Studies of compositional variations in germanium quantum dots grown on silicon

We report optical and scanning transmission electron microscopy studies of germanium dots grown on silicon. In an attempt to control the self-organized growth process and promote dot size uniformity the dot layers were grown on a 4.5nm Si_0.6Ge_0.4 alloy template layer. Photoluminescence results indicate the formation of carrier confining Ge rich islands, whilst Raman scattering results indicate the presence of an alloy throughout the structures formed. The samples were studied in the UK high resolution scanning transmission electron microscopy facility at Liverpool, UK. Energy dispersive analysis of individual line scans through the sample show that the structures are composed of an alloy throughout with an asymmetric distribution of Germanium in the dots and in the wetting layer close to the dots. We discuss the results in the light of the proposed growth mode for these dots and conclude that attempts to manipulate the composition of these dots during growth may be problematic due to the self-organized nature of their formation.