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)-6.3x6.3 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)-root 3x root 3 surface leads to a change of the Ga adsorption site from T-4 to substitutional and the formation of very small 6.3x6.3 domains with a local Ga coverage of 0.8 ML. Since the average Ga coverage of the root 3x root 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 6.3x6.3 domains. Thus, further Ge deposition on this surface leads to the formation of Ge islands of a uniform height.