A study was made, of the stability and migration of self-interstitials, using first-principles self-consistent pseudopotential calculations. The neutral Si interstitial was lowest in energy at a [411]-split site; with energy barriers of 0.15 of 0.18eV for migration into hexagonal and tetrahedral interstitial sites. The migration barrier from an hexagonal site to a tetrahedral site was lower (0.12eV). These migration barriers were further lowered via successive changes in the charge state at different sites; thus permitting the athermal diffusion of interstitials at very low temperatures. The [110]-split geometry was also the most stable structure for negatively charged states, while positively charged self-interstitials had the lowest energy at tetrahedral sites. Apart from the migration barrier, the formation energy of the [110]-split interstitial was estimated to be equal to about 4.19eV. The resultant activation enthalpy (about 4.25eV) was in good agreement with high-temperature experimental data. In order to explain some pre-exponential factors, it was necessary to calculate the formation entropy of self-interstitials. Previous calculations had indicated a large formation entropy (-6k) for the [110]-split interstitial, while a formation entropy of about 10k was required in order to explain the experimental data. However, this previous work had not considered a special diffusion path. It was pointed out here, without considering extended interstitials or the participation of several kinds of interstitial, that the [110]-split interstitial could give rise to a greater entropy. If the [110]-split interstitial diffused along a special path, an atom at an hexagonal site could migrate into one of 6 nearest-neighbors to form a [110]-split configuration. The interstitial atom thus increased the number of migration paths; hence contributing an additional migration entropy to the total entropy term.

First-Principles Study of the Self-Interstitial Diffusion Mechanism in Silicon. W.C.Lee, S.G.Lee, K.J.Chang: Journal of Physics - Condensed Matter, 1998, 10[5], 995-1002