Electromigration of Au on Ge(111): Adatom and island dynamics

International audienceWe have observed the motion of two-dimensional phases of Au on Ge(111) under an electric bias by in operando low energy electron microscopy. Electromigration of Au results in a complex dynamics that depends on the nature of the involved mobile phases: two-dimensional ad atom gas or two-dimensional islands. We show that the Au ad atoms move at the surface in the direction opposite to the electric current. The wind force induced by the electron ow is measured: the eective valence of Au (Z * =-82±15) is directly deduced from the coverage prole of a Au ad atom gas spatially retained by a strong Erlich-Schwoebel barrier at a downhill step edge. The velocity of two-dimensional Au islands versus island size reveals a mass transport by terrace diusion inside the islands. The energy barrier for diusion above 820 K is 1.16± 0.08 eV and it strongly increases up to 3.1±0.6 eV below. We attribute this change of regime to a modication of diusing species from single Au atoms at high temperature to Au clusters at low temperature. The strong shape uctuations of the 2D-islands is consistent with a nearly vanishing line tension of 1.2±0.4 meV/nm at 800 K