Control of strain-induced formation of self-assembled silicon germanium epitaxial nanostructures

Heteroepitaxial growth of semiconductor device structures typically results in large strains developing in the epitaxial layer due to the lattice mismatch. Depending on the growth conditions, this strain may be reduced through the formation of islands on the surface of the film, the injection of misfit dislocations, or interdiffusion. There has been recent widespread interest in using island morphologies produced by such strain relieving mechanisms as a possible way to form quantum structures for new devices. However, it is still necessary to find a way to control island size, shape, and placement in order to form useful devices. We have combined in-situ stress measurements with ex-situ microscopy to map out surface morphology and dislocation relaxation regimes as a function of the various growth parameters. We have found that growth of SixGe1-x films under kinetically limited conditions results in dramatically different surface morphologies than growth under conditions of high adatom mobility. For a given composition, the correct combination of growth rate and temperature can lead to formation of strain relieving (501) faceted pyramidal pits, and finally the cooperative nucleation of islands surrounding the edges of each pit. The size of the resulting self-assembled four-fold nanostructure is inversely proportion to strain and stable over a large thickness range and upon annealing. However, pits that are not surrounded by island walls elongate in a single dimension to form grooves in the film upon annealing. In order to control placement of such structures, we have used a focused ion beam (FIB) to fabricate holes of dimensions and spacings that create interstices in which a single four-fold nanostructure will form. These methods may provide a new route to hierarchical assembly of nanostructures with potential applications to novel nanoelectronic device architectures.