Sub-monolayer structures of Ag over layers on Ge (111): experimental observations and first-principles study

Experimentally determined structures of Ag adatoms on the Ge (111) surface using low energy electron diffraction, low energy electron microscopy and scanning tunneling microscopy as functions of coverages and temperature are presented. For clean Ge (111), c (2x8) and (2x1) phases occur, while Ag over layers cause (4x4), (v3xv3) R30 and (3x1) surface structural phases. The experimental data are used to determine a phase diagram. First-principles calculations are carried out to form atomic models of the Ge (111) surface and of over layer structures formed on Ag deposition at sub-monolayer coverage on the surface, to calculate their electronic and vibrational structures. The Ge (111)-c (2x8) configuration with Ge adatoms adsorbed on three-fold hollow T4 sites is found 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) over layer of the system has Ge atoms of missing top layer reconstructed surface forming a honeycomb chain with metal at 1/3 ML coverage in channel. The Ge (111)-Ag (v3xv3)R30 over layer contains one monolayer (ML) Ag forming an inequivalent Ag triangles in surface unit cell on missing top layer reconstructed Ge (111) surface. The Ge (111)-Ag (4x4) over layer contains one triangular subunit with six Ag adatoms and the other with three Ge adatoms on intact double layer Ge (111) surface. All those models are thermodynamically stable. The temperature-coverage dependent surface phase diagram obtained by minimizing the surface free energy of combination of stable phases captures the main features of experimental phase diagram.