Kapur, Sumeet
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Publication Entropic Origins of Stability in Silicon Interstitial Clusters(2008-12-05) Kapur, Sumeet; Sinno, TalidThe role of entropy in the thermodynamic properties of small interstitial clusters in crystalline silicon is investigated using an empirical potential. It is shown that both vibrational and configurational entropies are potentially important in setting the properties of small silicon interstitial clusters and, in particular, contribute to the formation of “magic” sizes that exhibit special stability, which have been inferred by experimental measurements of dopant diffusion. The results suggest that a competition between formation energy and entropy of small clusters could be linked to the selection process between various self-interstitial precipitate morphologies observed in ion-implanted crystalline silicon.Publication Detailed Microscopic Analysis of Self-interstitial Aggregation in Silicon. I. Direct Molecular Dynamics Simulations of Aggregation(2010-07-19) Kapur, Sumeet; Sinno, TalidA comprehensive atomistic study of self-interstitial aggregation in crystalline silicon is presented. Here, large-scale parallel molecular dynamics simulations are used to generate time-dependent views into the selfinterstitial clustering process, which is important during post-implant damage annealing. The effects of temperature and pressure on the aggregation process are studied in detail and found to generate a variety of qualitatively different interstitial cluster morphologies and growth behavior. In particular, it is found that the self-interstitial aggregation process is strongly affected by hydrostatic pressure. {111}-oriented planar defects are found to be dominant under stress-free or compressive conditions while {113} rodlike and planar defects are preferred under tensile conditions. Moreover, the aggregation pathways for forming the different types of planar defect structures are found to be qualitatively different. In each case, the various cluster morphologies generated in the simulations are found to be in excellent agreement with structures previously predicted from electronic-structure calculations and observed experimentally by electron microscopy. Multiple empirical interatomic potential models were employed and found to generally provide similar results leading to a fairly consistent picture of self-interstitial aggregation. In a companion article, a detailed thermodynamic analysis of various cluster configurations is employed to probe the mechanistic origins of these observations.Publication Carbon-Mediated Aggregation of Self-Interstitials in Silicon(2004-04-15) Kapur, Sumeet; Sinno, Talid R; Prasad, ManishThe carbon-mediated aggregation of silicon self-interstitials is investigated with a novel approach based on large-scale parallel molecular dynamics. The presence of carbon in the silicon matrix is shown to lead to concentration-dependent self-interstitial cluster pinning, dramatically reducing cluster coalescence and thereby inhibiting the nucleation process. The extent of cluster pinning increases with cluster size for the range of cluster sizes observed in the simulation. The effect of carbon on single self-interstitials is shown to be of secondary importance, and the concentration of single self-interstitials as a function of time is essentially unchanged in the presence of carbon. A quasi-single component mean-field interpretation of the atomistic simulation results further confirms these conclusions and suggests that the experimentally observed effect of carbon on transient-enhanced diffusion (TED) could be due to carbon-cluster interactions.Publication Detailed Microscopic Analysis of Self-interstitial Aggregation in Silicon. II. Thermodynamic Analysis of Single Clusters(2010-07-19) Kapur, Sumeet; Nieves, Alex M; Sinno, TalidWe analyze results generated by large-scale molecular-dynamics simulations of self-interstitial clusters in crystalline silicon using a recently developed computational method for probing the thermodynamics of defects in solids. In this approach, the potential-energy landscape is sampled with lengthy molecular-dynamics simulations and repeated energy minimizations in order to build distribution functions that quantitatively describe the formation thermodynamics of a particular defect cluster. Using this method, a comprehensive picture for interstitial aggregation is proposed. In particular, we find that both vibrational and configuration entropic factors play important roles in determining self-interstitial cluster morphology. In addition to the expected role of temperature, we also find that applied (hydrostatic) pressure and the commensurate lattice strain greatly influence the resulting aggregation pathways. Interestingly, the effect of pressure appears to manifest not by altering the thermodynamics of individual defect configurations but rather by changing the overall energy landscape associated with the defect. These effects appear to be general and are predicted using multiple, well-tested, empirical interatomic potentials for silicon. Our results suggest that internal stress environments within a silicon wafer (e.g., created by ion implantation) could have profound effects on the observed selfinterstitial cluster morphology.