Two-atom structures of Ge on Si(100): dimers versus adatom pairs

We present an ab initio study of the properties of structures composed of two and four Ge atoms adsorbed on the troughs of the Si(100) surface, and we conclude that these structures are all composed of dimers, with a chemical bonding between the adatoms. We compare our calculated local density of states with scanning tunneling microscope (STM) images, and we show that these Ge dimers adsorbed on the troughs between the substrate dimer rows can be identified with the adatom pairs observed experimentally. We also show that the local buckling of the substrate dimers can give rise to similar structures with very different STM images.