14 Nanoscale Res Lett (2006) 1:11–19 Fig. 4 Ge nanocluster formation on Si(100). (a) STM image (100 · 100 nm2) taken at room temperature following deposition of 0.5 ML Ge on 10 ML Xe buffer. Two dashed lines indicate possibility of a 3D carrier conﬁnement, which is desirable for applications in optoelectronics. The sizes of Ge nanoclusters can be tuned by changing the amount of deposited Ge when using the BLAG method. It has been shown that 4 ML Xe is sufﬁcient to buffer the substrate [18]. With a constant Xe buffer layer thickness of 6 ML, the average nanocluster size is approximately 2.5 nm laterally and 0.4 nm in the growth direction when the Ge coverage is 0.5 ML, while at a Ge coverage of 6 ML, the nanoclusters grow to approximately 7.5 nm wide and 1 nm high (Fig. 5). The nanocluster density increases slightly in the low coverage range of 0.5–1 ML and then decreases at higher coverage. This is consistent with conventional molecular beam epitaxy growth of the ﬁrst monolayer where the early stages exhibit nucleation of islands followed by a steady-state regime with constant island density and the later stages exhibit island coalescence. The weak coverage dependence of the nanocluster density in the low coverage regime indicates that the step edges of Si surface after the nanocluster formation. (b) Size distribution for Ge nanoclusters adatoms on the Xe buffer are mobile enough to reach the existing nuclei with very few new nanoclusters being nucleated. At high Ge coverage as shown in Fig. 5c, coalescence of nanoclusters results in a decrease of density compared to the low coverage case. The morphologies of the nanoclusters can also be modiﬁed by varying the Xe buffer-layer thickness. The initial shapes of nanoclusters on the buffer layer are determined by the competition between thermody- namics, which favors equilibrium structures with min- imal surface areas, and kinetics that imposes constraints due to low temperature, resulting in a high density of nearly round clusters. As shown by com- paring Fig. 6a and b, these nanoclusters enlarge as the Xe buffer thickness increases from 6 ML to 10 ML at the same Ge coverage of 0.5 ML. This phenomenon has been ascribed to desorption-assisted coalescence [18] in which the nanoclusters have more time to move and coalesce prior to contact with the Si substrate when landing from a thicker Xe layer. Moreover, Fig. 5 STM images showing morphology evolution of Ge nanoclusters as a function of deposited Ge atoms. (a) 0.5 ML, (b) 1 ML, and (c) 6 ML. The thickness of Xe buffer layer is the same in all cases. All images are 100 · 100 nm2