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 $n=1\mathrm{}–4$ substitutional C impurities in surface and subsurface sites of a $c(4\mathrm{}\times 4)$ surface cell. For $n=2\mathrm{}$ and $4,$ mixed configurations, with half of the C atoms at the surface and the others in $\alpha$ 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 $c(4\mathrm{}\times 4)$ reconstructed surface.