bluuebunny/crossref_metadata_2025 huggingface dataset:
Surfactant-Mediated Growth of SiGe/Si Quantum-Well Structures Studied by Photoluminescence Technique and Secondary Ion Mass Spectrometry
Authors: Nilsson S. , Zeindl H. P. , Krüger D. , Klatt J. , Kurps R.
Published: April 2011

In this investigation, surfactant-mediated growth of SiGe/Si single quantum-well structures is studied by photoluminescence and secondary ion mass spectrometry. The samples were grown by molecular-beam epitaxy and Sb was used as a surfactant. The photon energy of the SiGe-related near-band-edge photoluminescence was used to probe the action of Sb as a surfactant to promote two-dimensional growth and to reduce segregation of Ge during growth. First, the "growth-temperature window" at which Sb acts preferentially as a surfactant was determined. Then, at this optimized temperature of 700°C, the influence of different Sb coverages was investigated and it was found that 0.5 monolayer was a sufficient coverage to obtain full surfactant action. Ge concentration depth profiles obtained by secondary ion mass spectrometry confirmed the effect of surfactant-mediated growth and, in addition, the unintentional incorporation of the Sb surfactant during growth was determined quantitatively. In a final experiment the effect of deposition of Sb on either the lower or the upper heterointerface is addressed.
The use of Rheed Intensities for the Quantitative Characterization of Surfaces
Authors: Ma Y. , Lordi S. , Eades J. A.
Published: April 2011

RHEED is an important method for the in situ characterization of surfaces. Until recently, this characterization has not used the intensities of RHEED reflections. Improved methods of calculation have made it possible to simulate RHEED patterns from bulk-terminated, reconstructed and stepped surfaces. From the comparison between simulated and experimental patterns, it is now possible to refine the positions of atoms in surfaces and to determine the density of steps on surfaces (well almost).
Orientation Dependence of CdTe/Si Grown by MBE
Authors: Almeida L. A. , Chen Y. P. , Faurie J. P. , Smith David J. , Tsen S.-C. Y. , Sivananthan S.
Published: April 2011

In this study CdTe (111)B was grown by molecular beam epitaxy on vicinal Si(001) substrates, with a variety of substrate tilt angles (θ), and tilt directions (φ) relative to [110]. Layer quality, and content of double-domain and microtwin defects were evaluated by double crystal rocking curve (DCRC) full width at half maximum (FWHM) and x-ray diffraction, respectively. Transmission electron microscopy (TEM) was used to study interface quality and the nature of structural defects as a function of epilayer thickness. In the present investigation, substrate preparation and growth conditions, particularly initiation conditions, are correlated with Si (001) tilt. It has been found that oxide desorption processes can depend strongly on θ, especially for larger values of θ (> 4°). Currently, we routinely produce single domain, twin-free CdTe(111)B epilayers on vicinal Si (001) substrates.
Defect Structures in GaP/Si
Authors: Samavedam Srikanth B. , Kvam Eric P. , Ford Greg , Wessels Bruce W. , Chin T. P. , Woodall Jerry M.
Published: April 2011

The heteroepitaxial growth of gallium phosphide on silicon (GaP/Si) is a useful step towards integration of III-V based devices onto silicon. GaP layers grown on silicon substrates of different orientations using metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) were characterized using transmission electron microscopy (TEM) with an attempt to understand the epilayer growth characteristics. Despite the fact that the GaP/Si system has a low misfit (≈0.4 %), a high density of crystal stacking defects was commonly observed. Inversion domain boundaries (IDBs) were another common defect observed in regions where the fault density was reduced.
Formation of Interfacial Dislocations in Hetero-Epitaxial Layers Grown in Two-Dimensional Mode
Authors: Oktyabrsky S. , Narayan J.
Published: April 2011

High-resolution transmission electron microscopy has been used to study formation of interfacial defects related to misfit strain accommodation in Ge/Si heterostructures (mismatch 4%) grown in the two-dimensional mode. Special emphasis is placed on the conditions leading to a two-dimensional (layer-by-layer) growth mode. We discuss general features of a dislocation tangle resulted from glide-limited plastic relaxation, typical for highly mismatched (001)-diamond and zinc-blende heterostructures. The evolution of the dislocation network as a function of film thickness and thermal annealing is controlled by growth instabilities and dislocation interactions. The observed correlation in distribution of parallel misfit dislocations including pairing (at <2 nm) of misfit segments from intersecting glide planes and rearrangements in a nonequilibrium dislocation network driven by elastic interaction between 60° dislocation segments in the almost relaxed heterostructures are discussed in detail. Pairing of the 60° glide dislocations results either in their combination to form pure edge 90° dislocations or in the dissociation into partials. We propose and experimentally verify a model for the latter process involving the formation of extrinsic stacking faults in the heterolayers under compressive strain.
Twinning in epilayers of CdTe on <211> Si: Influence of ZnTe buffer layer and substrate misorientation
Authors: Gray A. , Dhar N.K. , Clark W. , Charlton P. , Dinan J.H. , Segnan R. , Mayo W.E.
Published: April 2011

X-ray diffraction spectra of CdTe epilayers grown with and without ZnTe buffer layers on <211> Si substrates by molecular beam epitaxy consist of 422 and 331 reflections. We interpret these as evidence for the existence of twins within the volume of a <211> oriented epilayer and show that twin volume is dependent on the ZnTe buffer layer and substrate misorientation.
A One-Dimensional Stochastic Model of Diamond Growth
Authors: Frenklach Michael
Published: April 2011

