Nanoscale Res Lett (2006) 1:11–19 Fig. 6 STM images showing coalescence of Ge nanoclusters with increasing Xe buffer-layer thickness. (a) 6 ML Xe buffer, (b) 10 ML Xe buffer, and (c) 40 ML Xe buffer. The quantity of Ge deposited (0.5 ML) is the same in all cases. All images are 100 · 100 nm2 15 clusters of nanoclusters develop when the Xe buffer thickness increases from 6 ML to 10 ML with an elongation of some nanoclusters appearing, possibly because of incomplete coalescence. Aggregation of nanoclusters into chains develops for a buffer-layer thickness of 40 ML as shown in Fig. 6c. A similar behavior was recently reported for Au na- noclusters on graphite grown with much thicker buffer layers [8]. The aggregated cluster structure can be attributed to a competition between the arrival rate of new clusters and the time needed to coalesce. Desorption of thicker buffer layers generates more cluster movement and thus leads to a more pronounced cluster aggregation that outpaces the coalescence pro- cess. The local structures at the Ge nanocluster/Si interface have been investigated using transmission electron microscopy (TEM). Figure 7a displays a cross- sectional Z-contrast TEM image of the sample with 6 ML Ge, showing the structure of Ge nanoclusters sandwiched between the amorphous Si cap and the crystalline Si substrate. A higher magniﬁcation image in Fig. 7b indicates that the Ge island makes an obtuse contact angle with the Si substrate. No coherent Ge layer between the Ge nanoclusters is observed, con- ﬁrming the absence of a wetting layer [11]. Moreover, no lattice coherence appears at the Ge nanocluster/Si interface. The observed shape of the amorphous is- lands is different than the hut clusters with {510} facets normally seen when Ge is deposited on the Si(100) surface [13]. This is explained by the BLAG environ- ment in which Ge adatoms interact and form clusters with essentially no inﬂuence from the Si substrate. After increased exposure to the electron beam of the microscope, the Ge clusters crystalline structure start to show up. In Fig. 7c, these Ge islands appear to adopt a pseudomorphic structure with the Si substrate. Fig. 7 Cross sectional Z- contrast images of 6 ML Ge deposited on 6 ML Xe buffer layer on Si(001). (a) Ge nanoclusters sandwiched between the Si substrate and an amorphous Si capping layer. (b) Higher magniﬁcation image showing a Ge cluster making an obtuse contact angle with the Si substrate. (c) Image of a Ge island after increased exposure to the electron beam of the microscope showing the dot has recrystallized to adopt a pseudomorphic structure with the Si substrate