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

Abstract

We 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 adatom gas or two-dimensional islands. We show that the Au adatoms move at the surface in the direction opposite to the electric current. The wind force induced by the electron flow is measured: the effective valence of Au (${Z}^{*}=\ensuremath{-}82\ifmmode\pm\else\textpm\fi{}15$) is directly deduced from the coverage profile of a Au adatom 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 diffusion inside the islands. The energy barrier for diffusion above 820 K is $1.16\ifmmode\pm\else\textpm\fi{}0.08$ eV and it strongly increases up to $3.1\ifmmode\pm\else\textpm\fi{}0.6$ eV below. We attribute this change of regime to a modification of diffusing species from single Au atoms at high temperature to Au clusters at low temperature. The strong shape fluctuations of the 2D islands is consistent with a nearly vanishing line tension of $1.2\ifmmode\pm\else\textpm\fi{}0.4$ meV/nm at 800 K.