Energetics of surface and subsurface carbon incorporation in Si(100)

We have studied the initial stages of carbon incorporation in Si(100) by means of first principles calculations for a large variety of atomic configurations involving n=1-4 substitutional C impurities in surface and subsurface sites of a c(4×4) surface cell. For n=2 and 4, mixed configurations, with half of the C atoms at the surface and the others in α sites of the fourth layer, are found to be energetically favored, suggesting that carbon penetration in the subsurface region already takes place at low coverages. Calculated C 1s core binding energies for these mixed configurations agree well with x-ray photoelectron spectroscopy results for the c(4×4) reconstructed surface.