Structural study of Sn-induced superstructures on Ge(111) surfaces by RHEED

When less than 0.3 monolayer of tin was deposited onto a clean Ge(111) surface at room temperature and then heated to high temperatures, the 2 × 8 superstructure characteristic of the clean Ge(111) surface changed to a 1 × 1 structure with twofold-splitting diffuse scattering. As the mean thickness of Sn exceeded 0.3 monolayer, (√3 × √3)R30° and 7 × 7 superstructures were formed on the surface. The periodicity of the former structure is familiar to many metal-induced superstructures on Ge(111) and Si(111) surfaces, while the latter is very interesting as having the same periodicity as a well known 7 × 7 superstructure of clean Si(111) surfaces. By increasing the amount of the deposit beyond {1}/{2} monolayer in mean thickness, a 5 × 5 superstructure appeared on the surface. This structure also is to be noted, because a surface structure with the same periodicity is observed on a homoepitaxial and a heteroepitaxial Si(111) surface. Structural models for the Ge(111)(7 × 7)-Sn and Ge(111)(5 × 5)-Sn structures are proposed.