Reversible place-exchange during film growth: a mechanism for surfactant transport

A mechanism for surface-active additives "floating" on the surface of a film during epitaxial growth is presented and applied to Sb-modified homoepitaxial growth on Ag(111). Scanning tunneling microscopy results for that system suggest that single, diffusing Ag atoms can transform "substitutional" Sb in the topmost Ag layer into Sb atoms adsorbed on top of that layer, thus causing the Sb to move up one atomic layer. This is in contrast to a model previously proposed which has Ag islands overgrowing buried Sb before Sb moves up to the next layer. The mechanism presented here is expected to be more generally valid for systems in which the surfactant atoms occupy substitutional sites