Influence of germanium ad-dimers on carbon incorporation in the Si(001) surface

We present a theoretical study of the influence of germanium ad-dimers on carbon incorporation in the Si(001) surface. Our ab initio energetic calculations show that the presence of a germanium ad-dimer clearly improves the single-carbon-atom penetration in the Si(001) subsurface layers with respect to the defectless surface. The energetic barrier observed in the case of the defectless Si(001) surface has disappeared, and the third-layer alpha sites are largely favored. Comparing our results to those obtained in the presence of silicon ad-dimers, we notice roughly similar trends, but our study emphasizes the role of the chemical nature of the ad-dimer following its orientation. On the other hand, the size difference between the germanium and silicon atoms in the ad-dimers is found to be of less importance. We eventually show that parameters such as the chemical nature, orientation, and location of the ad-dimers can be adequately monitored to improve carbon penetration in the Si(001) surface and allow a better control of the carbon atom positions in the subsurface layers.