bluuebunny/crossref_metadata_2025 huggingface dataset:
Microstructural Stability of Stressed Lamellar Eutectics
Authors: Sridhar N. , Rickman J.M. , Srolovitz D.J.
Published: April 2011

At elevated temperatures, stresses enhance mass transport at the microstructural scale and under certain conditions, lead to microstructural instabilities. Using a linear stability analysis, we examine the effect of stresses on the morphological stability of plate-like phases and lamellar eutectic composites. These stresses can be either due to misfit strains and/or due to externally applied loads. We find that for misfitting plates, the nominally flat plate-matrix interface is unstable with respect to the growth of perturbations with wavelengths greater than a critical wavelength, provided that the reinforcing plates are elastically suffer than the surrounding matrix. On the other hand, for stresses generated by externally applied loads, the flat interface is always unstable as long as the plate modulus and the matrix modulus are not identical. In addition, the analysis reveals that misfit strains can either counteract or enhance the destabilizing influence of applied loads depending on the elastic properties of the plate and the matrix. Finally, we have developed stability diagrams that identify material properties and operating conditions required to maintain a stable interface in these lamellar eutectic composites.
A Kinetic Model of Precipitate Evolution
Authors: Lane Rohrer C. , Asta M. D. , Foiles S. M. , Hyland R. W.
Published: April 2011

Chemical reaction rate theory is used to model the kinetics of precipitation reactions in Al alloys, including the effects of continuous cooling and thermally generated point defects. The computational method models the processes of nucleation, growth, and coarsening within a single framework. Calculated time and temperature dependent precipitate number densities and sizes during the homogeneous precipitation of the A13Sc phase in an Al-.11 at% Sc alloy are shown to compare favorably with experimental observations.
Ordering Kinetics and Magnetic Properties in the Pt1co1–xNix Alloy System
Authors: Tu Huan , Ruiz Eduardo , Orehotsky J.
Published: April 2011

Ordering kinetics were measured of various alloys in the order-disorder Pt1Co1–xNix (0 ≤ x<0.5) magnetic alloy system.The kinetics of all the investigated alloys were found to be characterized by an Arrhenius temperature dependence. The associated activation energy as a parametric representation of the kinetics was evaluated as a function of the nickel content. This result was correlated with measured values of the saturation magnetization and Curie temperature of these alloys in both the atomically ordered and disordered states. A strong correlation was discovered between the activation energy and the saturation magnetization of the disordered ternary alloys.
Investigation of Phase Transformations and Ordering During Combustion Synthesis
Authors: Kachelmyer C.R. , Varma A. , Khomenko I.O. , Rogachev A.S. , Merzhanov A.G.
Published: April 2011

Two complementary experimental techniques are presented that describe the mechanisms during the combustion synthesis of NiAl and Ti5Si3. The first involves quenching a reacting wedge-shaped sample imbedded in a copper block where the propagating combustion front extinguishes while traveling to the apex. The second technique, time-resolved X-ray diffraction (TRXRD), provides a direct in-situ observation of the sequence of high temperature phase transformations. The information obtained from this investigation will be useful in developing improved process models of combustion synthesis, which can lead to the production of advanced materials with tailored microstructure and properties.
Phase Separation of β-SN in Strained, Compositionally Metastable Ge1-xSnx Alloys
Authors: Kriesel Joshua W. , Lee Susanne M.
Published: April 2011

Using rf sputtering and post-deposition annealing in a differential scanning calorimeter (DSC), we manufactured bulk (4000 nm) films of crystalline Ge0.83Sn0.17. This Sn concentration is much greater than the solid solubility limit of Sn in Ge (x ≤ 0.01). Continued annealing thermally induces Sn phase separation from the alloy, limiting the ultimate attainable grain size in the metastable crystals. We examine, here, the mechanisms and kinetics of the processes limiting the size of the Ge0.83Sn0.17 polycrystals. From a combination of DSC, electron microprobe, and x-ray diffraction (XRD) measurements, we propose phase transformation mechanisms corresponding to crystallization of amorphous Ge0.83Sn0.17, crystallization of an as-yet unidentified phase of Sn, and phase separation of Sn from the Ge1-xSnx crystals. We were unable to observe the unidentified phase of Sn in XRD, but the phase must be present in the material to account for the quantitative discrepancies (as much as 8 at.%) in Sn percentages determined from each of the DSC, XRD, and electron microprobe measurements. Our models for the various transformation kinetics were corroborated by the subsequent phase-separated Sn melting behavior observed in the DSC: two Sn melting endotherms, one of which was 20–100°C lower than the bulk melting temperature of Sn. This depressed temperature endotherm we speculate represents liquefaction of nanometer-sized (β–Sn clusters.
Molecular Beam Epitaxy of InGaP Films Grown on Si(001) Substrates with Various Kinds of Initial Buffer Layers
Authors: Kawanami H. , Sekigawa T.
Published: April 2011

Single domain InxGa(1-x)P (x=0.4) films were successfully grown on Si(001) misoriented substrates by molecular beam epitaxy with solid phosphorous source. The effects of the initial 100 nm thick interfacial buffer layer such as InGaP (i.e. direct growth without buffer layer), GaP, AJP, and GaAs were examined. Superior surface morphologies were indicated for the films with GaP, A1P, and GaAs buffer layers compared with the films on InGaP buffer layer. The results indicated the necessity of some kind of initial buffer layer different from the film.
Interfacial Coarsening in an MBE Growth without Slope Selection
Authors: Golubović Leonardo
Published: April 2011

