Structural and Electronic Properties of the Methyl-Terminated Si(111) Surface

We have investigated the structural and electronic properties of the methyl-terminated Si(111) surface using first-principles calculations at the semilocal density functional theory level, inclusive of semiempirical dispersion forces between the methyl groups. In agreement with previous ab initio studies and consistent with low-energy electron diffraction measurements, we find that the most stable geometry corresponds to a (1 x 1), symmetric pattern, with methyl groups vertically bound to the Si substrate. Therefore, our computed scanning tunneling microscopy (STM) images exhibit three-fold symmetry at variance from those reported by recent experiments. The main contribution to the tunneling current comes from surface resonant states close to the Fermi level that are very weakly affected by self-energy corrections within many body perturbation theory. Our computed current voltage profiles are in qualitative agreement with experiment. We suggest that the differences between computed and measured STM patterns may arise from tip substrate interactions.