Influence of ad-dimers on the incorporation of carbon in the Si(100) surface

We present first principle total energy calculations of carbon incorporation in the Si(100) surface in the presence of silicon ad-dimers either parallel or perpendicular to the dimer rows of the clean surface. By modifying the local stress, the ad-dimers are found to make the carbon penetration in the subsurface layers easier. At low coverages (θ < 0.125), carbon atoms are preferentially adsorbed in the third layer directly below an ad-dimer, and the barrier at the crossing of the second layer on the clean surface is strongly reduced or completely vanishes. At higher coverage (0.125 < θ < 0.25), configurations with part of the carbons at the surface and part in the third layer are favored. These tendencies are enhanced with increasing ad-dimer density.