We study the dynamics of an MBE growth in the presence of the interfacial instabilities inducing a pyramidal or mound-type interfacial growth. By using a novel collective coordinates method similar to Bray’s theory of phase ordering, we discuss the statistics of the growing interfaces in the absence of slope selection, when the mounds slope indefinitely increases with time.
Anisotropic Lattice Relaxation and its Mechanism of ZnSe Epilayer Grown on (001) GaAs Substrate by Molecular Beam Epitaxy
Authors: Kim C.S. , Noh S.K. , Lee H.J. , Cho Y.K. , Kim Y.I. , Park H.S. , Kim T.I.
Published: April 2011

We have investigated anisotropic lattice relaxation and its mechanism of ZnSe epitaxial layer grown on (001) GaAs substrate by MBE. Double-crystal X-ray rocking curves for (004), {115} and {404} reflections were measured as a function of the azimuthal rotation angle of the sample. We observed the sinusoidal oscillation of the FWHM of the epilayer peak for (004) reflections due to the asymmetric dislocation density along two orthogonal <110> directions, and the direction of the maximum FWHM corresponding to high dislocation density is along [110]. In addition, the strain along [110] is smaller than that along [1-10], indicating that the layer suffered anisotropic lattice relaxation. The direction of larger relaxation([l-10]) is not consistent with that of high dislocation density([110]). The results suggest that the asymmetry in dislocation density is not responsible for the anisotropic relaxation of the ZnSe epilayer.
Molecular Beam Epitaxial Growth Mode Transitions on Vicinal Surfaces
Authors: Trofimov V.I. , Medvedev B.K. , Mokerov V.G. , Shumyankov A.G.
Published: April 2011

Kinetic model of MBE growth on vicinal surface is investigated. The model includes step propagation, nucleation and growth of islands on the terraces and Schwoebel barrier at descending step edges as -well. By numerical solution of kinetic rate equations for growth on stepped surface, adatom and island density profiles across a terrace are obtained. With using simple criterion for growth mode transition the "phase diagram" of growth modes in parametric space γ–β is constructed, γ∼J/D and β∼tan-2φ, where J is the atomic flux, D is the surface diffusion coefficient and φ is the substrate miscut angle. The transition curve in the γ–β plane separating step flow mode region from the mixed (step-flow+nucleation) growth mode region is well describded by a simple equation γ=A/β3 where constant A=10 and 100 with and without Schwoebel effect. The relations for critical terrace width (miscut angle) and transition temperature are derived and it is shown that these relations are in fairly well agreement with available experimental data on the MBE growth of GaAs.
Simulations of SI(100) Growth: Step Flow and Low Temperature Growth
Authors: Roland C. , Zhang Q.M. , Boguslawski P. , Bernholc J. , Gilmer G.H.
Published: April 2011

We have investigated Si (100) homoepitaxy in variety of different temperature regimes. In the high temperature step flow regime, the growth properties of the different steps play an important role. The binding sites and diffusion barriers for a Si adatom moving over the buckled Si(100) surface and single-height steps were investigated with the ab initio Car-Parrinello scheme. The SA step edge was found to be a relatively poor sink for adatoms. By contrast, growth takes place much more rapidly at the SB step edges, so that one can understand the relatively fast growth observed at the ends of dimer rows. We have also investigated Si(100) homoepitaxy with classical molecular dynamics simulations at very low temperatures, where typically an amorphous deposit forms. By tuning the energy of the incoming atomic beams, one can lower the temperature of the amorphous to crystalline transition considerably and thereby enhance epitaxial growth.
Low Density Titanium Alloys by Non-Equilibrium Processing.
Authors: Wilkes D.M.J. , Goodwin P.S. , Ward-Close C.M. , Bagnall K. , Steeds J.
Published: April 2011

High levels of Mg solid solution in Ti have been achieved by mechanical alloying in a clean atmosphere. The equilibrium solubility of magnesium in titanium is very low and processing of these alloys by conventional routes is not possible due to the high vapour pressure of Mg and alloys of Ti and Mg have only been previously achieved by vapour quenching.

Contamination during milling was avoided by processing under a high purity argon atmosphere in sealed vials. Lattice parameter measurements and SEM observations suggested that complete solid solutions were achieved in alloys up to 24wt% Mg after 48h milling. For one alloy this was confirmed by high resolution TEM which showed extremely fine Ti grains in which Mg was homogeneously distributed with no clustering or precipitation.
Microhardness Enhancement in Fully Dense Fe-Based Nanophase Metallic Alloys
Authors: He L. , Ma E.
Published: April 2011

Nano-grained Fe-29Al-2Cr intermetallic and Fe-Cu two-phase composites have been consolidated to full density from powders produced by high-energy ball milling, using a sinter forging procedure developed recently in our laboratory. Grain sizes remained within nanophase range (<100 nm) after consolidation. Microhardness tests of Fe-29Al-2Cr samples consolidated to different density levels indicate a significant strengthening effect due to nanoscale grain size and a monotonic microhardness increase with decreasing residual porosity. Fully dense Fe-Cu composites exhibit enhanced microhardness as compared with rule-of-mixtures predictions, which may be attributable to interface strengthening at fcc-bcc interphase boundaries.
Defect Generation During Epitaxial Growth of CoSi2 on Miniature Sized (100) Si Substrate and its Effect on Electrical Properties
Authors: Byun Jeong Soo , Seon Jeong Min , Park Jin Won , Hwang Hyunsang , Kim- Jae Jeong
Published: April 2011

