Ehrlich-Schwoebel effect for vacancies: Low-index faces of silver

Abstract

We employ surface-embedded-atom-method potentials to investigate the energy landscape of vacancies diffusing over, along, and near steps on the low-index faces of silver. We compare the vacancy diffusion barriers with those of adatoms. Barriers for vacancies diffusing in the surface of terraces near and up close-packed step edges are calculated for the (111), (100), and (110) surfaces. Large Ehrlich-Schwoebel (ES) barriers (>200 meV) for vacancies ascending step edges occur on all three faces, and these barriers are enhanced over those of adatoms. Barriers for their diffusion along step edges and around kinks and corners of vacancy islands are calculated for the (100) surface. Here a sizable vacancy ``corner'' ES barrier occurs (148 meV). The ``kink'' ES barrier, which proceeds by the exchange mechanism, is very small (7 meV). We develop an accurate scheme of estimating the vacancy energy landscape in terms of bond breaking and atomic coordination at the relevant surface lattice sites. We assess the role of the vacancy diffusion barriers on the structure of two-dimensional (2D) vacancy islands and 3D vacancy pits on Ag(100). The vacancy ES effect promotes a pitted morphology that persists above room temperature. The calculated vacancy diffusion barriers do not support the suggestion that vacancy diffusion on top of clusters can help account for the behavior of the observed scaling exponent for the coarsening or diffusion of Ag clusters on Ag(100).