Since newly elaborated optoelectronic and nanoelectronic devices require more and more small sizes of their structural components, the thickness of investigated silicon oxide layers was reduced to several nanometers. Three-dimensional Ge nanoclusters about 20 nm in height and 30 nm in the basis were prepared by molecular-beam epitaxy on initially amorphous SiO_х (х<2) film of the thickness varying from 2 to 8 nm. Their distribution density over the substrate surface exceeded 10¹¹ cm^-2. In contrast to traditional heteroepitaxial formation of Ge quantum dots on Si(100) or growth of Si nanoclusters on SiO_х, when the correlation between the nanocluster height and lateral size is usually about b~8-10 and pronounced facets {113} are formed, in our case the nanoclusters were almost hemispherical with  b~1-2.

       Formation mode of three-dimensional Ge nanoclusters on initially amorphous SiO_х film is a question under discussion. Taking into consideration the fact that adhesion of Ge adatoms may be rather essential, the mechanism of 2D®3D transformation corresponds to the conditions of Stranski-Krastanov epitaxial growth realized in the presence of a lattice mismatch between the growing surface and Ge. We suppose that a large number of randomly aligned compound nanocrystallites, for example Si_1-xGe_xO_y, can be formed on the surface in our case. The presence of oxygen in these nanocrystallites even at a low concentration may essentially enlarge the strain relaxation. That is why the forming Ge nanoclusters take a shape of hemispheres with a low b coefficient and high distribution density over the surface that is more typical to Volmer-Weber epitaxial growth.

The work has been performed in the framework of Austra-Ukrainian joint scientific and technical collaboration supported by the Austrian Academic Exchange service under project #UA05/2007. Also this work is supported by the program of fundamental investigations “Nanosystems, nanomaterials and nanotechnologies”.

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