Kinetic Role of a Surfactant in Island Formation

Diffusion in the presence of a surfactant on islands is investigated using ab initio molecular dynamics. We introduce a double layer step on the Si(001) surface that emulates the $\{311\}$ facet of an island and show how the diffusion barrier varies with introduction of an Sb surfactant at the step edge. In contrast with the role of surfactants on a flat surface, where diffusion of adatoms is suppressed by site exchange, we find that the surfactant significantly reduces the Schwoebel barrier near the step edge such that island formation is severely suppressed and thus layer-by-layer growth is promoted. We show that the surfactant favors the step edge site and is replaced by adatoms via a pushing-out mechanism, thus repeating the step growth until the island is flattened.