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 mono layer of Ge on Ag(lll) surprisingly induces a surface alloy forming a p(roo t 3x root3)R30 degrees superstructure observed in low energy electron diffr action patterns. Yet high-resolution scanning tunneling microscopy images d o not exhibit any chemical contrast between Ge and Ag atoms. This is interp reted with ab initio total-energy calculations, which also show that the Ge atoms are located in substitutional sites forming an ordered two-dimension al surface alloy with almost identical local electronic densities for both elements.