Probing the Structure and Energetics of Dislocation Cores in SiGe Alloys through Monte Carlo Simulations

We present a methodology for the investigation of dislocation energetics in segregated alloys based on Monte Carlo simulations which equilibrate the topology and composition of the dislocation core and its surroundings. An environment-dependent partitioning of the system total energy into atomic contributions allows us to link the atomistic picture to continuum elasticity theory. The method is applied to extract core energies and radii of 60 degrees glide dislocations in segregated SiGe alloys which are inaccessible by other methods.Comment: 5 pages, to be published in Physical Review Letter

Softening of ultra-nanocrystalline diamond at low grain sizes

Ultra-nanocrystalline diamond is a polycrystalline material, having crystalline diamond grains of sizes in the nanometer regime. We study the structure and mechanical properties of this material as a function of the average grain size, employing atomistic simulations. From the calculated elastic constants and the estimated hardness, we observe softening of the material as the size of its grains decreases. We attribute the observed softening to the enhanced fraction of interfacial atoms as the average grain size becomes smaller. We provide a fitting formula for the scaling of the cohesive energy and bulk modulus with respect to the average grain size. We find that they both scale as quadratic polynomials of the inverse grain size. Our formulae yield correct values for bulk diamond in the limit of large grain sizes.Comment: 5 pages, 3 figures, to be published in Acta Materiali

Structure and chemical ordering in amorphous silicon carbide alloys

Simulations of amorphous silicon carbide alloys indicate that the amorphous network deviates from an ideal tetrahedral geometry due to the presence of threefold- coordinated carbon atoms. All samples exhibit a significant degree of chemical ordering, most of it arising from fourfold-coordinated carbon atoms. The results show that there is no phase separation and that hydrogenation might promote tetrahedral carbon coordination, as well as chemical ordering. © 1991 IOP Publishing Ltd

Stress properties of diamond-like amorphous carbon

The problem of intrinsic stress in tetrahedral amorphous carbon is studied using as a probe the concept of atomic-level stresses. These are extracted from the local energetics within the empirical potential approach. The finite temperature statistics of the system are described by Monte Carlo simulations. The general finding of these simulations is that equilibrated, annealed films that relax the external constraints and pressure possess zero total intrinsic stress, but they still contain a high fraction of sp3 sites. The same holds for the hypothetical all-tetrahedral model of a-C. This finding is in contrast to the case of non-equilibrium as-grown structures, that are left intrinsically stressed by the deposition process

Energetics and stability of diamondlike amorphous carbon

Simulations of diamondlike amorphous carbon formed by quenching the liquid under high pressure indicate that two distinctly different dense phases exist. The as-quenched phase is metastable and mostly fourfold coordinated. Upon annealing, it converts into a more stable structure with the majority of atoms having threefold coordination. Both structures are consistent with experimental findings. The annealed phase is clearly distinguished from the low-coordinated network of evaporated ±-C

Structural properties and energetics of amorphous forms of carbon

We have made a comparative theoretical study of the most common forms of unhydrogenated amorphous carbon (α-C), namely, of the dense, diamondlike phase and the low-density evaporated α-C (e-C). Emphasis is given to the connection among the structure, energetics, and stability of these phases. To make the simulations of the amorphous structures (formed by quenching the liquid) tractable, we used the Monte Carlo method, combined with the empirical-potential approach. Our analysis employs a powerful total-energy-partitioning scheme, which is proved very useful in treating the energetics of disordered systems. It is found that threefold sp2 sites are the energetically favorable geometries in e-C, and thus they are by far more numerous. The nonplanar character of sp2 sites and the absence of sixfold rings indicate that medium-range order is rather not significant in e-C. The increasing graphitic character of e-C, as the temperature is raised, is explained by resorting to the effective temperatures T*, at which the atoms freeze in their lattice positions. For diamondlike α-C, the simulations show that there exist two distinctly different dense structures. The ''as-quenched'' one (i-C) is mostly sp3 bonded, but it is metastable. Upon annealing, it converts into a second phase (i-C*), mostly sp2 bonded, with a significant energy gain. A specific mechanism is proposed for this transition. The insensitivity of density to annealing is explained if we use the concept of the ''glass transition temperature'' T*. Finally, by introducing an isotropic bulk modulus for the amorphous phase, it is found that e-C has a much lower compressibility than i-C*, enhancing the distinguishability among the two low-coordinated forms of α-C

Intrinsic stress and stiffness variations in amorphous carbon

We have studied the problem of intrinsic stress in tetrahedral amorphous carbon. Our methodology was based on the concept of atomic level stresses. These are extracted from the local energetics within the empirical potential approach. The finite temperature statistics of the system are described by Monte Carlo simulations. The universal finding of our investigations was that equilibrated, annealed films that relax the external constraints and pressure possess zero total intrinsic stress, but still contain a high fraction of sp3 sites. This is in contrast to the case of non-equilibrium as-grown structures that are left intrinsically stressed by the deposition process. We also studied the variation of stiffness in the amorphous carbon network as a function of the average coordination. It was found that this variation deviates from a mean-field-like behavior

A constrained-equilibrium Monte Carlo method for quantum dots - the problem of intermixing

Islands grown during semiconductor heteroepitaxy are in a thermodynamically metastable state. Experiments show that diffusion at the surface region, including the interior of the islands, is fast enough to establish local equilibrium. I review here applications of a Monte Carlo method which takes advantage of the quasi-equilibrium nature of quantum dots and is able to address the issue of intermixing and island composition. Both, Ge islands grown on the bare Si(100) surface and C-induced Ge islands grown on Si(100) precovered with C are discussed. In the bare case, the interlinking of the stress field with the composition is revealed. Both are strongly inhomogeneous. In the C-induced case, the interplay of strain and chemical effects is the dominant key factor. Islands do not contain C under any conditions of coverage and temperature

Bonding and short-range order in liquid silicon-carbon alloys

Simulations of liquid silicon-carbon mixtures show that, at low pressures, the tendency of carbon towards low coordination dominates the overall structural characteristics. As a result silicon coordination is lower than in pure l-Si, accompanied by an enhancement of tetrahedral order and covalent bonding which persist in the liquid. The distribution of atoms is random in contrast to amorphous Si-C alloys. Under high pressures (1 Mbar) carbon coordination remains lower than four. At this pressure pure l-C is less dense than diamond