Ge/Ag(111) semiconductor-on-metal growth: Formation of anAg2Gesurface alloy

We study a semiconductor on a close-packed surface of a metal for a system that tends to phase separation. At room temperature, deposition of 1/3 monolayer of Ge on Ag(111) surprisingly induces a surface alloy forming a $p(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ superstructure observed in low energy electron diffraction patterns. Yet high-resolution scanning tunneling microscopy images do not exhibit any chemical contrast between Ge and Ag atoms. This is interpreted with ab initio total-energy calculations, which also show that the Ge atoms are located in substitutional sites forming an ordered two-dimensional surface alloy with almost identical local electronic densities for both elements.