Self-aligned silicide (salicide) formation of epitaxial CoSi2, using a Co/Ti bilayer, on linear oxide (SiO2) patterned (100)Si substrate has been investigated. Rapid thermal annealing (RTA) at 550°C resulted in the lateral encroachment of silicide in the Si under the edge of the oxide. After RTA at 900°C, even though an epitaxial CoSi2 layer was formed on the Si substrate, defects such as lateral encroachment and voids were generated under the edge of the oxide. It was found that such defects lead to device failure due to the deterioration of the gate oxide and the shallow junction.
Hall Effect Investigation of Doped and Undoped ßB-FESI2
Authors: Brehme S. , Ivanenko L. , Tomm Y. , Reinsperger G.-U. , Staulß P. , Lange H.
Published: April 2011

Polycrystalline ß-FeSi2 layers prepared by codeposition of Si and Fe on cold and hot Si substrates and ß-FeSi2, crystals grown by chemical vapor transport were investigated. Resistivity and Hall effect measurements revealed the p-type conductivity of undoped material and the influence of some dopants of the iron group. The activation energy of a Cr-related acceptor was determined to about 85 meV. The mobility data were found to depend significantly on the purity of the preparation process.
Morphologies and Growth Modes of Fesi and β-Fesi2 Layers Prepared by Rapid Thermal Annealing
Authors: Amesz P. H. , Jørgensen L. V. , Libezny M. , Poortmans J. , Nijs J. , van Veen A. , Schut H. , de Hosson J. Th. M.
Published: April 2011

This paper reports on the formation of FeSi and β-FeSi2by rapid thermal anneal (RTA). Sputtered Fe layers with a thickness of 25 nm and 40 nm on (100) Si wafers were subjected to RTA-treatment at temperatures ranging between 500 and 700 °C. This leads to three different types of growth. Two types of lateral growth and a hillock like growth. All the growth modes were nucleation controlled. The lateral growth rates of β-FeSi2 were measured at temperatures ranging from 625 to 700 °C, this provided a value for the activation energy of this process of about 1.35 eV. Positron annihilation analysis and scanning electron microscopy were used to study the onset of the transformation.

It is suggested that the lateral growth appears when the Fe layer is completely transformed to FeSi before FeSi2 nucleates and that the hillock-like growth emerges when FeSi2 nucleates before the complete transformation of the Fe layer to FeSi.The RTA formed P3-FeSi2 layers have a roughness of the order of the thickness of the layer and a high defect density as observed by cross-section transmission electron microscopy. In order to decrease the surface roughening a-Si and Si3N4 capping layers were deposited with plasma enhanced vapour deposition before RTA.
Diffusion Processes in Silicides: A Comparison Between Bulk and Thin Film Phase Formation.
Authors: Gas Patrick , D'Heurle Francois
Published: April 2011

Diffusion processes in silicide thin films play a key role both during their formation by reactive diffusion or during their use. However our unique source of information was provided by indirect analysis: growth of thin films and dopant redistribution which are quite difficult to analyse in terms of diffusion mechanisms. Recent tracer experiments conducted in bulk silicides are presented. They allow a determination of both volume (v) and grain boundary (gb) diffusion coefficients. Contrary to what is observed in certain intermetallic compounds no fast volume diffusion mechanism was found. The main difference with the behaviour of pure metals is a slightly higher value of the ratio Qgb/Qv which makes gb diffusion an efficient process in a wider temperature range.

A quantitative analysis of diffusion processes during silicide formation is then possible. As an example we propose a comparison between the kinetics of growth of thin films and bulk diffusion couples in the Co/Si and Ti/Si systems. Providing that attention is paid to: i) the laws of growth which are slightly different for a phase growing simultaneously with others (bulk) and one phase growing alone (thin films), and ii) the grain size of the growing phase which is strongly dependant on temperature and thicknesses excellent agreement is obtained between the two sets of measurements. Moreover the growth rates may be calculated quite accurately from the values of the volume and gb tracer diffusion coefficients. This stresses and quantifies the role of interfacial diffusion in thin films behaviour.
Ion Beam Processed Ir/SiGe Structures
Authors: Curello G. , Gwilllam R. , Harry M. , Sealy B. J. , Rodriguez T. , Clement M.
Published: April 2011

The thermal reaction of Ir/SiGe structures following ion implantation induced Ir grain boundary mixing has been studied. The morphology of the final interface has been investigated by Cross-sectional Transmission Electron Microscopy (XTEM) and Rutherford Backscattering Spectrometry (RBS) has been used to detect the redistribution of constituent atoms in the reacted layers. A 20 nm iridium film was deposited in vacuum by electron beam evaporation onto p-Si1-xGex (x = 0.25, 0.33) fully relaxed MBE grown layers. Implantation conditions used were effective in amorphizing the interface-close region of the Ir film and the SiGe substrate. After regrowth, XTEM results show that the interface quality is improved with respect to the material that had not been implanted. RBS results on the other hand show Ge incorporation in the reacted layer to occur in contrast to the non implated case where the Ge piles up at the silicide/SiGe interface. The effect of a second anneal step at higher temperatures (in the range 800°C – 900°C) is also investigated.
Silicide Metallization of Aluminum Nitride Substrates for High-Temperature Microelectronics
Authors: Savrun E. , Sarikaya M. , Luan A. , Pearsall T.
Published: April 2011

