Isolated Ge islands, i.e., islands not connected by a wetting layer can be obtained by selective epitaxial growth in voids of ultrathin oxides of thickness 1–2 nm. Voids of 30–600 nm size were created before epitaxy during a high temperature anneal of the ultrathin oxide. The formation of one island per window was investigated at 700 °C as a function of Ge thickness and void size. Islands nucleate mainly at the edge of the void and for this reason they have an anisotropic shape. In voids smaller than 300 nm only one island is nucleated. Islands form only in voids greater than a critical size (30–80 nm) which depends on the total amount of Ge deposited. We observe that height, width, and aspect ratio of isolated islands increase with void size for a given Ge thickness. A metastable state of Ge in small windows was observed. Moreover, the Si interdiffusion is strongly reduced with decreasing island size (i.e., with void size) reaching only ∼10% in comparison with ∼50% in islands on large areas. © 2004 American Institute of Physics.