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

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×{}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)-√3×√3 surface leads to a change of the Ga adsorption site from T₄ to substitutional and the formation of very small 6.3×{}6.3 domains with a local Ga coverage of 0.8 ML. Since the average Ga coverage of the √3×√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×{}6.3 domains. Thus, further Ge deposition on this surface leads to the formation of Ge islands of a uniform height.