and high molar mass is a biocompatible material [1] used as bulk material in medical implants. We developed a process to coat surfaces with PTFEP films and performed ELISA experiments designed to understand their blood compatibility. We observed that PTFEP adsorbs preferentially albumin from plasma, and only small amounts of coagulation or inflammation stimulating proteins. In general, there is a good correlation between increasing content of albumin in the adsorbed protein film and reduced platelet adhesion. Another important prerequisite of blood compatibility is the stabilization of the native state of adsorbed proteins, since denaturated proteins stimulate platelet adhesion. The elutability of adsorbed proteins by sodiumdodecylsulfate solution was used to quantify the amount of irreversible attached and presumably denaturated proteins. PTFEP showed a low amount of irreversibly adsorbed proteins of the coagulation cascade. Circular dichroism measurements of adsorbed fibrinogen and albumin showed only weak distortions of the secondary structure of these proteins on the surface of PTFEP.

We conclude that PTFEP has a unique blood compatibility because of the favorable composition and the stabilization of the protein layer against denaturation.
The Influence of Intracellular Transport Developmenttransport on Axonal Development
Authors: Hentschel H.G.E. , Fine A.
Published: April 2011

We describe how axonal differentiation during neuronal growth is dependent on intracellular transport. The interaction of diffusion and active transport of a morphogen whose concentration at the neurite tips influences the growth rate can lead to an instability involving winner-take-all growth, forming a single, quickly growing neurite (the axon) and inhibiting the growth of the other neurites. We discuss the dynamics of this process and its dependence on cell parameters.
Movement of Vesicles in Eucariotic Cells. Role of Intravesicle Protons as a Fuel and Modulation of Their Concentration by Drugs or Metabolic Changes.
Authors: Pescarmona G. P. , Morra E. , Aldieri E. , Ghigo D. , Bosia A.
Published: April 2011

Import (endocytosis) and export (secretion) of molecules from the cells is mediated by vesicles (lysosomes, endosomes) sliding along microtubules or actin filaments. These vesicles share a common feature: an internal pH of about 5, with an inner protons concentration 1000 fold higher than in the surrounding cytoplasm. As the proton gradient in mitochondria is able to drive ATP synthesis we can expect a similar role (energy supplier) for protons in all acidic vesicles. To experimentally test the vesicles' transport we have loaded them with a fluorescent dye (chloroquine) and then measured its efflux over 5 hours. This efflux was reduced by all treatments lowering the actual concentration of protons in the vesicles, independently of the properties. Treatments included lowering intracellular NADH, inhibitors of ATP-dependent proton translocase and/or the Na+/H+antiport, drugs that accumulate into lysosomes, buffering its acidity (chloroquine, doxorubicin). These results support the idea of a role of a proton gradient as a fuel for protein motors.
Viscoelasticity and its Microscopic Characterization in Semiflexible Biopolymer Solutions
Authors: Mackintosh F. C. , Gittes F. , Schnurr B. , Olmsted P. D. , Schmidt C. F.
Published: April 2011

Plant and animal cells contain a complex polymeric network known as the cytoskeleton. A principal component of this is the actin cortex, a gel-like network of F-actin protein filaments. Recently, solutions of reconstituted F-actin have provided in vitro models of the actin cortex, as well as excellent model systems in which to study semiflexible polymers. We describe models of viscoelasticity in semifexible polymers, and report theoretical and experimental results for thermal fluctuations of embedded particles, which act as local viscoelastic probes of soft materials such as biopolymer solutions. Specifically, we report high-frequency scaling behavior of the shear modulus, as the 3/4 power of frequency, in contrast with the behavior of flexible polymer systems.
Semi-Flexible Polymers Grafted to Surfaces: “Doughnut” and “Tower” Micelles
Authors: Bright J. N. , Williams D. R. M.
Published: April 2011

