We investigated surface reconstructions induced by monolayer coverages
of Cu on Ge(111) using scanning tunneling microscopy (STM) at room te
mperature (RT) and 50 K. Following annealing to moderate temperatures
(approximate to 100 degrees C) a discommensurate, two-dimensional doma
in superlattice with hexagonal symmetry is observed. Within the domain
s, 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 thi
s to dynamic processes happening at RT which are prohibited at 50 K. T
hree 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 fa
ult 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 a
nd H-3 sites. The increased lattice constant produces the discommensur
ations which are thus the result of adsorbate induced stress. Upon rai
sing the annealing temperature, larger and larger parts of the surface
are depleted of Cu and exhibit the c(2 x 8) reconstruction of clean G
e(111). Decreasing areas show the domain superstructure, which thus pr
oves to be thermodynamically unstable and most of the Cu appears to be
incorporated in 3-D islands, probably consisting of an epitaxial Cu g
ermanide.