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 small er 'hut' clusters (width 30-60 nm, {105} side facets) and the larger 'domes ' (width 60-100 nm, mostly {113} side facets) have been investigated. We ob tain 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 tra nsition towards the shape of truncated pyramids. Even for the large dome cl usters an abrupt decrease in the aspect ratio z/x can be observed at Si cov erages as low as a few monolayers. We discuss the mechanisms, which can lea d to this behaviour and compare with the observations of other studies. (C) 2000 Elsevier Science S.A. All rights reserved.