Si overgrowth of self-assembled Ge clusters on Si(001) — a scanning tunnelling microscopy study

The first stages of the Si overgrowth of self-assembled Ge quantum dots on Si(001) have been studied by scanning tunnelling microscopy. Both the smaller ‘hut’ clusters (width 30–60 nm, {105} side facets) and the larger ‘domes’ (width 60–l00 nm, mostly {113} side facets) have been investigated. We obtain direct evidence of a distinct change of the morphology of the clusters even for low Si coverages. In case of the hut clusters this leads to a transition towards the shape of truncated pyramids. Even for the large dome clusters an abrupt decrease in the aspect ratio z/x can be observed at Si coverages as low as a few monolayers. We discuss the mechanisms, which can lead to this behaviour and compare with the observations of other studies.