(1+1)-dimensional stochastic simulations were performed representing elementary processes underlying chemical vapor deposition of diamond films. The results exhibit different growth regimes, depending on the values assigned to kinetic rates, and generally support the critical role of surface migration suggested earlier for the growth of diamond.
Characterization of Particles Synthesized by Aerosol Processes for Various Pb:Si Molar Feed Ratios
Authors: Owens Timothy M. , Biswas Pratim
Published: April 2011

A detailed characterization was performed of the particles produced under various Pb:Si molar feed ratios in a flow reactor at a maximum temperature of 1000 °C. The silica particles formed in the high temperature region coagulated and only partially coalesced to form large agglomerate structures of high specific surface area. For a lead only feed, the resulting particles were hydrocerussite with small but detectable amounts of massicot. As the silica precursor was inlet in excess amounts (Pb:Si ≤ 1:12), the crystalline lead compounds disappeared and amorphous lead-silica complexes predominated. The particle morphology also changed from cylindrical, polygonal and spherical shapes to large agglomerate structures composed of several size modes of primary particles. At Pb:Si molar feed ratios of 1:12 and 1:29, the particles making up the chain-like agglomerate structure were primarily spherical with larger lead silicate spherical particles (≈ 0.5 μm) attached to the agglomerate. The lead was found to be distributed throughout the large agglomerate structures.
Determination of the Chemical Potentials of Highly Supersaturated Cu-Co Alloys
Authors: Gärtner F. , Busch R. , Haasen P. , Bormann R.
Published: April 2011

With the CALPHAD approach based on the regular solution model, the thermodynamic functions of the Cu-Co system can be extrapolated to compositions where the homogeneous fee solid solution is thermodynamically unstable. Up to now it has been unclear how reliable such extrapolations are in view of the driving forces for decomposition and whether the Gibbs energy is well described by the model in the unstable region. Therefore, electromotive force measurements were performed to determine directly the chemical potentials of highly supersaturated Cu-Co alloys. Under the prerequisite that time scales of the electrochemical exchange reaction are fast compared with the kinetics of decomposition, the thermodynamics of the alloys can be investigated quasi statically. Within the experimental accuracy the results for both a metastable and an unstable alloy agree well with the calculated chemical potentials. In addition, the experiments demonstrate that the electrochemical cell reaction is reversible upon small external concentration fluctuations of the investigated alloys. Possible reasons for this unexpected stability are discussed taking the kinetics of the decomposition into account.
Diffusion Enhancement in Glassy Metals Under Non-Equilibrium Conditions
Authors: Bellini S. , Mazzone G. , Montone A. , Vittori-antisari Enea M. , Casaccia C.R.
Published: April 2011

The diffusion properties of a Ni-Zr metallic glass formed at the interface of a bulk diffusion couple have been studied in conditions far from a fully relaxed state. The growth kinetics of the interface film have been enhanced by both plastic deformation and high energy electron irradiation. Different results have been obtained in the two cases, since in the first case the film grows exponentially with time, while in the second case the usual square root dependence on time is observed. This behaviour has been interpreted as a consequence of the annihilation kinetics of the excess free volume introduced in the glass by the above methods. Two different mechanisms of free volume annihilation , namely exchange with a crystal vacancy at the glass-crystal interface and structural relaxation in the bulk glassy phase have been considered to be operative so that the nature of the growth kinetics has been found to depend on the mechanism predominant in each experimental condition.
Effect of In-Situ Formation of Nanoscale γ-Al2O3 and AlN on Thermal Stability of Cryomilled Nanocrystalline Fe
Authors: Perez R.J. , Huang B. , Lavernia E.J.
Published: April 2011

Cryogenic attritor milling (cryomilling) is used to synthesize nanocrystalline Fe-10wt.%Al powders. Following consolidation in a rigid die at 823 K and heat treatment for 1 hour at 1223 K, an average grain size of 16±7 nm is maintained. This level of thermal stability is shown to exceed that of pure Fe processed under identical conditions. The significant increase in thermal stability is attributed primarily to the presence of nanometer-scale γ-Al2O3 and AIN dispersoids formed during cryomilling and subsequent heat treatment.
Synthesis of Granular Systems by Cluster Beam Deposition
Authors: Pollack Margolita , Herr Ulrich , Averback R.S.
Published: April 2011

Granular films were prepared by simultaneous cluster beam and physical vapor deposition. The clusters are formed by high pressure sputtering and have an average size between 2 and 5 nm. This method makes it possible to control the granule size and concentration, which is not possible using precipitation synthesis of granular alloys. Most importantly, a diverse set of granule/matrix systems are accessible by this system. The method is illustrated for systems of interest for giant magneto-resistance applications: Co/Cu (miscible), and Co/Cr (immiscible.) Synthesis of films with isolated embedded clusters of more complex structures, e.g. intermetallics Ni3Al and Cu3Au, are also discussed.
Nanostructured Silver Particles Embedded in a Silica Matrix
Authors: Lee M. H. , Dobson P. J. , Cantor B.
Published: April 2011

