Three-dimensional Ge islands between 15 and 200 nm in size were found to grow only on Si(111) surface windows in ultrathinSiO2film after Ge deposition and subsequentSiO2decomposition. The size of Ge islands gradually decreased as the Ge thickness decreased. Pseudomorphic two-dimensional Ge layers with the5×5structure formed in surrounding areas of the windows. The windows were produced by selective thermalSiO2decomposition induced by focused electron beam irradiation. These results suggest a new technique for nanometer-scale Ge island fabrication at given points on Si surfaces. ©1998 American Institute of Physics.