Growth kinetics and thermodynamic equilibrium can both be determining factors at different stages of III-V semiconductor heteroepitaxy. We study their interplay, employing kinetic Monte Carlo simulations for the InAs(001) surface. The simulation contains atomistic details of both species, including the stability of different reconstructions and their kinetics. The behavior of the surface in thermodynamic equilibrium, including different reconstructions, is determined exclusively by extensive total energy calculations employing ab initio density functional theory. The continuous phase transition between the α2(2x4) and β2(2x4), predicted by theory, is confirmed by experiment. At full layer coverage, a recovery of the stable reconstruction is observed. The different time scales associated with As2 and In are discussed with respect to equilibrium and kinetics.

Crystal-melt interfaces of binary hard spheres are investigated using molecular-dynamics simulation. For a diameter ratio α=0.414, two crystal phases coexisting with the fluid are possible, depending on the pressure. At low pressures, the liquid coexists with a pure fcc crystal of the larger particle, while at high pressures a 1:1 binary crystal of “NaCl” type is the coexisting phase. For both of these systems, we study the structural and dynamical changes as the interface is traversed from bulk crystal to bulk fluid through the calculation of density and diffusion coefficient profiles. It is observed that the total width of the interfacial region is narrower in the “NaCl”/binary fluid interface than in the corresponding lower pressure fcc/binary fluid system.

The wetting and spreading of liquid Ag drops on Cu (001) surfaces are studied with molecular dynamics simulations. As the liquid Ag drop spreads on the Cu surface, reactive wetting occurs in that a Ag/Cu alloy is formed. The results for the reactive wetting case are compared to a frozen substrate for which there is no alloying. At the highest temperature studied, T = 1300 K, the radius of the drop spreads faster than for the frozen substrate, indicating the driving force for spreading is greater when the liquid and solid mix. Lowering temperature in reactive systems results in behavior more similar to non-reactive systems due to suppressed mixing.

The continued safe operation of nuclear reactors and their potential for lifetime extension depends on ensuring reactor pressure vessel integrity. Reactor pressure vessels and structural materials used in nuclear energy applications are exposed to intense neutron fields that create highly non-equilibrium defect concentrations, consisting of a shell of self-interstitial atom and clusters surrounding a vacancy-rich core, over picosecond time scales. This spatially correlated defect production initiates a long chain of events responsible for microstructure evolution and hence irradiation embrittlement. In this paper, we describe the combined use of molecular dynamics (MD) and kinetic lattice Monte Carlo (KMC) to simulate the long-term rearrangement (aging) of displacement cascades in dilute Fe-Cu alloys. The simulations reveal the formation of a continuous distribution of three dimensional cascade vacancy-Cu cluster complexes and demonstrate the critical importance of spatial, as well as short and long-time correlated processes that mediate the effective production of primary defects. Finally, this approach can generate production cross-sections for vacancy-Cu clusters that can then be used in rate theory type models of long term global micro and microstructural evolution.

Efficient and faithful parallel simulation of large asynchronous systems is a challenging computational problem. It requires using the concept of local simulated times and a synchronization scheme. We study the scalability of massively parallel algorithms for discrete-event simulations which employ conservative synchronization to enforce causality. We do this by looking at the simulated time horizon as a complex evolving system, and we identify its universal characteristics. We find that the time horizon for the conservative parallel discrete-event simulation scheme exhibits Kardar-Parisi-Zhang-like kinetic roughening. This implies that the algorithm is asymptotically scalable in the sense that the average progress rate of the simulation approaches a non-zero constant. It also implies, however, that there are diverging memory requirements associated with such schemes.

Micro-Raman spectroscopic investigations of arsenic-implanted silicon show lines characteristic of silicon crystallites even at implant doses above the amorphization threshold. The intensity and frequency of occurrence of the lines increase with the implanted dose. Polarization/orientation Raman studies indicate the crystallites are silicon in the hexagonal phase (Si-IV) and silicon in the diamond phase (Si-I). The latter are oriented differently than the substrate silicon. Monte Carlo simulations of the arsenic ion energy loss and published molecular dynamics studies suggest that each arsenic ion deposits sufficient energy to locally melt the silicon lattice. This is taken as the basis of the present attempt to explain the origin of the crystallites. A one-dimensional numerical model is developed to determine the time scale for the liquid silicon to solidify. The effect of amorphous silicon on the solidification is also investigated.

We outline a theoretical framework for estimating the evolution of the particle size distribution in colloid and nanoparticle synthesis, when the primary growth mode is by externally controlled addition of singlet building blocks. The master equations, analyzed in the leading “non-diffusive” expansion approximation, are reduced to a set of easily numerically programmable relations that yield information on the time evolution of the particle size distribution.