A series of different nano-sized silver particles has been prepared using r.f. magnetic plasma co-sputtering apparatus. A combination of high-resolution lattice images and optical diffractograms has been used to characterize the atomic structures during nucleation and growth of silver particles embedded in a silica matrix. For particles with diameter of 2 ∼ 5 nm, most have single crystal structures, but some have singly twinned morphologies and a few have multiply twinned morphologies. In those particles, there are many defects such as dislocations, stacking faults, twin boundaries and lattice fringe bends. For particles with diameter of 5 ∼ 9 nm, we can detect some single crystals, singly twinned particles and multiply twinned particles including decahedral and icosahedral particles. Anomalous meta-stable structured particles with 90° cross {111} lattice fringes have been also found. For particles with diameters of larger than ∼ 9 nm, most have multiple lamellar twins and sometimes there are macroscopically irregular shaped particles.
Titanium Silicidation Induced Point Defects in SI
Authors: Herner S. B. , Gossmann H.-J. , Jones K. S.
Published: April 2011

Antimony and B-doping superlattices were employed to study the population of Si native point defects after the formation of a TiSi2 film at 800 to 890°C. Formation of a TiSi2 film at these temperatures results in interface roughness on the order of 300 Å (root mean square). Artifacts in SIMS data arising from sputtering through such a rough interface were avoided by chemomechanical polishing of the underlying Si after chemically etching off the TiSi2 film. The resulting Si surface had a roughness of 0.7 Å. Enhancement in the diffusion of Sb and retardation in the diffusion of B over control samples without a TiSi2 film indicate a vacancy supersaturation and an interstitial undersaturation.
Beem and UHV-TEM Studies of PtSi/Si(001)
Authors: Kavanagh K. L. , Morgan B. A. , Talin A. A. , Ring K. M. , Williams R. S. , Reuter M. C. , Tromp R. M.
Published: April 2011

Recent work using ballistic electron emission microscopy (BEEM) and ultra-high vacuum transmission electron microscopy (UHV-TEM) to study structure-property correlations of PtSi/Si(001) contacts is described. The spatial uniformity of the Schottky barrier height (SBH) for diodes grown at room temperature and 350°C was compared with the interface microstructure using TEM. Narrow SBH distributions with standard deviations of 7–12 meV were obtained independent of the deposition system and temperature. Analysis of the distributions compared well with standard current-voltage and capacitance-voltage measurements on the same diode. These low temperature depositions produced fine grained PtSi films (10–20 nm) with an interface roughness dependent on the cleaning process. Probable orientation effects on the SBH distribution were apparently screened out because of the low doping of the substrates that were used. As the deposition temperature is increased the PtSi tends towards polycrystalline epitaxy with island growth beginning at 700°C. Grains are rectangular and develop a large shape anisotropy of great interest for future investigations.
Electrical and Structural Characterization of Ti Contacts to Si0.89 Ge0.11/Si(001) Epilayers
Authors: Lyakas M. , Beregovsky M. , Moskowitz I. , Eizenberg M.
Published: April 2011

The properties of thin (350 Å) Ti layers deposited on Si0.89Ge0.11 layers epitaxially grown on Si(001) were studied as a function of isochronal (30 min.) thermal treatments in the temperature range Ta=550–800°C. Both as-deposited and annealed at Ta up to 750°C Schottky diodes revealed near-ideal I–V and C–V characteristics with the same flat-band barrier height eV. The results indicate that at these Ta the Fermi level is pinned with respect to the conduction band.

Annealing at 800°C resulted in an improvement of the Schottky diodes quality and a drop in and the series resistance Rs of the contacts. The values of the ideality factor n and ( measured were 1.03±0.02 and 0.56±0.007 eV, correspondingly. The electrical parameters of these metal/semiconductor contacts were correlated with the dynamics of interfacial reactions due to the applied heat-treatments.
Barrier Height Control and X-Ray Diffraction Study of Metal on Si1−Gex/Si Grown by Very Low Pressure CVD
Authors: Shiâ Z. Q. , He L. , Zheng Y.
Published: April 2011

The potential application of epitaxial Si1−xGex, on Si in electronic and optoelectronic devices has led to an increased study of metal - Si1−xGex interaction and barrier height control technique. In this paper, we report the epitaxial growth of Si1−xGex on Si and the Schottky barrier formation processing. The Si1−xGex (x=0.17 and 0.20) layers were grown by rapid radiant heating, very low pressure chemical vapor deposition (VLPCVD). The crystal structure and epitaxial nature of the Si1−xGex, layers were studied by X-Ray diffraction. The value of full width at half maximum (FWHM) was found to be 0.34° for the as grown Si1−xGex (400) peak. The metal-Si1−xGex/Si Schottky diodes were formed by depositing Pd on Si1−xGex/Si at room temperature (RT=300K) and low temperature (LT=77K). The Schottky barrier heights and current transport mechanisms were determined by current-voltage-temperature (I-V-T) measurements. The interface property of Pd/ Si1−xGex/Si were studied as a function of metal deposition and post annealing temperatures.
Iridium Silicides Formed by RTA in Vacuum
Authors: Rodríguez T. , Almendra A. , Botella M. , Silva M. F. Da , Soares J. C. , Wolters H. , Ballesteros C.
Published: April 2011