A novel metallization for aluminum nitride substrates to package silicon carbide integrated circuits for use at temperatures up to 600°C was investigated. Chemical equilibrium calculations were used to determine the chemical compatibility of several refractory and transition metal disilicides with AIN. Tungsten disilicide, niobium disilicide, and titanium disilicide were selected for thin film deposition studies. WSi2, NbSi2, and TiSi2 thin films were deposited by RF sputtering on AIN substrates and heat treated at 900°C, 1000°C, and 1200°C in an argon atmosphere. Sheet resistivities were measured and interface stabilities and structures were characterized by scanning and transmission electron microscopy imaging, electron diffraction, and energy dispersive x-ray microanalysis spectroscopy. The results show that metal silicides appear to be promising as metallization for aluminum nitride for use at temperatures above 600°C.
Temperature Dependence and Annealing Behaviour of Hf Implanted (100)Si: HfSi 2 Synthesis
Authors: Silva M. R. Da , Melo A. A. , Soares J. C. , Silva M. F. Da , Moons R. , Langouche G. , Vantomme A. , Conde O. , Lu Y.-C. , Kung H.
Published: April 2011

We investigated the formation of a buried HfSi2 layer by high fluence ion implantation of isotopically mass separated 179–180 Hf+ on heated silicon (100) substrates. It is shown that for the substrate temperature of 600°C a buried HfSi 2 layer is formed. By subsequent annealing at 1000 °C a continuous 12 nm HfSi2 layer on the Si surface is formed followed by 130 nm big almost spherodized HfSi2 ellipsoid and 80 nm small HfSi2 and Si grains. The annealing of samples implanted at lower temperatures show that HfSi2 is also formed but with reduced yield. A summary of the relevant data is presented.
High Resolution Electron Microscopy of Sputter-Deposited Zirconia-Alumina Nanolaminates
Authors: Schofield M. A. , Whig R. , Aita C. R. , Gajdardziska-Josifovska M.
Published: April 2011

High resolution electron microscopy is employed to study the crystallography and morphology of zirconia nanocrystallites in zirconia-alumina nanolaminates and zirconia films. Unity volume fraction of tetragonal zirconia formed when the zirconia layer thickness was less than 6.2 nm, a theoretically predicted critical size for tetragonal-to-monoclinic zirconia (t -> m-ZrO2) transformation. In thicker layers, monoclinic zirconia formed, accompanied by renucleation and void formation which caused roughness to the zirconia nanolayers. The average position of the voids in the layers was 6.3 nm from the growth interface, coinciding with the critical dimension for t -> m-ZrO2 transformation.
Polysilicon Waveguides for Silicon Photonics
Authors: Agarwal Anuradha M. , Black Marcie R. , Foresi James S. , Liao Ling , Liu Yaping , Kimerling L. C.
Published: April 2011

Photonic integrated circuits in silicon require waveguiding through a material compatible with silicon VLSI technology. Polysilicon (polySi), with a high index of refraction compared to SiO2 and air, is an ideal candidate for use in silicon optical interconnect technology. Inspite of its advantages, the biggest hurdle to this technology is that losses of 350dB/cm have been measured in as-deposited polySi waveguide structures, as against ldB/cm losses measured in waveguides fabricated in crystalline silicon. We report methods for reducing scattering and absorption, which are the main sources of losses in this system. To reduce surface scattering losses we fabricate waveguides in smooth recrystallized amorphous silicon and Chemo-Mechanically Polished (CMP) polySi, both of which reduce losses by about 40dB/cm to 15dB/cm. Atomic Force Microscopy (AFM) and spectrophotometry studies are used to monitor surface roughness which has been reduced from an RMS roughness value of 19–20nm down to about 4–6nm. Bulk absorption/scattering losses can depend on size, structure, and quality of grains and grain boundaries which we investigate by means of Transmission Electron Microscopy (TEM). Although the lowest temperature deposition has twice as large a grain size as the highest temperature deposition, the losses appear to not be greatly dependent on grain size in the 0.1pm to 0.4pm range. Additionally, absorption/scattering at dangling bonds is investigated before and after a low temperature Electron-Cyclotron Resonance (ECR) hydrogenation step. After hydrogenation, we obtain the lowest reported polySi loss values at λ = 1.54μm of about 15dB/cm.
Fabrication of Polycrystalline Silicon on Glass from Fluorinated Precursors with the Aid of Atomic Hydrogen
Authors: Akasaka T. , He D. , Shimizu I.
Published: April 2011

High quality polycrystalline silicon was made on glass from fluorinated precursors by two step growth, i.e., (1) formation of seed crystals on glass by layer-by-layer(LL) technique and (2) grain-growth on the seeds. In LL technique, deposition of ultra-thin films and treatment with atomic hydrogen was repeated alternately. Columnar grains with 200 nm dia were grown epitaxy-like on the seeds by optimizing the deposition parameters under in situ observation with spectroscopic ellipsometry.
Engineering Desired Grain Orientation and Boundaries Separating Adjacent Grains in Polycrystalline Thin Films
Authors: Rauf I. A. , Sayer M.
Published: April 2011

