Electronic effects of isolated halogen atoms on the Ge(001) surface

The asymmetric slab approach and density functional theory (DFT) are used to discuss the interaction of isolated halogen atoms (F, Cl, Br and I) with the Ge(001)c(4 × 2) surface. For each adsorbate on thick slabs we find two stable adatom configurations separated by ~ 0.1 eV. In both configurations, the unsaturated dangling bonds (DB) of the chemisorbed dimers are fully charged due to the interaction of the DB-derived surface states with the bulk states. This behavior is found to be insensitive to whether the calculations are performed using standard ground-state DFT, or DFT supplemented by MBJLDA and spin-orbit corrections to produce an accurate surface electronic structure and energy gap. On chemisorbed ultrathin Ge slabs, however, these unsaturated DBʹs are empty for the lowest energy configurations. This results in different geometries and energetics for the chemisorbed dimers. A detailed analysis of the predicted scanning tunneling microscopy images of halogen-chemisorbed surfaces is also presented. It is predicted that the halogen atoms may image darkly, yielding filled-state images similar to that of the clean surface, while the empty-state images should be noticeably different.