The growth kinetics of the iridium silicides, IrSi and IrSi1.75 formed by RTA in vacuum has been characterized as a function of the processing temperature, up to 675°C, and annealing time for iridium films of different thickness. The sample temperature was measured using as a reference the SPE growth rate of amorphized silicon processed in the same RTA conditions. The samples were characterized by RBS, HREM, XRD and microdiffraction. RBS spectra show that the transition region between IrSi and IrSi1.75 is slightly gradual, while the interface between the IrSi and the iridium metal is sharp. The layer thickness of each silicide as a function of temperature and time was obtained from the RBS spectra. The activation energy of the diffusion coeficient of Si in IrSi and IrSi1.75 are about 1.3 eV and 3.9 eV respectively. Phase kinetics are the same for both thin and thick samples processed at the same temperature and annealing time when unreacted iridium remains in the film. However, the phase evolution is different when the annealing time is long enough to convert all the iridium metal into silicide in the thin film sample but not in the thick one. In this case, IrSil.75 is the only phase found if the annealing time is long enough. Silicides formed at 525°C consist mainly of polycrystalline IrSi, with grain size between 3 and 10 nm, and some isolated grains of IrSi 1.75. The interface between the IrSi and the Si is flat, very sharp, and well oriented grains grow from it. In order to avoid the IrSi1 75 formation and to obtain IrSi layers at reasonable rates, an RTA process at 500°C is proposed.
Study of the Hall-Petch Dependence in an Annealed Nanocrystalline Iron Thin Film
Authors: Savader J. B. , Scanlon M. R. , Cammarata R. C. , Smith D. T. , Hayzelden C.
Published: April 2011

The hardness dependence on grain size of a nanocrystalline iron thin film prepared by sputtering was investigated. The as-deposited film had an initial grain size of 11.4 nm and sections of the film were annealed in a reducing atmosphere furnace to increase the grain size up to 31.5 nm. Hardness was measured by nanoindentation for different grain sizes, and a Hall-Petch plot of hardness versus reciprocal square root of grain size was obtained. This Hall-Petch plot gave a positive slope that was about a factor of seven smaller than the slope obtained from ball-milled iron with grain sizes between 6.0 and 8.5 nm [8].
Thermodynamics of Hole and Hillock Growth in Thin Films
Authors: Srolovitz D. J. , Yang W. , Goldiner M. G.
Published: April 2011

This paper provides a thermodynamic analysis of hole and hillock growth in stressed thin films in simplified geometries. Two limiting cases were considered. In the first, matter is transported to (from) the growing hillock (hole) from (to) the surface of the film. In the second case, the material transported to (from) the growing hillock (hole) comes from (goes to) the interior of the film in the vicinity of the hillock (hole). When bulk relaxation is dominant, hillock growth is favored by a compressive stress in the film and hole growth is enhanced when the film is in tension. This is the commonly observed situation. On the other hand, when surface relaxation effects dominate, hillocks and holes can grow in either tension or compression.
Effects Of Impurities on the Crystallization and Grain Growth of Polycrystalline Si Films
Authors: Paik Jae- Cheol , Park Kwang-Hyun , Choi Woon , Nam Seung-Eui , Kim Hyoung-June
Published: April 2011

Device-quality poly-Si films are used as the active material of poly-Si thin film transistors. Poly-Si films with high crystallinity, which have large grains and/or low intragranular defects, lead to a high device performance. The presence of O or C impurities in the deposited Si films can greatly affect the behavior of crystallization and grain growth of these films, and their resulting crystallinities. A substantial amount of O or C can be introduced in the films during deposition from residual gases in the deposition chamber. Control of base pressure during Si deposition, therefore, will be an important process parameter determining the crystallinity of these films. The effects of the base pressure on the crystallization and grain growth of deposited Si films were investigated using a high vacuum Chemical Vapor Deposition system. Lower base pressure decreases the deposition temperature for the amorphous/crystalline transition of as-deposited films. Crystallization of amorphously deposited films is also affected by base pressure. The kinetics of crystallization and crystallinities of poly-Si films after crystallization are substantially increased by reducing base pressure. Enhanced crystallization kinetics and film crystallinities can be explained by reduced inclusions of O or C impurities in Si films, thus enhancing the atomic mobility. The improved film crystallinity of poly-Si films leads to higher device performance of poly-Si TFT's.
Improved Spray-Pyrolysis Deposition System for Polycrystalline Conductive SnO2 Thin Films
Authors: Foglietti V. , Galbato A. , Bearzotti A. , Galloppa A. , Maltese P.
Published: April 2011

Bismuth-doped tin oxide films have been deposited using a new spray-pyrolysis deposition system. Among the properties are the possibility of depositing thin films over very large areas with good uniformity and the relatively small cost compared with other types of deposition systems. The apparatus is divided in two major parts, the aerosol chamber and the deposition chamber. The deposition chamber consists of a flat rotating hot plate that can house substrates up to 18 cm in diameter. Deposition temperature ranges from 290 °C to 350 °C. The SEM pictures show a phase transition from amorphous to polycrystalline which also strongly affects the transport properties of the film. The high values of the surface resistivity obtained are consistent with those ones required to develop an analog grey scale ferroelectric liquid crystal display. The sample shows a 20% uniformity in the value of surface resistivity over an area of 140 cm2, and the results obtained until now show a satisfactory reproducibility. Bismuth and Tin concentration versus deposition temperature have been determined by ICP mass spectrometry. Our work is supported by the ESPRIT project “PROFELICITA”.
Structure and Properties of W and Mo-Subcontact Layers Influenced by Thermal Annealing
Authors: Gesheva K. A. , Stoyanov G. , Harizanova A. , Stefanov R.
Published: April 2011

