Discommensurations, epitaxial growth and island formation in Ge(111):Cu

We investigated surface reconstructions induced by monolayer coverages of Cu on Ge(111) using scanning tunneling microscopy (STM) at room temperature (RT) and 50 K. Following annealing to moderate temperatures (approximate to 100 degrees C) a discommensurate, two-dimensional domain superlattice with hexagonal symmetry is observed. Within the domains, atomic resolution showing a hexagonal arrangement of surface atoms with slightly increased lattice parameter compared to the substrate is observed by STM, but only at cryogenic temperatures. We attribute this to dynamic processes happening at RT which are prohibited at 50 K. Three alternative microscopic models for the interior of the domains of the discommensurate phase are proposed. The first with stoichiometry CuGe and Cu substituting for Ge in the surface layer and a stacking fault in half of the domains. The second with stoichiometry CuGe2 and Cu imbedded in the Ge surface layer. The third model with stoichiometry Cu2Ge and two Cu atoms per(1 x 1)surface unit cell in substitutional and H-3 sites. The increased lattice constant produces the discommensurations which are thus the result of adsorbate induced stress. Upon raising the annealing temperature, larger and larger parts of the surface are depleted of Cu and exhibit the c(2 x 8) reconstruction of clean Ge(111). Decreasing areas show the domain superstructure, which thus proves to be thermodynamically unstable and most of the Cu appears to be incorporated in 3-D islands, probably consisting of an epitaxial Cu germanide.