Introduction

The behaviour of polymer chains grafted to a surface has attracted considerable interest due to its broad range of applications in both colloid science and biological systems.
Electrostatically Induced Bundle Formation of Rodlike Polyelectrolytes: Comparison of Predictions from Monte Carlo Simulations with Experiments on Fd And M13 Virus Particles.
Authors: Nordenskiöld Lars , Lyubartsev Alexander , Tang Jay X. , Janmey Paul A.
Published: April 2011

This work compares the electrostatic bundling of two Inovirus particles: fd and M13, that are structurally identical except that the effective axial charge density of M 13 is approximately 30% lower than that of fd. The electrostatic force (or osmotic pressure) between these ordered biopolyelectrolytes, as a function of inter-rod separation, has been calculated with Monte Carlo simulations. Comparison is made with experiments on the bundling of fd and M 13 caused by divalent ions, as detected by light scattering. In the theoretical calculations, the bundling results from electrostatic attraction between the neighbouring polyelectrolytes, caused by the correlated interactions between the ion clouds. The importance of this effect (i. e. capacity to induce bundling) is governed by surface charge density of the polyelectrolyte, amount of multivalent ion present, charge and size of the hydrated multivalent ion. These predictions are in very good agreement with the presented experimental results on bundling of fd and M 13 caused by Ca2+ and Mg2+
Comparison of Experimental with Theoretical Melting of the Yeast Genome and Individual Yeast Chromosome Denaturation Mapping Using the Program Meltsim
Authors: Bizzaro J. W. , Marx Kenneth A. , Blake R. D.
Published: April 2011

The yeast S. cereviseae represents the first eukaryotic organism whose genome has been entirely sequenced as a result of the Human Genome Project(1). In this report we demonstrate the good agreement between an experimental high resolution melting curve of total nuclear S. cereviseae DNA and the theoretical melting calculated for the complete yeast DNA genome (12,067,277 bp: Saccharomyces Genome Database) by the statistical thermodynamics program MELTSIM, parameterized for long DNA sequences(2,3). The experimental and theoretical melting curves are both fairly symmetrical and possess nearly identical Tmvalues. Calculated melting of coding and flanking DNA regions indicates that flanking DNAs are more (A+T)-rich than coding sequences and account for the earliest melting DNA. Calculated melting curves of the 16 individual yeast chromosomes are very similar and with few exceptions exhibit symmetric melting curves. MELTSIM was also used to calculate a theoretical denaturation map of Chromosome III DNA. The agreement between MELTSIM calculated and experimental melting data demonstrates our ability to accurately simulate long DNA sequence melting in complex eukaryotic genomes, whose sequences are becoming increasingly available for study in public databases. This has important consequences for the understanding of sequence dependent energetic properties of DNA in their biological sequence context and also for their potential use in biomaterials applications.
Influence of RTP on Vacancy Concentrations
Authors: JACOB M. , Pichler P. , Wohs M. , Ryssel H. , Falster R.
Published: April 2011

The increase and reduction of the vacancy concentration by rapid thermal processing (RTP) was investigated by platinum diffusion. Direct experimental evidence is presented for the consumption of vacancies during oxidation and for the introduction of vacancies during processing in ammonia and nitrogen. These results confirm the indirect conclusions from dopant diffusion and from the growth and shrinkage of extended defects. In addition, it was possible to establish an upper limit for the equilibrium concentration of vacancies at 1180 °C which is lower than previously reported values.
A New Practical Approach to Implement a Transient Enhanced Diffusion Model into an Fem-Based 2-D Process Simulator
Authors: Sugiyasu Noriyuki , Suzuki Kaina , Kojima Syuichi , Ohyama Yasushi
Published: April 2011