One of the most important problems to solve in order to improve the CdTe solar cell performance is the deposition of stable ohmic contacts. W and Mo with their high work function, high temperature stability and good mechanical properties are prospective materials for contacts to p-CdTe. This paper presents data for the electrophysical characteristics of W or Mo/CdTe and W or Mo/ CdS-CdTe heterostructure systems. W and Mo films are deposited by a W(CO)6 based CVD process at atmospheric pressure and low temperatures (250–300)°C. We emphasize the improvment of the contact properties by developing a transitional layer heavily doped with acceptors. The defects arrangement in the layer should promote the diffusion of such impurities as Cu, P, As etc. A proper balance between the impurities and the defects could be achieved by suitable thermal treatment. Rapid thermal or traditional annealing in different gas atmospheres – N2, Ar or air were applied, followed by chemical or electrochemical treatment. Electrical characteristics measurements and structural studies were performed for CdTe layers as well as for the SnO2 - CdS - CdTe structures
Electron Transport in Highly Textured Metal Films Grown by Partially Ionized Beam Deposition
Authors: Soss S. R. , Gittleman B. , Mello K. E. , Lu T.-M. , Lee S. L.
Published: April 2011

In principle, the resistivity of bulk FCC cubic materials should not depend on the orientation due to the fact that the conductivity tensor is single valued. However, we show that this conclusion is not valid for thin films. Deposition of highly oriented Al, Ag, and Cu films on amorphous substrates using the partially ionized beam (PIB) technique exhibit a resistivity which is strongly correlated with the texture, i.e., the tighter the texture, the lower the film resistivity. We model the film as an array of grains whose grain boundaries can be considered as delta function potentials for electron scattering and the strength of the potentials can be calculated from the measured resistivity of the films. On the other hand, the fiber texture distribution of the the films is obtained from X-ray pole figure measurements, and Monte-Carlo simulations are then performed using this data to determine the average dislocation density at the grain boundaries due to the grain to grain crystallographic mismatch. We show that the transmittance coefficient for electron scattering, and therefore the film resistivity, is a monotonically increasing function of the average dislocation density. We therefore conclude that the structure of grain boundaries in a thin film provides the necessary mechanism by which the resistivity of an FCC cubic metal can depend on the texture.
Characterization of Copper Diffusion into Al and Al-1% Si Polycrystalline Thin Films
Authors: Walsh L. H. , Ramseyer G. O. , Beasock J. V. , Helbig H. F. , MacWilliams K. P.
Published: April 2011

Al and AI-1%Si 900 nanometer thin films were deposited on 100 nanometer Cu films on thermally oxidized (100 nanometer) Si wafers. The Al and Cu films were deposited using evaporation techniques, and the Al-1%Si film was sputter deposited. Different thin film samples were heated in vacuum to 175, 250, 330 and 400°C for 1 hour. The various annealed and original samples were compared using surface morphology, as well as composition versus sample depth. Differences between the Al and Al-1%Si samples are discussed.
Independent Optimization of Nucleation and Growth Processes of Titanium-Alloy Underlayers for Cobalt-Alloy Perpendicular Recording Media
Authors: Nolan T. P. , Hirayama Y. , Futamoto M.
Published: April 2011

Magnetic recording performance of cobalt-alloy perpendicular media depends upon microstructural features including preferred c-axis out-of-plane orientation, grain size and grain separation. These depend on the titanium-alloy underlayer surface structure, which in turn depends on film nucleation and growth. The Ti-alloy nucleation and growth stages are independently controlled by changing argon pressure during sputter deposition. Ti and Ti90Cr10 underlayers have qualitatively different microstructures, but in each case a two-step deposition process produces a better combination of separated magnetic layer grains and strong preferred orientation than constant argon pressure underlayer media. Correspondingly, media magnetic performance is improved over single pressure depositions. Observed microstructure and magnetic properties are consistent with simple growth models.
Large Scale Molecular Dynamics Study of Amorphous Carbon and Graphite on Parallel Machines
Authors: Yu J. , Omeltchenko Andrey , Kalia Rajiv K. , Vashishta Priya , Brenner Donald W.
Published: April 2011

Using a reactive empirical bond-order potential (REBOP) model for hydrocarbons1, large scale molecular dynamics simulations of carbon systems are carried out on parallel machines. Structural and dynamical correlations of amorphous carbon at various densities are studied. The calculated structure factor agrees well with neutron scattering experiments and the results of tightbinding molecular dynamics simulations. The dynamic behavior of crack propagation through graphite sheet is also investigated with the molecular-dynamics method. Effects of external stress and initial notch shape on crack propagation in graphite are studied. It is found that graphite sheet fractures in a cleavage-like or branching manners depending on the orientations of the graphite sheet with respect to the external stress. The roughness of crack surfaces is analyzed. Two roughness exponents are observed in two different regions.
Ab-Initio Molecular Dynamics of Organic Compounds on a Massively Parallel Computer
Authors: Gygi François
Published: April 2011

