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 (thetasles0.125), 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 (0.125lesthetasles0.25), 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