We have proposed a coupled and de-coupled combined method to solve partial differential equations for a transient enhanced diffusion model. In the case of a boron diffusion process, the sum of concentrations of interstitial Si and of impurity-interstitial pair, the sum of concentrations of vacancy and of impurity-vacancy pair and each chemical impurity concentration are kept constant. The charge neutrality law is also applied. This procedure has realized a robust solution system which is implemented into our in-house FEM-based 2-D process simulator, and transient enhanced diffusion simulations for a sub-quarter micron nMOSFET have been demonstrated.
Use of Rigorous Three-Dimensional Electromagnetic Simulation to Evaluate the Effectiveness of Optical Proximity Correction for Nonplanar Lithography
Authors: Yeung M. S. , Barouch E.
Published: April 2011

A rigorous three-dimensional lithography simulator based on a recently developed single integral equation formulation for electromagnetic scattering is used in a simulation study of the effectiveness of using pre-distortions of mask patterns to compensate for linewidth variations in the resist patterns on nonplanar substrate topography. An efficient simulation methodology is used to reduce the number of required scattering calculations. Simulation results for different mask shapes and defocus conditions are presented.
Finite Element Analysis of a MOCVD Reactor Having a Close-Spaced Injector
Authors: Sun Yang Ren , Weyburne David W. , Paduano Qing S.
Published: April 2011

A 2-D axisymmetric finite element analysis has been performed on a vertical MOCVD reactor having a close-spaced water-cooled injector for which the gas is injected only 1.6 cm above the rotating disk. The analysis showed that the system has a much higher flow stability, in terms of suppression of unwanted thermally driven convection, than conventional reactors. In addition, the flow study has identified two kinds of flow recirculation, a purely thermally driven convection and a geometry assisted one. The former is initiated with gas flowing upward near the center of the disk circulating clockwise, the latter one is located near the chamber wall circulating counterclockwise. This result is important in interpreting experiments.
The Effect of a Thin Sample on the Extended Defect Evolution in Si+ Implanted Si
Authors: Li Jing-Hong , Jones Kevin S.
Published: April 2011

The annealing kinetics of implant damage in Si+ implanted Si has been investigated using in-situ and ex-situ annealing of transmission electron microscopy (TEM) samples prepared prior to annealing. The defect evolution at 800°C was studied for a Si wafer implanted with Si+ at 100keV to a dose of 2×1014 cm-2. This implant was above the sub-threshold loop formation threshold allowing one to study simultaneously the {311} defect dissolution and dislocation loop nucleation and growth. In order to study the effect on the defect evolution of using a thin sample for an in-situ annealing experiment, a pair of samples, one thick and one thinned into a TEM sample, were annealed in a furnace simultaneously. It was found that the presence of a second surface 2000Å below the implant damage did not affect the extended defect evolution. For the in-situ annealing study it was found that the {311} dissolution process and sub-threshold dislocation loop formation process was not affected by the TEM electron beam at 160kV as long as an 800°C furnace pre-anneal was done prior to in-situ annealing. The dissolution rate of the {311} defects was used to confirm the TEM holder furnace temperature. The results of both the in-situ the {311} defects is released during the 311 dissolution process and 30% comes to reside in dislocation loops. Thus, the loops appear to contain a significant fraction of the total interstitial concentration introduced by the implant.
Physical Model and Numerical Results of Dissociation Kinetics of Hydrogen-Passivated Si/SiO2 Interface Defects
Authors: Gadiyak G. V.
Published: April 2011

A simple model of thermal dissociation and recovery of hydrogen-passivated silicon defects at the Si/SiO2 interface, such as Pb - centers, during vacuum thermal annealing has been suggested. This model considers reactions of hydrogen with defect states at the Si/SiO2 interface and diffusion of liberated atomic and molecular hydrogen in a silicon dioxide film. An excellent agreement was obtained between the experimental and numerical simulation results for oxides with different thickness (200–1024 Å), grown both (111) and (100) samples and annealed in the temperature range (480–700° C).
Nucleation and Growth of Voids in Silicon
Authors: Plekhanov P. S. , Gösele U. M. , Tan T. Y.
Published: April 2011