We present results of ab-initio electronic structure calculations and molecular dynamics simulations of organic molecules carried out using adaptive curvilinear coordinates, within the local density approximation of density functional theory. This approach allows for an accurate treatment of first-row elements, which makes it particularly suitable for investigations of organic compounds. A recent formulation of this method relies on a real-space approach which combines the advantages of finite-difference methods with the accuracy of adaptive coordinates, and is well suited for implementation on massively parallel computers. We used molecular dynamics simulations to obtain the fully relaxed structures of nitrosyl fluoride (FNO), and of the aromatic heterocycles furan and pyrrole. The equilibrium geometries obtained show excellent agreement with experimental data. The harmonic vibrational frequencies of furan and pyrrole were calculated by diagonalization of their dynamical matrix and are found to agree with experimental data within an rms error of 25 cm-1 and 28 cm-1 for furan and pyrrole respectively. This accuracy is comparable to that attained for smaller organic molecules using elaborate quantum chemistry methods.
Electronic Structure Calculations on a Real-Space Mesh with Multigrid Acceleration
Authors: Sullivan D. J. , Briggs E. L. , Brabec C. J. , Bernholc J.
Published: April 2011

We have developed a set of techniques for performing large scale ab initio calculations using multigrid accelerations and a real-space grid as a basis. The multigrid methods permit efficient calculations on ill-conditioned systems with long length scales or high energy cutoffs. We discuss the design of pseudopotentials for real-space grids, and the computation of ionic forces. The technique has been applied to several systems, including an isolated C60 molecule, the wurtzite phase of GaN, a 64-atom cell of GaN with the Ga d-states in valence, and a 443-atom protein. The method has been implemented on both vector and parallel architectures. We also discuss ongoing work on O(N) implementations and solvated biomolecules.
Atomic and Electronic Structure of Germanium Clusters at Finite Temperature Using Finite Difference Methods
Authors: Chelikowsky James R. , ÖĞüt Serdar , Jingc X. , Wu K. , Stathopoulos A. , Saad Y.
Published: April 2011

Determining the electronic and structural properties of semiconductor clusters is one of the outstanding problems in materials science. The existence of numerous structures with nearly identical energies makes it very difficult to determine a realistic ground state structure. Moreover, even if an effective procedure can be devised to predict the ground state structure, questions can arise about the relevancy of the structure at finite temperatures. Kinetic effects and non-equilibrium structures may dominate the structural configurations present in clusters created under laboratory conditions. We illustrate theoretical techniques for predicting the structure and electronic properties of small germanium clusters. Spefically, we illustate that the detailed agreement between theoretical and experimental features can be exploited to identify the relevant isomers present under experimental conditions.
Prediction of a Very Hard Triclinic Form of Diamond
Authors: Benedek G. , Facchinetti M. , Miglio L. , Serra S.
Published: April 2011

In a theoretical search for new hypotetical sp3-bonded carbon structure containing five-fold rings as a possible result of fullerene transformation under pressure, we have found a triclinic form of diamond with 16 atoms per unit cell which we called tcl-16. We have calculated the ground state structure, the cohesive energy, the bulk modulus and the electronic density of states by means of tight binding molecular dynamics (TBMD). Finally we have compared the phonon spectra at F to existing Raman data for a non-cubic- phase of diamond.
First-Principles Calculation of the Structure of Mercury
Authors: Mehl Michael J.
Published: April 2011

Mercury has perhaps the strangest behavior of any of the metals. Although the other metals in column IIB have an hcp ground state, mercury's ground state is the body centered tetragonal βHg phase. The most common phase of mercury is the rhombohedral αHg phase, which is stable from 79K to the melting point and meta-stable below 79K. Another rhombohedral phase, γ71Hg, is believed to exist at low temperatures. First-principles calculations are used to study the energetics of the various phases of mercury. Even when partial spin-orbit effects are included, the calculations indicate that the hexagonal close packed structure is the ground state. It is suggested that a better treatment of the spinorbit interaction might alter this result.
Ab Initio Calculations for SiC-Al Interfaces by Conjugate-Gradient Techniques
Authors: Kohyama Masanori
Published: April 2011

Conjugate-gradient (CG) techniques for ab initio calculations of large complex systems have been examined for SIC-Al interfaces. The CG method by Bylander, Kleinman and Lee is more efficient than the Teter-Payne-Allan (TPA) method, the modified TPA method and the block Davidson method, although the TPA method is efficient for SiC surfaces. From the relaxed configurations, we have found strong attractive interactions between C and Al atoms, which should play a favorable role for adhesion between SiC and Al.
EELS Studies of B2-Type Transition Metal Aluminides: Experiment and Theory
Authors: Bottonm G. A. , Guo G. Y. , Temmerman W. M. , Szotek Z. , Humphreys C. J. , Wango Yang , Stocks G. M. , Nicholson D. M. C. , Shelton W. A.
Published: April 2011

The electronic structure and bonding character of intermetallic alloys are investigated by a combination of electron energy loss spectroscopy (EELS) experiments and ab initio electronic structure calculations. A detailed comparison is made between experimental spectra and calculations. The changes in electronic structure within a transition metal alurninide series and also due to alloying are studied using EELS spectra. The Korringa-Kohn-Rostoker coherent-potential-approximation method and large supercell models are used to investigate changes in composition and the effect of dopants on the electronic structure.
The Magnetic Structure of Cu0.2Ni0.8 Alloys
Authors: Wang Yang , Stocks G. M. , Nicholson D. M. C. , Shelton W. A. , Szotek Z. , Temmerman W. M.
Published: April 2011

