Exchange-Barrier Effects on Nucleation and Growth of Surfactant-Mediated Epitaxy

The initial stage of nucleation and growth of two-dimensional (2D) Ge islands on Pb covered Si(111) surfaces at room temperature is studied using scanning tunneling microscopy. The Pb overlayer is found to greatly enhance surface diffusion of deposited Ge adatoms. There is a critical deposition time above which the observed island density increases sharply while the average island size decreases. We believe nucleation as well as growth is initially hindered by the high energy barriers for Ge clusters to exchange with Pb atoms until Ge clusters reach a critical size. A mechanism is proposed to explain the observed phenomena.