Nucleation of voids and vacancy-type dislocation loops in Si under vacancy supersaturation conditions has been considered. Based upon nucleation barrier calculations, it has been found that voids can be nucleated, but not dislocation loops. The homogeneous nucleation rate of voids has been calculated for different temperatures by assuming different enthalpy values of Si vacancy formation. The process of void growth due to precipitation of vacancies has been numerically simulated. Comparing results of the nucleation and the growth modeling and taking into account the competition between the two processes, the limited time available, and the crystal cooling rate after growth, it has been shown that homogeneous nucleation of voids to experimentally observed densities and void growth to observed sizes is possible if enthalpy of Si vacancy formation is within the range of 2.9 to 3.6 eV with the nucleation temperature in the range of 980–1080 °C.
Modeling of Atom Diffusion and Segregation in Semiconductor Heterostructures
Authors: Bracht Hartmut , Walukiewicz Wladek , Haller Eugene E.
Published: April 2011

We propose a new approach for modeling of impurity diffusion at semiconductor heterointerfaces. The approach is based on the notion of a common energy reference for highly localized defects. It is shown that in the kick-out process, the segregation of group II acceptors is controlled by the valence band offsets among different constituent layers of the heterostructure. Extensive numerical modeling of the diffusion provides an explanation for the experimentally observed strong segregation of Zn and Be acceptors in the lattice matched InP/InGaAs, InP/InGaAsP and GaAs/AlGaAs heterostructures.
Simulation of Under- and Supersaturation of Gallium Vacancies in Gallium Arsenide During Silicon in- and Outdiffusion
Authors: Chen C.-H. , Gösele U. , Tan T. Y.
Published: April 2011

The diffusivity of Si in GaAs shows a dependence on the cubic power of its concentration or the concentration of electrons n under both in- and outdiffusion conditions. Hence, the diffusion of Si in GaAs is consistent with the Fermi-level effect model invoking the triply-negatively-charged Ga vacancies, , as the point defect species responsible for diffusion to occur on the Ga sublattice under n-doping condition. However, the Si diffusivity values of the indiffusion cases is several orders of magnitude smaller than those of the outdiffusion cases at the same Si concentrations. This means that the two apparent Si diffusivity values under intrinsic conditions will contain also the same discrepancy, which has been previously assessed to be due to a undersaturation in indiffusion cases and a supersaturation in outdiffusion cases. In this study we have calculated the under- and supersaturation values using the known Si diffusivities. We found that the GaAs surface states play a key role in the development of the under- and supersaturations.
Interactions of Point and Extended Defects in Structural Intermetallics: Real-Space Lmto-Recursion Calculations
Authors: Kontsevoi O. Yu. , Mryasov O. N. , Gornostyrev Yu. N. , Freeman A. J.
Published: April 2011

A real-space TB-LMTO-recursion method for electronic structure calculations is applied to the study of interacting extended and point defects in NiAl. Results of calculations for the pure intermetallic and with ternary additions (within a supercell model) show good agreement with band structure results. Further, electronic structure and total energy calculations of point (single impurity, M=Ti, V, Cr, Mn, Fe and Co) and planar defects such as anti-phase boundaries (APB) were carried out and the interaction between them was determined. We found that for the ½〈111〉{110} APB in NiAl, ternary additions occupy exclusively the 3d-metal sublattice and decrease the APB energy (except for Co). Finally, we employ TB-LMTO-REC to study the electronic structure of the most complex extended defect, a dislocation. We demonstrate for the 〈100〉{010} edge dislocation in NiAl that: (i) quasi-localized states may exist as a result of specific lattice distortions in the dislocation core with a type of “broken” bonds; (ii) the electronic structure changes appreciably in the process of dislocation motion; (iii) van-Hove singularities present in the ideal crystal may be shifted to E;r as a result of the dipolar character of the deformations in the dislocation core.
Let there be Light in Tight Binding
Authors: Vogl P. , Graf M. , Görling A.
Published: April 2011

