Ge nanostructures grown by self-assembly; influence of substrate orientation

The present paper discusses structural and optical properties of self-assembled random and non-random Ge dots on finite-area mesa substrates with facets with high-index orientation under conditions of strong surface diffusion. The island formation on the (001) top surface of the mesas was found to be different if other crystallographic planes are within the diffusion length. The growth on mesas is dominated initially by preferential nucleation of ordered islands on {hk1} facets and later by random nucleation of islands of more uniform sizes on the (001) top surface. The shape of ordered islands is influenced by the anisotropic properties of high-index surfaces. A transition of ordered islands from trapezoid-based to elongated domes with high aspect ratio was identified, with a mono-modal distribution in each stage of growth. We discuss these features in terms of strong island-island interaction and fast surface diffusion. The relevant change of the optical properties of ordered islands with respect to the random (001) case is the narrowing of photoluminescence peaks.