Electronic structure of substitutional chalcogen impurities in silicon

We report the results of a first-principles calculation of the electronic structure of substitutional, unrelaxed, and neutral chalcogen impurities (O, S, and Se) in silicon. We have employed the recently developed quasi band crystal-field defect Green's-function method. We find that whereas atomistic models predict that the binding energies of donor levels in semi-conductors increase with the ionization potential of the free impurity atoms, a special enhancement of the screening in the solid predicts, for chalcogen impurities in silicon, a reversal in this order. Our results are in excellent agreement with recently reported optical excitation data available for Si:S and Si:Se. We demonstrate that whereas oxygen shows the expected sp bonding to the host crystal, sulfur and selenium exhibit also significant d bonding by utilizing their virtual d states. We discuss the relevance of effective-mass---type calculations to chalcogen impurities in the light of our results.