Empirical tight-binding theory is generalized to incorporate time-dependent electromagnetic fields in a systematic and gauge-invariant manner that does not introduce any extra adjustable parameters. It is shown that this approach successfully predicts a wide range of solid state properties that have not been accessible within the tight binding method so far. We present applications such as optical constants, luminescence in heterostructures, properties in ultra-high magnetic fields and lattice dynamics in polar materials.
Ordering effects in Disordered Metallic Alloys
Authors: Pasturel A.
Published: April 2011

This paper reviews recent progress in the theory of the atomic and electronic structure of disordered metallic alloys containing transition metals (TM). Realistic structural models are obtained using molecular dynamics simulations based on pair interatomic forces derived from a tight-binding-bond method. This approach describes quantitatively compositional trends in chemical and topological short-range order in agreement with diffraction experiments.
How Far to Use Tight-Binding Potentials for Bimetallic Surface Modelling?
Authors: Tréglia G. , Legrand B.
Published: April 2011

Modelling in a realistic way both equilibrium and dynamical processes on bimetallic surfaces requires the availability of interatomic potentials sufficiently simple (i.e. analytical) although derived from the electronic structure. This is possible in the framework of Tight-Binding formalism. We present here a review of the applications of such potentials, together with some reflexions about their limitations.
Semi-Empirical Tight-Binding Parameters for Total Energy Calculation in Zinc
Authors: Bere A. , Hairie A. , Nouet G. , Paumier E.
Published: April 2011

The semi-empirical tight-binding method is used to build up an interatomic potential in zinc. Using relaxed structures, the parameters are fitted to the lattice parameters, the elastic constants and the vacancy formation energy. The total energy calculation predicts the stability of the h.c.p. structure. The potential is used to calculate the energy of some extended defects: the basal stacking fault and two twin boundaries.
Effects of Grain-Boundaries in Superconducting Materials
Authors: Hogan-O'Neill J. J. , Martin A. M. , Annett James F.
Published: April 2011

We examine the effects of grain-boundaries on the order-parameter and critical-currents in superconductors. We use a geometrical model of the lattice structure of grain-boundaries. We solve the Bogoliubov-de Gennes equation using the Recursion Method to obtain the self-consistent BCS gap function Δ and the local density of states N(E) near the boundary. Imposing a phase difference across the boundary we calculate the supercurrent and hence obtain the critical-current, Ic, of the junction.
A Comparison of Linear Scaling Tight Binding Methods
Authors: Horsfield A. P. , Bowler D. R. , Goringe C. M. , Pettifor D. G. , Aoki M.
Published: April 2011

Four linear scaling tight binding methods (the density matrix method, bond order potentials, the global density of states method, and the Fermi operator expansion) are described and compared to show relative computational efficiency for a given accuracy. The density matrix method proves to be most efficient for systems with narrow features in their energy gaps, while recursion based moments methods prove to be most efficient for metallic systems.
Total Energy Differences Between Silicon Carbide Polytypes and their Implications for Crystal Growth
Authors: Llmpijumnong Sukit , Lambrecht Walter R. L.
Published: April 2011

The energy differences between various SiC polytypes are calculated using the full-potential linear muffin-tin orbital method and analyzed in terms of the anisotropie next nearest neighbor interaction (ANNNI) model. The fact that J1 + 2J2 < 0 with J1 > 0 implies that twin boundaries in otherwise cubic material are favorable unless twins occur as nearest neighbor layers. Contrary to some other recent calculations we find J1 > |J2|. We discuss the consequences of this for stabilization of cubic SiC in epitaxial growth, including considerations of the island size effects.
Copper Corrosion Mechanisms of Polysulfides
Authors: Chaka Anne M. , Harris John , Li X. P.
Published: April 2011