For studying systems with a cubic anisotropy in interfacial energy σ, we extend the Cahn-Hilliard model by including in it a fourth rank term, which leads to an additional linear term in the evolution equation for the compositioneld. It also leads to an orientation-dependent effective fourth rank coeffcient γ(hkl) in the governing equation for the one-dimensional composition prole across a planar interface. The main effect of a non-negative γ(hkl) is to increase both σ and interfacial width w, each of which, upon suitable scaling, is related to γ(hkl) through a universal scaling function. The anisotropy in the interfacial energy can be large enough to give rise to corners in the Wul. shapes in two dimensions. In particles of finite sizes, the corners get rounded, and their shapes tend towards the Wul. shape with increasing particle size. In the study of unmixing of concentrated alloys, the anisotropy not only leads to non-spherical particle shapes, but also to strongly elongated morphologies.

A phase-field model is developed to simulate free dendritic growth coupled with fluid flow for a pure material in three dimensions. The preliminary results presented here illustrate the strong influence of convection on the three-dimensional (3D) dendrite growth morphology. The knowledge of the flow and temperature fields in the melt from the simulations allows for a detailed understanding of the convection effects on dendritic growth.

A model for the description of strain hardening and hysteresis at different temperatures and strain rates in ductile single crystals is introduced. The theory accounts for: and arbitrary number and arrangement of dislocation lines over a slip plane; the long-range elastic interactions between dislocation lines; the core structure of the dislocations; the interaction between the dislocations and applied resolved shear stress field; and the dissipative in teractions with short-range obstacles and lattice friction, resulting in hardening, path dependency and hysteresis. We introduce a variational formulation for the statistical mechanics of dissipative systems. The influence of finite temperature as well as the mechanics are modeled with Metropolis Monte Carlo simulations and a mean field approximation. The theory predicts a range of behaviors which are in qualitative agreement with observation, including: hardening and dislocation multiplication under monotonic loading and hysteresis loops under under cyclic loading. The flow stress was found to be dependent on the temperature and on the strain rate only at finite temperature.

The deposition of AlGaAs in the presence of HCl was investigated at the macroscopic and mesoscopic scales. Fluid dynamics simulations were first performed in order to study the dependence of the deposition rate on the operating conditions. Unknown gas phase and surface kinetic parameters were estimated by quantum chemistry and transition state computations. The fluxes of all species to the surface were thus computed and provided the input to a kinetic Monte Carlo model used to investigate the morphology evolution of the film.

We describe progress on the issue of pathological elastic wave reflection in atomistic and multiscale simulation. First we briefly r eview Coarse-Grained Molecular Dynamics (CGMD). Originally CGMD was formulated as a Hamiltonian system in which energy is conserved. This formulation is useful for many applications, but recently CGMD has been extended to include generalized Langevin forces. Here we describe how Langevin dynamics arise naturally in CGMD, and we examine the implication for elastic wave scattering.

The stress-velocity relation of a single dislocation moving in a 2-D lattice has been studied using interactive molecular dynamic simulations. A hybrid interatomic model potential which couples Lennard-Jones(LJ) potential and the Embedded Atom Model (EAM) potential, is used to include radial and many body interactions. Both parts are assembled by a parameter so that the potential can be changed to describe a pure radial interaction to a strong many body interaction in a continuous way. Setting up a constant-stress scenario, the movement of a single dislocation is tracked from zero velocity state, up to a terminal velocity state. The external stress vs. terminal velocity curves have been obtained in the subsonic regime for different values of the coupling parameter. Non-linear relations are found velocity regime 0.1ct to 0.6ct, where ct is the transverse speed of sound. Results have been analyzed using an augmented Peierls model to seek the connection between atomic scale, continuum variables and the limiting speed of dislocations

We examine a class of step ow models of epitaxial growth obtained from a Burton-Cabrera-Frank (BCF) type approach in one space dimension. Our goal is to derive a consistent contin uummodel for the evolutionof the film surface. Away from peaks and valleys, the surface height solves a Hamilton-Jacobi equation (HJE). The peaks are free boundaries for this HJE. Their evolution must be specified by boundary conditions reecting the micro- scopic physics of nucleation. We inv estigate this boundary condition by numerical simulation of the step ow dynamics using a simple nucleation law. Our results rev ealthe presence of special structures in the profile near a peak; we discuss the relationship between these structures and the contin uumequation. We further address the importance of evaporation for matching the local behavior near the peak to the solution of the contin uum equation.