Cui et al. Nanoscale Res Lett 2011, 6:59 http://www.nanoscalereslett.com/content/6/1/59 Page 4 of 5 Figure 3 Cross-sectional schematic diagrams of strain distribution in the Si capping layer and Si migration during annealing. a At the beginning of annealing, both the strain energy relaxation and the surface energy minimization drive Si atoms at the rim migrate outwards. The Si atoms at the central part of the mound migrate outwards at a much slower rate. b When the surface at the rim is below the horizontal plane, the surface energy minimization drives Si atoms at the surrounding area to migrate inwards, the strain energy relaxation drives the Si atoms to migrate outwards. The trench is formed when the two opposite fluxes compensate each other. c After the trench is formed, the Si atoms at the central part still migrate outwards at a much slower rate driven by both strain energy relaxation and surface energy minimization. d When the surface area begins to increase, the surface energy minimization turns to induce Si to migrate inwards. When the inward Si flux induced by surface energy minimization compensates the outward Si flux induced by strain energy relaxation, the nanopit with well-defined shape is formed finally. The red line indicates the final profile of the nanopit. The curly arrows indicate the Si migration directions induced by strain energy relaxation or surface energy minimization. well-defined shape are formed finally. If only the strain energy is considered, no inward flux exists, the nanopits will become deeper and deeper with annealing time. No significant difference can be observed between the pro- files of the nanopits formed after annealing for 10 and 30 min at 640°C, as shown in Figure 4, further confirm- ing that the formed nanopits are stable at the tempera- ture of 640°C, and both strain energy and surface energy are responsible for the formation of the nanopits. QD molecules are interesting for their applications in quantum computation. The fabrication of QD molecules is thus interesting for both fundamental research and practical applications. Here, we report a growth method of SiGe QD molecules on the nanopit-patterned surface formed by the process described above. Figure 5 shows the surface morphology of QD molecules by depositing 0.6 nm Ge on the nanopit-patterned surface at 650°C. The subsequently formed QDs are located at the sides around the nanopits. The boundaries are along 〈100〉 rather than 〈110〉 for the nanopits, which is induced by the preferential nucleation of Ge at the corners of the nanopit for smaller surface energy at the initial stage and subsequent strain energy relaxation during the islanding growth [12]. Compared with the work of Deng and Krishnamurthy [12], in which the nanopits are induced by carbon impurities, the remarkable advantage of the present work over theirs is that in the present growth process no other elements are involved, except Si and Ge. However, planar defects or stacking faults are formed in the capping layer in our case because the thickness of capping layer is larger than 20 nm, a critical thickness to generate stacking faults [5]. If the thickness of the capping layer is less than 20 nm, it is possible to grow defect-free QD molecules. Figure 4 Profiles of nanopits formed after annealing at 640°C for 10 and 30 min, respectively. Figure 5 AFM image (1 × 1 μm 2) of the QD molecules grown on the nanopit-patterned template.