Ge/Ag(111) semiconductor-on-metal growth:: Formation of an Ag2Ge surface 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(lll) surprisingly induces a surface alloy forming a p(root 3x root3)R30 degrees 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.