Molecular dynamic simulations of self-interstitials in silicon
Ab initio molecular-dynamics simulations in periodic supercells containing 64 up to 216 Si host atoms were used to study the static and dynamic properties of neutral self- interstitial aggregates, In,in silicon. The lowest energy configurations for In, n<5, have been identified. Each of I and I2 has one local minimum, while the potential energy surface for aggregates of three or more Ifs have 2 several local minima, leading to a range of metastable configurations. Constant temperature runs show that I2 and the most stable I3‹ clusters are highly mobile. In these complexes, all the self-interstitials are located around a single bond centered site, a feature that greatly facilitates exchange process and is responsible for the rapid diffusion. Simulations of Ib3 show that the three Is exchange sites with each other, but the center of the defect remains at the same place. Simulations of I and I4 show no diffusion or exchange on the same time scale. Next, the highly mobile Ia3 units' are assumed to be the building blocks for self-interstitial precipitates. We study the interactions Ia3 + Ia3 ¨I6 and I6 + Ia3¨I9 by bringing an Ia3 toward either Ia3 or I6 along various crystalline directions in 216 host silicon atoms supercells. The calculations show that these reactions occur at a substantial gain in energy and that the stacking along some directions is energetically preferred over others. The results suggest that precipitation mechanisms involving rapidly moving self-interstitial clusters could play an important role in the formation of extended defects.