Ab initio calculations of the self-interstitial in silicon

Using the ab initio pseudopotential approach it is shown that the lowest-energy configuration for the neutral silicon interstitial is the (011) dumbbell. Another local minimum energy atomic configuration with very similar energy was found, which has very low symmetry and hence is multiply degenerate. Other high-symmetry configurations were found to be mechanically unstable. We have further performed ab initio molecular-dynamics simulations to study self-interstitial migration. We find several possible mechanisms and a tendency for an atom to become "excited" and perform several correlated jumps through the structure before being recaptured into the minimum energy state. Both excitation and recapture processes appear to be thermally activated, but the number of jumps is larger at low temperatures and the overall migration is athermal. The molecular-dynamics simulations also allow us to evaluate local phonon densities of states for the defects.