A simple process, involving a controlled temperature gradient on the surface of the substrate during the deposition of a thin film is described. This technique is similar to zone-refining but produces chains of impurity free grains. Simultaneously, dopants and other defects are confined to the grain boundaries separating two such chains. For a small temperature gradient only preferred orientation is obtained within a chain. For medium values of temperature gradient the grain boundaries within a chain are engineered to be either on a low energy coincidence site lattice (CSL) or a twin depending on the magnitude of gradient. For high magnitudes of temperature gradient, the chain is predicted to be a single crystal strip.
Size Effects in BaTiO3 Thin Films
Authors: Lu X. M. , Zhu J. S. , Li P. , Jiang W. , Liu X. , Wang Y. N.
Published: April 2011

The size effects were studied by the measurement of optical transmittance, Raman spectra and mechanical dissipation in BaTiO3 films. The variation of energy gap, Raman peaks, Curie temperature with film thickness and grain size was observed and the possible origin was analyzed.
Annealing Characteristics of Al-Light-Rare-Earth Alloy Thin Films for Microelectronic Conductor Lines
Authors: Takayama Shinji , Tsutsui Naganori
Published: April 2011

The addition of La and Pr to Al thin films markedly decreases the grain size of the Al matrix and largely suppresses growth of thermal defects of hillocks and whiskers at high temperatures (350°C – 450°C). A large number of fine metallic compounds of Al11RE3 and/or Al3RE (RE = La and Pr) were segregated in an Al matrix, mostly at grain boundaries, after annealing at 350°C. The resistivities of the films after annealing at the above temperatures show very low values of less than 6 μωcm, without the salient formation of hillocks or whiskers on the film surfaces.
Variational Monte Carlo on a Parallel Architecture: An Application to Graphite
Authors: Menchi M. , Bosin A. , Fahy S.
Published: April 2011

We present the parallelization strategy adopted to perform Variational Quantum Monte Carlo calculations on solids on distributed-memory architectures and the issues involved in the development of the parallel programs. Results obtained by the calculation of total electronic energy of graphite using nonlocal pseudopotentials in conjunction with the Variational Monte Carlo approach will be shown.
Dynamics and Morphology of Cracks in Silicon Nitride Films: A Molecular Dynamics Study on Parallel Computers
Authors: Nakano Aiichiro , Kalia Rajiv K. , Vashishta Priya
Published: April 2011

Multiresolution molecular dynamics approach on parallel computers has been used to investigate fracture in ceramic materials. In microporous silica, critical behavior at fracture is analyzed in terms of pore percolation and kinetic roughening of fracture surfaces. Crack propagation in amorphous silicon nitride films is investigated, and a correlation between the speed of crack propagation and the morphology of fracture surfaces is observed. In crystalline silicon nitride films, temperature-assisted void formation in front of a crack tip slows down crack propagation.
Environment Sensitive Embedding Energies of Impurities, and Grain Boundary Stability in Tantalum
Authors: Krasko Genrich L.
Published: April 2011

Metalloid impurities have a very low solubility in Tantalum, and therefore prefer to segregate at the grain boundaries (GBs). In order to analyze the energetics of the impurities on the Tantalum GB, the LMTO calculations were performed on a simple 8-atom supercell emulating a typical (capped trigonal prism) GB environment. The so-called “environment-sensitive embedding energies” were calculated for Hydrogen, Boron, Carbon, Nitrogen, Oxygen, Phosphorus, and Sulphur, as a function of the electron charge density due to the host atoms at the impurity site. The calculations showed that, at the electron density typical of a GB, Carbon has the lowest energy (followed by Nitrogen and Boron) and thus would compete with the other impurities for the site on the GB, tending to displace them from the GB. The above energies were then used in a modified Finnis-Sinclair embedded atom approach for calculating the cohesive energies and the equilibrium interplanar distances in the vicinity of a (111) Σ3tilt GB plane, both for the clean GB and that with an impurity. These distances were found to oscillate, returning to the value corresponding to the equilibrium spacing between (111) planes in bulk BCC Tantalum by the 10th-12th plane off the GB. Carbon, Nitrogen and Boron somewhat dampen the deformation wave (making the oscillations less than in the clean GB), while Oxygen, Phosphorus and Sulphur result in an increase of the oscillations. The cohesive energies follow the same trend, the GB with Carbon being the most stable. Thus, Carbon, Nitrogen and Boron may be thought of as being cohesion enhancers, while Oxygen, Phosphorus and Sulphur result in decohesion effects.
Melting of Aromatic Compounds: Molecular Dynamics Simulations
Authors: Kung P. W.-C. , Books J. T. , Freeman C. M. , Levine S. M. , Vessali B. , Newsam J. M. , Klein M. L.
Published: April 2011

We have used constant pressure molecular dynamics calculations to explore the behavior at various temperatures of two molecular crystals: benzene and a brominated phenyl compound. We observed a melting transition by heating the crystals from a low temperature. In the case of benzene, we performed one heating run of about 1 ns and obtained agreement with the experimental melting point to within some 8%. We have also simulated the melting of a more complex molecular crystal that contains bromine and phenyl groups. We performed four heating runs, with different rates of heating. For total simulation times of about 100, 220, 770, and 1 I50ps, the heating runs predicted melting temperatures that differed from the experimental melting temperature by 53%, 33%, 25%, and 9% respectively.
Growth Mechanism of Si Dimer Rows on Si(001)
Authors: Yamasaki T. , Uda T. , Terakura K.
Published: April 2011

