ELECTRONIC-STRUCTURE OF INTERSTITIAL CARBON IN SILICON

We report the results of electronic structure calculations for the carbon interstitial in silicon in the positive, neutral, and negative charge states. We have used two self-consistent complementary cluster approaches. The modified neglect of diatomic differential overlap method produces reliable total energies as a function of the atomic configuration. In the total energy we have included the compressive or tensile energy of parametrized springs attached to the outer silicon atoms of the cluster. For all charge states there is significant host relaxation. The minimum-energy geometry is then used to find the single-particle electronic structure using the scattered-wave X-alpha method. We determine that the unpaired electron for the positive charge state is localized on the nonbonding carbon p orbital, consistent with experiment. Our results for the negative charge state are quite sensitive to the host-atom relaxations, but suggest the presence of a delocalized unpaired electron, also consistent with recent observation.