Corrosive behavior of commercial organopolysulfides of the type R-(S)n-R is known to increase dramatically towards copper-based alloys when n ≥ 4, whereas di- and trisulfides are essentially inert. Three possible reasons for this behavior are examined using local and nonlocal density-functional theory as well as post-Hartree-Fock theory at the MP2 level. The hypotheses are: the shorter chains are protected from the surface by steric hindrance of the terminal organic groups, longer polysulfides can chelate copper atoms and remove them from the surface, and S-S bonds become weaker and more reactive as the polysulfide chains become longer. We find the predominant reason for the increase in corrosive behavior to be a dramatic decrease in S-S bond dissociation energy when n ≥ 4 due to stabilization of the unpaired electron via delocalization in RSS• thiyl radicals. The thiyl radicals are thus the species which is capable of attacking and corroding the copper surface. Chelation is eliminated as a possible mechanism, and steric protection and oil solubility afforded by the organic terminal groups are found to play a minor role.
Pressure Induced Phase Transformations in Silica
Authors: Çağin Tahir , Demiralp Ersan , Goddard William A.
Published: April 2011

Silica, SiO2, is one of the most widely studied substance because of its complex and unusual properties. We have used a recently developed 2-body interaction force field [1] to study the structural phase transformations in silica under various pressure loading conditions. The specific transformations we studied are the a-quartz to stishovite, coesite to stishovite and fused glass to a dominantly six-coordinated dense glassy phase. Molecular dynamics simulations are performed under constant loading rates ranging from 0.1 GPa/ps to 1.0 GPa/ps, with final pressures upto 100 GPa and at temperatures of 300, 500, and 700 K. We observe the crystal to crystal transformations to occur reconstructively, whereas it occurs in a smooth and displacive manner from glass to a stishovite-like phase confirming earlier conjectures.[2] We studied the dependence of transition pressure on the loading rate and the temperature to elucidate the shock loading experiments. We also studied the unloading behavior of each transformation to assess the hysterisis effect.
Theoretical Studies of the Structures of the Liquid-Vapor Interfaces of Metals and Binary Alloys
Authors: Rice Stuart A. , Zhao Meishan , Chekmarev Dmitriy
Published: April 2011

We report the results of self-consistent quantum Monte Carlo simulations of the structure of the liquid-vapor interfaces of Ga and of alloys of Bi in Ga and In in Ga. The single particle density distribution along the normal to the interface is predicted to display stratification, with a spacing of about an atomic diameter, and the pair structure function in the plane of the liquid-vapor interface is predicted to be essentially the same as that in the bulk liquid. In all of the cases considered the qualitative and quantitative character of these predictions are in good agreement with the results of experimental studies.
Spin-Polarized Density of States and Electron Tunnelling from the CO/Al2O3 Interface
Authors: Nguyen-Manh D. , Tsymbal E.Yu. , Pettifor D. G. , Arcangeli C. , Tank R. , Andersen O. K. , Pasturel A.
Published: April 2011

In order to elucidate the mechanism of spin-polarized electron tunneling in thin-film ferromagnet-insulator junctions, self-consistent band structure calculations of the CO/Al2O3 interface have been performed using a new LMTO technique. Since the results of the calculations are very sensitive to the distance between the Co and Al planes, we have minimised the total energy with respect to this distance. Our calculations show that at the Fermi energy a strong bonding between the 3d-electrons of Co with the sp-electrons of Al at the interface can have an important influence on the spin polarization of the layer-projected density of states (LPDOS) of inner Al and O layers. Since the Fermi energy lies within the minority-spin d-band of Co but above the majority-spin d-band, the sp-d bonding results in a smaller LPDOS of the minority-spin electrons of the interfacial Al layers in comparison to that of the majority-spin electrons. This asymmetry in the LPDOS extends to the inner Al2O3 layers implying a positive spin polarization of the tunneling density of states. The result is consistent with experimental observations on tunnelling from cobalt through alumina where positive values of the spin polarization of the tunnelling current were measured.
Comparison Between Atomistic and Continuum-Mechanics Modelling of Grain-Boundary Fracture
Authors: Cleri F. , Phillpot S. R. , Wolf D.
Published: April 2011

