Nucleation and Growth of Ge at Pb/Si(111) Surfaces: Reaction-Limited Aggregation

Abstract

The nucleation and growth of two-dimensional (2D) Ge islands at Pb-layer-covered Si(111) surfaces is studied using scanning tunneling microscopy (STM). Ge adatoms are highly mobile on the Pb layer. There is a threshold deposition coverage for nucleation of Ge islands to occur. Above it, the observed island density increases rapidly but the average island size decreases rapidly first and then increases slowly again. Our results indicate that the nucleation and growth processes are hindered by high energy barriers required for Ge clusters to exchange with the Pb atoms. A compact-to-fractal island shape transition is observed as the deposition flux is lowered, as the temperature is raised, or at a low Ge coverage, which contradicts previous theoretical predictions and experimental findings. Our results may shed much needed light on the fundamental mechanism in surfactant-mediated epitaxy (SME).