Initial processes of Si dimer row growth on Si(001) surface is studied by the first principles molecular dynamics method. We optimize several different ad-Si clusters composed of one to four atoms on the surface and estimate activation energies for some important growth processes. At lower temperatures, a metastable ad-Si dimer in the trough between substrate dimer rows attracts monomers and tends to grow into a short diluted-dimer row in the perpendicular direction to the substrate dimer rows. In high temperatures as ad-Si dimers can diffuse, a direct dimer condensation process is possible to elongate the dense-dimer rows also in the perpendicular direction.
DFT Study of the Monocyclic and Bicyclic Ring Geometries of C20
Authors: Wang Zhiqiang , Day Paul , Pachter Ruth
Published: April 2011

The monocyclic and bicyclic ring geometries of C20 are optimized using both the local density functional approximation (LDA) and gradient-corrected density functional theory (BLYP). The energy of the bicyclic ring is found to be higher than that of the monocyclic ring in both LDA and BLYP calculations. The BLYP results confirm the previous single point calculation based on Hartree-Fock geometries[1], which is in favor of the monocyclic ring geometry, while the LDA results still favor the cage geometry. LDA frequencies of both ring geometries are also presented.
A Semi-Empirical Methodology to Study Bulk Silica System
Authors: Chen Ai , Corrales L. René
Published: April 2011

A semi-empirical methodology [1,2] developed to model and simulate covalently bonded networked systems is modified to study the heteroatomic mixtures of silica. This methodology is capable of grasping the essential qualitative and quantitative features of the coupling between the electronic coordinates and the geometric structure. The methodology is used to simulate and to probe the structural and thermodynamic properties of the bulk crystalline, amorphous solid and the melt states of silica.
Triboemission And Wear Of Hydrogenated Carbon Films
Authors: Nakayama Keiji
Published: April 2011

It is suggested that perfluoropolyether lubricating oil coatings applied to the carbon overcoat film of magnetic recording layers become decomposed by electrons emitted from frictional surfaces. However, no work has at yet been reported as to triboemission of electrons from frictional carbon films.

This paper describes the behavior of triboemission of electrons and the friction coefficient during wear of sputtered hydrogenated carbon films (with various hydrogen contents on the glass substrate). The triboemission of electrons, together with friction coeficient, was measured in a frictional system of Al2O3 sliding on carbon films in a reduced dry air atmosphere. The worn surfaces of the carbon films were then observed using both a SEM and an AFM. The results showed that intense triboemission of electrons were observed during wear of hydrogenated carbon films. The electron emission intensity and friction coefficient transit from low to high with hydrogen content in the film. These results are discussed including physical properties of the carbon films such as internal stress and surface wettability.
Thermal Expansion of β-Sic, Gap and Inp
Authors: Reeber Robert R. , Wang Kai
Published: April 2011

Thermal expansion is important for predicting residual stresses in epitaxial films, composites and electronic devices as well as for providing information relevant to an understanding of interatomic potentials and the equation of state of materials. Model calculations have many assumptions, both inherent and implicit, and have difficulty accurately representing thermal expansion at high temperatures and pressures. We utilize a semi-empirical quasi-harmonic model to evaluate available data for β-silicon carbide, gallium phosphide and indium phosphide. The model allows prediction of the thermal properties of these semiconductors from near 0 K to the vicinity of their melting points. The approach, consisting of a simplified frequency spectrum with several Einstein terms, provides a convenient mathematical method where a minimum of empirical parameters represent the thermal property.
A Fetish for Gallium Arsenide
Authors: Barron Andrew R.
Published: April 2011

An overview of the development of a new dielectric material, cubic-GaS, from the synthesis of new organometallic compounds to the fabrication of a new class of gallium arsenide based transistor is presented as a representative example of the possibility that inorganic chemistry can directly effect the development of new semiconductor devices. The gallium sulfido compound [(tBu)GaS]4, readily prepared from tri-tert-butyl gallium, may be used as a precursor for the growth of GaS thin films by metal organic chemical vapor deposition (MOCVD). Photoluminescence and electronic measurements indicate that this material provides a passivation coating for GaAs. Furthermore, the insulating properties of cubic-GaS make it suitable as a the insulating gate layer in a new class of GaAs transistor: a field effect transistor with a sulfide heterojunction (FETISH).
Synthesis and Oxidation Kinetics of Sol-Gel and Sputtered Tantalum Nitride Thin Films
Authors: Kraus Gerald T. , Oldweiler Cory S. , Giannelis Emmanuel P.
Published: April 2011

Tantalum nitride thin films were produced by nitridation of sol-gel tantala thin films. The oxidation kinetics and the activation energy for oxidation were investigated and compared to those of sputtered tantalum nitride thin films. Data was gathered from in situ sheet resistance measurements taken between 550 and 720 K. Sol-gel films exhibited parabolic oxidation kinetics and had an activation energy of 1.9 eV. Sputtered films displayed quartic oxidation kinetics at lower temperatures tending toward cubic kinetics at higher temperatures and had an activation energy of 1.6 eV.
Real Time Ellipsometric Study of Boron Nitride Thin Film Growth
Authors: Bertran E. , Canillas A. , Campmany J. , Kasmi M. El , Pascual E. , Costa J. , Andújar J. L.
Published: April 2011