The locally self-consistent multiple scattering method is applied to ab initio spin-polarized local density approximation calculations for ferromagnetic CuNi alloys. The samples used to model the alloys are constructed with experimentally measured short range order parameters. These large cell calculations are performed using the Intel Paragon XP/S massively parallel processing supercomputer. The neutron magnetic diffuse scattering cross sections are calculated and compared with experiment. The dependence of magnetic moment formation on the local environment is discussed.
Derivation of Interatomic Potentials by Inversion of Ab Initio Cohesive Energy Curves
Authors: Bazant M. Z. , Kaxiras Efthimios
Published: April 2011

An analytic procedure for inverting ab initio cohesive energy vs. volume curves to obtain a parameter-free interatomic potential is presented. The procedure determines the radial functions in a cluster potential, assuming an angular dependence. The method is a nonlinear generalization of the ab initio pair potential formula of Carlsson, Gelatt and Ehrenreich to higher orders of cluster expansion [1]. We demonstrate our method by deriving an ab initio cluster potential for silicon with the Stillinger-Weber angular dependence [2]. A novel property of the potential is that bond-bending forces are not monotonic in the bond length, as is the case with most empirical potentials, but rather are peaked at the first neighbor distance. The validity of the inverted potential is discussed along with its use in the development of new interatomic potentials.
Mixed Approach to Incorporate Self-Consistency into Order-N LCAO Methods
Authors: Ordejon Pablo , Artacho E. , Soler J. M.
Published: April 2011

We present a method for selfconsistent Density Functional Theory calculations in which the effort required is proportional to the size of the system, thus allowing the aplication to problems with a very large size. The method is based on the LCAO approximation, and uses a mixed approach to obtain the Hamiltonian integrals between atomic orbitals with Order-N effort. We show the performance and the convergence properties of the method in several silicon and carbon systems, and in a DNA periodic chain.
Fast Algorithms for Composite Materials
Authors: Greengard L.
Published: April 2011

We briefly review recently developed fast algorithms for the evaluation of electrostatic fields in composite materials consisting of a collection of piecewise homogeneous inclusions embedded in a uniform background. These algorithms are based on combining a suitable boundary integral equation with the fast multipole method and a conjugate gradient-like iterative method. The CPU time required grows linearly with the number of points in the discretization of the interface between the inclusions and the background material, bringing large-scale calculations within practical reach.
Isotropic Md Simulations Of Dynamic Brittle Fracture
Authors: Español Pep , Rubio Miguel A. , Zúñiga Ignacio
Published: April 2011

We present results obtained by molecular dynamics simulations on the propagation of fast cracks in triangular 2D lattices. Our aim is to simulate Mode I fracture of brittle isotropic materials. We propose a force law that respects the isotropy of the material. The code yields the correct imposed sound c║, shear c┴ and surface VR wave speeds. Different notch lengths are systematically studied. We observe that initially the cracks are linear and always branch at a particular critical velocity c* ≈0.8VR and that this occurs when the crack tip reaches the position of a front emitted from the initial crack tip and propagating at a speed c = 0.6 8VR.
Dynamic Simulation Of Crack Propagation With Dislocation Emission And Migration
Authors: Zacharopoulos N. , Srolovitz D.J. , LeSAR R.A.
Published: April 2011

We present a simulation procedure for fracture that self-consistently accounts for dislocation emission, dislocation migration and crack growth. We find that the dislocation microstructure in front of the crack tip is highly organized and shows a complex temporal-spatial evolution. The final dislocation microstructure and the number of emitted dislocations immediately proceeding fracture varies rapidly with the loading rate. For high loading rates, fracture occurs at smaller loads with increasing loading rate. However, the load at fracture shows a maximum with respect to loading rates.
Criteria For Progressive Interfacial Debonding With Friction In Fiber-Reinforced Ceramic Composites
Authors: Hsueh Chun-Hway
Published: April 2011

Criteria for progressive debonding at the fiber/matrix interface with friction along the debonded interface are considered for fiber-reinforced ceramic composites. The energy-based criterion is adopted to analyze the debond length, the crack-opening displacement, and the displacement of the composite due to interfacial debonding. The analytical solutions are identical to those obtained from the mismatch-strain criterion, in which interfacial debonding is assumed to occur when the mismatch in the axial strain between the fiber and the matrix reaches a critical value. Furthermore, the mismatch-strain criterion is found to bear the same physical meaning as the strength-based criterion.
Interfacial Fracture Between Boron Nitride And Silicon Nitride And Its Applications To The Failure Behavior Of Fibrous Monolithic Ceramics
Authors: Kovar D. , Brady G.A. , Thouless M.D. , Halloran J.W.
Published: April 2011

