Shape of epitaxial Ge islands on Si(100) surfaces

The surface morphology of Ge layers, obtained by Ge deposition on the Si(100) surface and on the Si(100) surface covered with ultrathin SiO₂ films, is studied with scanning tunneling microscopy. The SiO₂ film is partly decomposed at temperatures above 500 °C and, therefore, it does not prevent the Ge epitaxial growth with respect to the crystalline structure of the Si substrate. We found that the presence of the SiO₂ film leads to the formation of relaxed Ge epitaxial islands, which differ from strained Ge islands that form on bare Si(100) surfaces and contain a significant amount of Si due to the strain-induced Si diffusion from the substrate. The results show that the use of the SiO₂ film allows one to control the Ge-Si composition of the islands and their shape.