We use the Peierls-Nabarro continuum mechanics model of dislocation nucleation, modified according to the results of atomistic simulations, to interpret the experimental results of fracture response in symmetric-tilt grain boundaries in Cu. We then directly perform Molecular Dynamics simulations of fracture propagation and dislocation emission from a microcrack placed in the interface plane of the symmetric-tilt (221)(221) grain boundary in fee Cu. Direction-dependent fracture response is observed in agreement with experiments, namely the microcrack advancing by brittle fracture along the [114] direction and being blunted by dislocation emission along the opposite [114] direction. Moreover, we are able to quantify important differences with respect to the continuum model due to the shielding of the stress field at the crack-tip and to the presence of the excess stress at the interface.
Negative Magnetoresistance in (Bi,Pb)2Sr3Co2O9 Layered Cobalt Oxides
Authors: Tsukada I. , Yamamoto T. , Takagi M. , Tsubone T. , Uchinokura K.
Published: April 2011

Transport and magnetic properties of layered cobalt oxide (BiPb)2Sr3Co2O9 are investigated in detail under magnetic field up to 8 T. Parent compound, Bi2Sr3Co2O9, is a typical band insulator with Co ions being in a low-spin 3+ state because of the well-separated dε and dγ levels possibly due to a strong crystal field. We have tried to introduce holes mainly by Pb substitution for Bi. The hole-doped sample shows metallic behavior in a resistivity measurement between 300 and 30 K. Below 30 K, however, the resisitivity increases. Under the magnetic field the resistivity is strongly suppressed in this region. We observed more than 30% resistivity drop at 2 K under H = 8 T, which is comparable to insulating (La,Sr)CoO3 system. We discuss the mechanism of hole doping and the origin of negative magnetoresistance with tranport and magnetic properties, and point out that the conventional double-exchange mechanism cannot be applied to this system. This means that some new mechanism is necessary to explain this phenomenon.
Surface Morphology and Lattice Misfit in Yig and La: Yig Films Grown by Lpe Method on GGG Substrate
Authors: Choi Duk-yong , Chung Su-jin
Published: April 2011

Y3Fe5O12(YIG) and La-doped YIG films were grown on the {111} GGG substrate using the PbO-B2O3 flux system. Pb, La incorporation and lattice misfit and annealing behaviors were studied. In the case of LPE growth of YIG film, lead ions from flux are substituted inevitably, and they play an important role in controlling film misfit. For a complete lattice matching, high supercooling is necessary in pure YIG growth, but this induces high defect concentration. In this experiment, La ions were added in the solution to sufficiently increase lattice parameter of the film grown under low supercooling. The concentration of substituted Pb and La were increased as the growth temperature was lowered and growth rate increased. The effective distribution coefficient of La was about 0.2 at a supercooling of 30 °C. The optimum growth conditions which bring about very small misfit were determined by measuring the misfit by double crystal diffractometer. Strain distributions of pre-annealed and annealed samples were investigated by triple crystal diffractometer.
Room Temperature Magnetoresistive Response in CMR Perovskite Manganite Thin Films
Authors: Todd Michael A. , Seegel Charles , Baum Thomas H.
Published: April 2011

