First-principles quantum transport theory of the enhanced wind force driving electromigration on Ag(111)

Herein we examine the low-bias electromigration wind force acting on quasi-one-dimensional nanoscale features within the Landauer-Buttiker conduction picture. Ordinarily the electromigration force is calculated under the approximation that the nonequilibrium carrier distribution in the vicinity of a defect is the same as that in the bulk. However, this approximation is rooted in the assumption that atomic scale defects scatter all incident electrons weakly (just as electrons weakly and diffusely scatter in the bulk). We examine this assumption by calculating the mode-resolved transmission against Ag(111) step edges and atomic wires using density-functional theory within the single-particle Green's function Landauer scattering picture. Furthermore we show that those modes that scatter strongly give rise to a nonequilibrium electrochemical potential drop across a defect and an increased wind force. The results quantitatively explain previously not understood experimental observations of an enhanced electron wind force against Ag(111) step edges [O. Bondarchuk et al., Phys. Rev. Lett. 99, 206801 (2007)]. In general, the results underscore the challenging nanoscale reliability problem posed by surface electromigration in nanostructures and the need for a nonequilibrium quantum transport description of the electron wind force.