We present anin situstudy of the growth of boron nitride thin films by real time ellipsometry. Films were produced in a PECVD reactor by rf glow discharge decomposition of ammonia (pure) and diborane (1% in hydrogen), on Ni-Cr coated c-Si substrates placed either on the powered electrode or on the grounded electrode of the reactor. A fast phase-modulated ellipsometer performed the real time monitoring of the growth processes at 350 nm. The ellipsometric angle trayectories were obtained through an autocalibrated method, especially suitable for thein situoptical analysis of transparent thin films. We applied several thin film growth optical models (homogeneous, two-layer, surface roughness) to analyze parameters of the films such as refractive index, extinction coefficient, roughness and deposition rate. In all the cases studied, the two-layer model fits well with the ellipsometric measurements, but a more sofisticated model considering a variable refractive index could better describe these films.
Passivation of Si(100)2×l Surfaces With Elemental Sulfur
Authors: Papageorgopoulos Aris
Published: April 2011

Deposition of elemental S on Si(100)2×1 surfaces at room temperature changes the reconstructed Si(100)2×1 to its original bulk-terminated Si(100)l×l surface. Sulfur forms initially a (2×l) on the Si(100)2×l surface and subsequently a (l×1) on the Si(100)l×l. Above 1ML, sulfur is diffused into the Si bulk near the surface. The sticking coefficient of S on Si(100) surface is constant up to 2 ML. Deposition of S at RT up to 1 ML increases the work function of the surface by about.3±0.05 eV. Above I ML, as the S is diffused into the Si bulk, the work function decreases.
P-type Doping of SIC by Aluminum Implantation for Advanced Device Applications
Authors: Caleca Maria A. , Du Honghua , Flemish Joseph R. , Withrow Stephen P.
Published: April 2011

6H-SiC epitaxial layers with a background n-type dopant concentration of 1 × 1016/cm3 were hot implanted to doses ranging from 4.0 × 1013 to 1.8 × 1014 Al ions/cm2 at 65, 135, and 220 keV to achieve a box-type implant distribution to a depth of 300 nm. Electrical activation of dopants was carried out using a proximity annealing method at 1500°C in a buffer environment to retard surface degradation of the SiC samples. Measurements using atomic force microscopy illustrated the morphological stability of the SiC surface during the high-temperature annealing. Transmission line measurements showed some degree of dopant activation. Characterization of fabricated p-n junction diodes demonstrated p-type conduction in the aluminum-implanted SiC samples.
In Vivo Evaluation of Pulsed Laser Deposited Hydroxyapatite Coating for Prosthesis-Bone Bonding
Authors: Cotell C. M. , Conklin J. A. , Auyeung R. C. Y. , Wong S. S. , Klapperich C. M. , Spector M.
Published: April 2011

The bone-implant bond strength to hydroxyapatite-coated cylindrical Ti-6A1–4V implants was evaluated in torsional tests using a canine model. Experimental hydroxyapatite (HA) coatings were deposited using pulsed laser deposition (PLD) under conditions that yield highly crystalline, phase-pure coatings on Ti-6A1–4V. Commercially available plasma-sprayed (PS) HA coatings of lower crystallinity and purity served as positive controls (for bone bonding) and uncoated polished and grit-blasted Ti-6A1–4V samples as negative controls. The torsional shear strength of the PLD HA-coated specimens implanted in cancellous bone was substantially lower than that of the PS HA-coated implants, at both 2 and 6 weeks post-implantation. Within statistical limits, the shear strength of the PLD HA-coated samples was similar to that of the polished Ti-6A1–4V control implants and lower than the grit-blasted Ti-6A1–4V control implants. Examination of PLD HA-coated samples after mechanical testing revealed many areas from which the coating had detached. Previous results from laboratory solubility studies showed that (a) PLD films released less calcium and phosphate than the PS controls, (b) rates of dissolution were much lower and (c) the PLD coatings contained fewer decomposition products. Taken with these previous findings, the present results suggest that the surface morphology and/or grain size of implants may be more important for bone bonding than the phase purity of hydroxyapatite coatings. Furthermore, the local chemical environment resulting from the dissolution of the more soluble phases present in the PS HA coatings may enhance bone bonding.
Electric Field Mediated Deposition of Bioactive Polypeptides on Neural Prosthetic Devices
Authors: Buchko Christopher J. , Kozloff Kenneth M. , Sioshansi Atisa , O'shea K. Sue , Martin David C.
Published: April 2011

We have developed processing schemes for depositing three-dimensionally tailored layers of protein polymers on a variety of solid substrates. One of our goals is to create stable, biocompatible coatings on silicon devices for implantation in the central nervous system. Our research has identified several candidate coatings whose morphologies lie in the biologically significant 0.1 to 100 micrometer length scale. Using electric field mediated deposition, we are able to process polypeptides into biologically-responsive films and coatings. Quantitative analysis of the structural evolution of the coating enables us to fine-tune its morphology by varying the field strength and geometry or solution concentration. The interaction of the coated substrates with neurons and glial cells are examined in vivo and in vitro. Data collected from light optical microscopy, atomic force microscopy, transmission electron microscopy, and scanning electron microscopy provide insight about the relationship between the microstructure of these coatings and their macroscopic properties.
A Symposium Overview: Surface Properties of Implant Materials
Authors: Meyer A. E.
Published: April 2011

Like most thriving human endeavors, the field of implant materials and devices intersects with a universe of populations and professions. Nearly everyone will “get into the act” in one way or another, resulting in a variety of perspectives that have more, or less, foundation in fact and genuine concern for the patient. Even the scope of the points of view of scientists and engineers is relatively broad. One reason for the richness of the field in which we work is the increasing number of sites in the body to which implants are being delivered for the treatment of disease and replacement or augmentation of natural tissues. Another reason is the diversity of materials used in implants and diagnostic devices.
Formation of Stacked Self-Assembled InAs Quantum Dots in GaAs Matrix for Laser Applications
Authors: Ustinov V. M. , Egorov A. Yu. , Zhukov A. E , Ledentsov N. N. , Maksimov M. V. , Tsatsul'nikov A. F. , Bert N. A. , Kosogov A. O. , Kop'ev P. S. , Bimberg D. , Alferov Zh. I.
Published: April 2011