Perovskite-structured LaxSryMnO3 thin-films have been deposited onto LaAlO3 substrates via liquid delivery chemical vapor deposition (LD-CVD) using metal(β-diketonato) precursors, M(thd)x [where M= Ca, Sr, La and Mn, thd = 2,2,6,6-tetramethyl-3,5-heptanedionato and x = 2–3]. Thin films were deposited at temperatures between 500 and 700 °C and subsequently annealed at 1000 °C under O2. These films possess stoichiometries that are: i) vastly different from the La0.67Sr0.33MnO3 compositions commonly reported in the literature and ii) display high temperature, low field responses that may be technologically important. Resistance versus temperature measurements revealed a metal to semiconductor transition at room temperature and above. Hall measurements on a film of La0.35Sr0.24MnO3 displayed a magnetoresistive response (MR) of -10% at 57 °C in a fixed magnetic field of 780 Oe. Based upon our research, the observed film properties are directly related to the deposited film stoichiometry and the best results were observed at Sr / La ratios between 0.30 and 1.0 for A-site deficient LaxSryMnO3 thin-films after thermal annealing.
Spin Tunneling in Conducting Oxides
Authors: Bratkovsky Alexander
Published: April 2011

Different tunneling mechanisms in conventional and half-metallic ferromagnetic tunnel junctions are analyzed within the same general method. Direct tunneling is compared with impurity-assisted, surface state assisted, and inelastic contributions to a tunneling magnetoresistance (TMR). Theoretically calculated direct tunneling in iron group systems leads to about a 30% change in resistance, which is close to experimentally observed values. It is shown that the larger observed values of the TMR might be a result of tunneling involving surface polarized states. We find that tunneling via resonant defect states in the barrier radically decreases the TMR (down to 4% with Fe-based electrodes), and a resonant tunnel diode structure would give a TMR of about 8%. With regards to inelastic tunneling, magnons and phonons exhibit opposite effects: one-magnon emission generally results in spin mixing and, consequently, reduces the TMR, whereas phonons are shown to enhance the TMR. The inclusion of both magnons and phonons reasonably explains an unusual bias dependence of the TMR.

The model presented here is applied qualitatively to half-metallics with 100% spin polarization, where one-magnon processes are suppressed and the change in resistance in the absence of spin-mixing on impurities may be arbitrarily large. Even in the case of imperfect magnetic configurations, the resistance change can be a few 1000 percent. Examples of half-metallic systems are CrO2/TiO2 and CrO2/RuO2, and an account of their peculiar band structures is presented. The implications and relation of these systems to CMR materials, which are nearly half-metallic, are discussed.
Fabrication Of La0.7Sr0.3MnO3/La0.5Sr0.5CoO3/La0.7Sr0.3MnO3 Heterostructures for Spin Valve Applications
Authors: Robson M. C. , Ogale S. B. , Godfrey R. , Venkatesan T. , Johnson M. , Ramesh R.
Published: April 2011

Epitaxial growth of oxide heterostructures, which may be utilized in spin valve applications, has been demonstrated. The heterostructures consist of two ferromagnetic layers separated by a nonmagnetic metallic interlayer. The ferromagnetic material used is the manganese perovskite oxide, La0.7Sr0.3MnO3, while the metallic oxide interlayer is La0.5Sr0.5CoO3. X-ray diffraction spectra demonstrate the high structural quality of the heterostructures. The magnetization of the heterostructure as a function of magnetic field measured at room temperature yields a double hysteresis loop that is characteristic of this type of spin valve structure. The behavior of this double hysteresis loop is also examined as a function of the metallic interlayer thickness.
Fabrication of High Temperature Superconductor-Colossal Magnetoresistor Spin Injection Devices
Authors: Kim J. , Stroud R. M. , Auyeung R. C Y. , Eddy C. R. , Koller D. , Osofsky M. S. , Soulen R. J. , Horwitz J. S. , Chrisey D. B.
Published: April 2011

Trilayer YBa2Cu3O7-δ/(SrTiO3, CeO2)/La0.67Sr0.33MnO3-δ devices have been fabricated for the study of supercurrent suppression due to the injection of spin-polarized quasiparticle current. The critical current for a YBa2Cu3O7-δ/100 Å SrTiO3/La0.67Sr0.33MnO3-δ device was found to decrease from 118 mA to 12.6 mA, for an injection current of 60 mA. The effect of film microstructure on the critical current suppression was investigated. Defects in the SrTiO3 and CeO2 layers were found to control the device properties.