Surfactant adsorption site and growth mechanism of Ge- on Ga-terminated Si(111).

Abstract

X-ray standing waves have been employed to study the microscopic mechanism of surfactant mediated epitaxy of Ge on Si(111). After deposition of 0.5 BL Ge on the Ga:Si(111)-6.3\ifmmode\times\else\texttimes\fi{}6.3 surface we find Ga floating on the surface in a Ga-Ge bilayer with Ga in a substitutional adsorption site. Deposition of 0.5 BL Ge on the Ga:Si(111)-$\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ surface leads to a change of the Ga adsorption site from ${T}_{4}$ to substitutional and the formation of very small 6.3\ifmmode\times\else\texttimes\fi{}6.3 domains with a local Ga coverage of 0.8 ML. Since the average Ga coverage of the $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ surface is 0.33 ML only, the change of adsorption site is accompanied by the formation of locally Ga free surface areas in coexistence with the 6.3\ifmmode\times\else\texttimes\fi{}6.3 domains. Thus, further Ge deposition on this surface leads to the formation of Ge islands of a uniform height.