Control of semiconductor quantum dot nanostructures: Variants of Si _x Ge_1-x/Si quantum dot molecules

We examine variations in the basic structure of quantum dot molecules (fourfold quantum dot nanostructures forming around a central facetted pit) in the Si_x Ge_1-x/Si (100) system. Arrays of quantum dot molecules are seeded by Ga+ focused ion beam (FIB) prepatterning of the Si substrate prior to epitaxial Si buffer layer growth and Ge_x Si _1-x film deposition. Five main variants to the regular quantum dot molecule structure are observed. The populations of these variant structures depend on the initial FIB processing conditions; their frequencies generally increase with increasing prepatterned pit depth and with increasing incident ion energy. This work suggests both routes to improving uniformity of regular quantum dot molecule arrays as well as routes to enabling synthesis of a wider range of nanostructure geometries.