Cobalt containing nano-islands on Ge(111)-c(2 x 8)

The structural and electronic properties of Co containing islands on Ge(111)-c(2 × 8) surfaces have been studied using scanning tunneling microscopy and spectroscopy. Room temperature deposition of a sub-monolayer amount of Co and subsequent annealing at 500 K lead to the formation of (√13 × √13)R13.9° reconstructed islands, whereas annealing at 750 K results in (2 × 2) reconstructed islands. The latter type of islands has heights up to several atomic layers. We find that, in analogy with Co on Si(111), the (2 × 2) islands consist of CoGe2, condensed in a fluorite structure. Each Co(111) layer is encapsulated between two close packed Ge layers, while the outermost trilayer is terminated by an additional layer of Ge adatoms in a (2 × 2) registry. The (√13 × √13)R13.9° structure can be considered as a precursor to the CoGe2 crystallites with triangular trilayer patches, composed of three embedded Co atoms terminated by six Ge adatoms. The remaining dangling bond transfers charge to surrounding Ge atoms in the bilayer. The different domains in the (2 × 2) islands are separated by three types of boundaries. Two types of domain boundaries are dynamic, whereas the third type of domain boundary is static. The (2 × 2) domains show a faint (6 × 2) superstructure, which is tentatively attributed to the aligned Co defects at the interface. Scanning tunneling spectroscopy measurements indicate the presence of Co atoms in the (2 × 2) and (√13 × √13)R13.9° reconstructed islands.