Isolated Interstitials in Silicon

An isolated interstitial in silicon at the nominal site is investigated using a procedure based on the expansion of the wave function in terms of the Wannier functions of the perfect crystal and using a pseudopotential for the defect potential. Scattering phase shifts are calculated for states within the valence band and a search is made for bound states within the band gap. We discover that there is no bound state in the band gap associated with this defect. The change in the one-electron energy arising from the interstitial is calculated and combined with the corresponding quantity for a single vacancy. We find this major contribution to the formation energy of the vacancy interstitial pair to be 13.6 eV.