Mechanical tests show that fibrous monolithic ceramics can exhibit graceful failure in bending due to marked crack deflection at silicon nitride/boron nitride interfaces. Model sandwich specimens were manufactured from polycrystalline silicon nitride with interphases consisting of mixtures of silicon nitride and boron nitride. The interfacial fracture resistances of the specimens were measured, and the results were correlated to the failure behavior of fibrous monolithic ceramics. It was found that the three modes of failure observed in fibrous monoliths can be correlated with the interfacial fracture resistance. In addition, anisotropy in the properties of the interphases were found to have a profound influence on crack deflection.
Effect Of Aggregate Content On Fracture Behavior Of Concrete
Authors: Xi Y. , Amparano F.E. , Li Zongjin
Published: April 2011

Effect of aggregate content on fracture behaviors of concrete is studied by testing on geometrically similar three-point bend beams. The results are analyzed by using a size effect method in which the fracture behavior of concrete is characterized by two parameters, fracture energy Gf and effective fracture process zone cf. Test results showed that with increasing volume fraction of aggregate in the range 45% - 75%: (1) the modulus of elasticity of concrete decreases slightly, (2) fracture energy Gf increases, but the rate is very small; (3) the size of the fracture process zone, cf, decreases, which may be explained by changes in coarseness of grain structures defined in terms of mosaic patterns.
Study Of Fiber Composite Failure Criterion
Authors: Zhou S. J. , Blumenfeld R. , Curtin W.A. , Holian B. L.
Published: April 2011

With a recently developed numerical technique, we have investigated failure processes in real fiber composites. This technique utilizes 3D lattice Green's functions to calculate load transfer from broken to unbroken fibers, and also includes the important effects of fiber/matrix sliding. It is found that the failure processes are more complex than previously thought: The fibers that break or slide are not necessarily located around the largest defect cluster. Rather, the breaking/sliding fibers accumulate, form several large clusters, coalesce and finally cause the fiber composite to fail. Only in some cases does one large defect dominate the damage evolution. Based on our observations of the irregular evolution of the damage, a simple model is developed to describe these different failure modes, and the failure criterion of the fiber composite is also discussed.
Kinetics Of Dislocation Emission From Crack Tips And The Brittle To Ductile Transition Of Cleavage Fracture.
Authors: Argon A.S. , Xu G. , Ortiz M.
Published: April 2011

Several activation configurations of dislocation embryos emanating from cleavage crack tips at the verge of propagating have been analyzed in detail by the variational boundary integral method, as central elements of the rate controlling process of nucleation governed fracture transitions from brittle cleavage to tough forms, as in the case for BCC transition metals. The configurations include those on inclined planes, oblique planes and crack tip cleavage ledges. Surface ledge production resistance is found to have a very strong embrittling effect. Only nucleation on oblique planes near a free surface and at crack tip cleavage ledges are found to be energetically feasible to explain brittle-to-ductile transition temperatures in the experimentally observed ranges.
Effect Of Crack Blunting On Subsequent Crack Propagation
Authors: SchiØtz J. , Carlsson A. E. , Canel L. -M. , Thomson Robb
Published: April 2011

Theories of toughness of materials depend on an understanding of the characteristic instabilities of the crack tip, and their possible interactions. In this paper we examine the effect of dislocation emission on subsequent cleavage of a crack and on further dislocation emission. The work is an extension of the previously published Lattice Greens Function methodology[1, 2, 3]. We have developed a Cavity Greens Function describing a blunt crack and used it to study the effect of crack blunting under a range of different force laws. As the crack is blunted, we find a small but noticeable increase in the crack loading needed to propagate the crack. This effect may be of importance in materials where a dislocation source near the crack tip in a brittle material causes the crack to absorb anti-shielding dislocations, and thus cause a blunting of the crack. It is obviously also relevant to cracks in more ductile materials where the crack itself may emit dislocations.
Novel Applications Of Ceramic Precursors - Tin Coating On Alumina And Functionally Gradient Materials
Authors: Seyferth D. , Narula C. K. , Czubarow P.
Published: April 2011

There are very few demonstrated applications of ceramic precursor technology. Here, we describe two new applications of known ceramic precursors, thin film deposition and the fabrication of functionally gradient materials [FGM]. To demonstrate the thin film deposition, we prepared titanium nitride film on an alumina substrate using (CH3)3SiNHTiCl3precursor by a single dipcoat-fire cycle. The fabrication of copper and aluminum based FGMs was demonstrated using Nicalon®fiber polycarbosilane and poly(methylsilane) precursors as binders and in situ sources of ceramics.
Microstructure and interfaces in TiB2/Ti-46Al-3Cr Alloy Composites
Authors: Si Weimin , Dudley Michael , Li Pengxing , Wu Renjie
Published: April 2011

A ternary titanium aluminide alloy, Ti-46Al-3Cr (at%), was discontinuously reinforced with 5 vol% titanium diboride (TiB2), by an in-situ synthesis technique, resulting in a two phase γ(TiAl) (mainly) and α2(Ti3Al) matrix with randomly dispersed TiB2 particle. Interfaces of TiB2-TiAl were investigated by Analytical Electron Microscopy (AEM) and High Resolution Electron Microscopy (HREM). No consistent crystallographic orientation relationship was observed between TiB2 particle and TiAl matrix, and there was no evidence of alloying elements (such as Cr) segregation or interphase formation at the TiB2-TiAl interface. HREM results indicated that no semi-coherent interface between TiB2 and TiAl has been observed. There existed a thin amorphous layer (0.5 to 1.3 nm) at the TiB2-TiAl interface, which may accommodate the large lattice misfit across the interface and enhance the interfacial bonding.