  We present a joint experimental and theoretical determination of structures
of Ag adatoms on the Ge(111) surface using low energy electron diffraction, low
energy electron microscopy, scanning tunneling microscopy, and density
functional theory-based calculations, as functions of coverages and
temperature. Experimentally for clean Ge(111), c(2X8) and (2X1) phases occur,
while Ag overlayers cause (4X4), (V3XV3)R30 and (3X1) surface structural
phases. The dependence of the growth behavior of these different phases was
examined as a function of temperature, Ag deposition rate and coverage,
substrate step density, and history of temperature cycling. First-principles
calculations of the electronic and geometric structures and vibrational
dynamics show the Ge(111)-c(2X8) configuration with Ge adatoms adsorbed on
three-fold hollow (T4) sites to be the energetically most favored phase of the
Ge(111) surface, among unreconstructed Ge(111), reconstructed Ge(111)-2X1, and
Ge(111)-c(2X8) structures. The Ge(111)-Ag(3X1) overlayer of the system has Ge
atoms forming a honeycomb chain on a missing top layer reconstructed surface,
with metal at 1/3 ML coverage in channel. The Ge (111)-Ag(V3XV3)R30 overlayer
contains one monolayer Ag forming inequivalent Ag triangles in a surface unit
cell on the missing top layer reconstructed Ge(111) surface. The
Ge(111)-Ag(4X4) overlayer formed at low Ag coverage contains two triangular
subunits at different heights: one with six Ag adatoms and the other with three
Ge adatoms on the intact double layer Ge(111) surface. The temperature and
coverage dependent surface phase diagram, obtained by minimizing the surface
free energy, captures the main features of the experimental phase diagram.
