Film growth of germanium on Ru(0001) studies by scanning tunneling microscopy

Using in situ scanning tunneling microscopy, we observed that germanium deposited on the Ru(0001) surface near room temperature forms a two-dimensional wetting layer in the submonolayer regime, followed with growth of a segregated layer of Ge three-dimensional (3D) clusters of heights within about 1 nm. The growth of the first flat wetting layer can be understood in terms of optimal surface energy reduction by coating the Ru surface with a Ge layer which has a lower surface free energy. The nucleation and growth kinetics agrees with that derived from the conservative Ising model. Domains of a (root21xroot21)R10.9degrees superstructure are observed on the wetting layer. Formation of a layer consisting of 1-nm-high clusters above the wetting layer indicates that the Ge wetting layer is extremely inert so that Ge adatoms can migrate large distances on the top of the wetting layer. The 3D Ge clusters seem to have a relatively narrow size distribution.