Vertically coupled InAs quantum dots have been synthesized by MBE through successive deposition of InAs dot sheets and thin GaAs spacers. The energy of ground state transition in PL spectra has been found to depend on a number of dot sheets and the spacer width. Injection laser based on vertically coupled quantum dots demonstrated lasing via the ground state of quantum dots in the entire 80K-300K temperature range. Lower threshold current density and wider range of the thermal stability of threshold current density as compared to the single sheet quantum dot laser have been observed.
Optical Properties of Ordered Gainp
Authors: Ernst P. , Geng C. , Hahn G. , Scholz F. , Schweizer H. , Zhang Yong , Mascarenhas A.
Published: April 2011

We use the information from low temperature PLE spectra measured on a set of samples with different degree of order for an investigation of the correlation between the order dependent valence band splitting and band gap reduction. The experimental values are compared with predictions from recent ab initio band structure calculations.

Moreover, we show how the low temperature optical properties of ordered GaInP2 are related to the microstructure of the material. We find a close correlation between the size of the ordered domains and the relative intensity of the intrinsic PL-peak from excitonic recombination compared with the rapidly shifting, below band-gap luminescence emission.
Spontaneous Lateral Modulations in InAlAs Buffer Layers Grown by MBE on InP Substrates
Authors: Peiró F. , Cornet A. , Ferrer J. C. , Morante J. R. , Halkias G. , Georgakilas A.
Published: April 2011

Transmission Electron Microscopy (TEM) and X-ray Diffraction (XRD) have been used to analyze the spontaneous appearance of lateral composition modulations in InyAl1−yAs (yIn.≅ 50%) buffer layers of single quantum well structures grown by molecular beam epitaxy on exact and vicinal (100) InP substrates, at growth temperatures in the range of 530°C–580°C. The influence of the growth temperature, substrate misorientation and epilayer mismatch on the InAlAs lateral modulation is discussed. The development of a self-induced quantum-wire like morphology in the In0.53Ga0.47As single quantum wells grown over the modulated buffers is also commented on.
Dependence of Optical Properties of Semiconductor Alloys on Long Range Order, Strain and Pressure
Authors: Wei Su-Huai , Franceschetti Alberto , Zunger Alex
Published: April 2011

Many III – V semiconductor alloys exhibit spontaneous (111) alternate monolayer ordering when grown from the vapor phase. We have studied theoretically the ordering induced changes in the optical properties of the semiconductor alloys. We describe (i) the change of the band gap ΔE9, and the valence band splitting ΔE12 as functions of the long range order parameter η, (ii) the consequence of coexistence of (001) epitaxial strain and (111) chemical ordering on the optical properties, (iii) optical anisotropy and spin polarization effects in the ordered alloy, (iv) theory of reflectance-difference spectroscopy in ordered alloy, and (v) the ordering-induced changes on high energy E1 and E2 transitions. Specific, experimentally testable predictions are listed in the summary section.
Atomic Ordering of III-V Alloy Semiconductors Grown on (110)InP and its Influence on Electron Mobility
Authors: Ueda O.
Published: April 2011

Atomic ordering in InGaAs, InAlAs, and GaAsSb crystals grown on (110) InP substrates by molecular beam epitaxy, has been studied by transmission electron microscopy. In the electron diffraction pattern from these crystals, superstructure spots associated with CuAu-I type ordered structure are found. When the tilting angle of the substrates increases, the ordering becomes stronger. The ordering is also stronger in crystals grown on substrates tilted toward the <001> or the <001> direction than those on substrates tilted toward the <110> direction. From these results, one can conclude that atomic steps on the growth surface play an important role in the formation of ordered structures. The ordering becomes stronger when the growth temperature increases. In high resolution images of the crystal, doubling in periodicity of 220 and 200 lattice fringes is found, which is associated with CuAu-I type ordered structure. Moreover, anti-phase boundaries are very often observed in the ordered regions. It is also found that ordering is not perfect, and that ordered regions are plate-like microdomains lying on planes slightly tilted from the (110) plane. On the basis of these results together with considerations of the atomic arrangement of the ordered structure and surface reconstruction on the (110) plane, we propose four possible models for the ordering. Through precise evaluation of these models, two models are selected as the most probable ones: these involve formation of the ordered structures on surfaces dominated by two monolayer steps. These models have been experimentally proven by the analyses of electron diffraction patterns from different InGaAs crystals grown on different growth surfaces. We have fabricated InGaAs/N-InAlAs heterostructures with a strongly ordered InGaAs channel layer. The measured two-dimensional electron gas mobilities from these structures are found to be 100,000 cm2/Vs in the <110> direction and 161,000 cm2/Vs in the <110> direction with a sheet electron concentration (Ns) of 9.5 × 1011 cm−2 at 6 K. The latter mobility is much higher than both calculated alloy scattering limited mobility and the best experimental results for lattice-matched InGaAs/N-InA]As systems. The mobility enhancement in the <110> direction is considered to be achieved by the suppression of alloy scattering due to the occurrence of ordering in the InGaAs channel layer.