Cui et al. Nanoscale Res Lett 2011, 6:59 http://www.nanoscalereslett.com/content/6/1/59 Page 3 of 5 Figure 2 a AFM image of the Si capping layer after in-situ annealing at 540°C for 7 min. b Cross-sectional profiles of an as grown QD (dashed), a Si mound before annealing (dotted), the transitional states from mounds to nanopits after annealing at 540°C for 7 min (solid lines 1–6), and a Si nanopit formed after annealing at 640°C for 10 min (red dash dot). Cross-sectional TEM images of c a Si mound before annealing, d a Si mound after annealing at 540°C for 7 min, and e a Si nanopit formed after annealing at 640°C for 10 min, the buried QD is also shown. The scale bar in (d) applies to (c) and (e). The two blue lines in (b) and two white lines in (c)–(e) indicate the deepest position of trenches. Si atoms and the capping layer is thick enough for rul- ing out Ge surface segregation (50 nm). The diffusion of Ge atoms in Si at the temperature lower than 650°C can be neglected. The whole process for the nanopit forma- tion is only related to Si atomic migration. Due to the existence of SiGe QD beneath the capping layer, the strain in the capping layer is tensile over the QD [23], compressive at the rim and over the wet- ting layer. The strain energy gradient in the compressive region is much larger than that in tensile region [5], so the Si migration is faster at the rim than at the central part over the QDs. The migration processes for the Si atoms at the rim and at the central part may be distin- guished as fast migration and slow migration, respec- tively. During annealing, the Si atoms at the rim migrate rapidly outwards onto the unstrained flat surface via the fast migration process, which results in the formation of the trench. At the same time, the Si atoms at the central part also migrate outwards via the slow migration pro- cess, which decreases the height of the mounds. Besides, the surface energy may play another important role due simply to the change in the surface area. At the begin- ning, the surface energy is also reduced due to the shrinkage of the surface area, which enhances the out- ward migration additionally, the scenario at this stage is depicted in Figure 3a. With the migration processing, when the surface at the rim is below the horizontal plane, the surface area and thus the surface energy turn to increase, inducing an inward flux, as shown in Figure 3b. As a result, the net outward migration of the Si atoms at the rim is reduced. When the inward flux induced by the surface energy minimization at the rim compensates the outward flux by the strain gradient, the trench is formed and in thermal equilibrium, as shown in Figure 3b. After the trench is formed, the slow outward migra- tion is still processing because the Si atoms at the cen- tral part of the mound is still strained. When Si atoms pass by on the surface of the trench, those atoms would experience a fast migration process due to the large strain energy gradient induced by adding Si atoms, and migrate quickly to the unstrained surface region. Thus the trenches maintain almost the same profiles during the slow migration process, as shown by lines 1–6 in Figure 2b, with all the lines merging to one at the trenches. Similarly, at the beginning, the surface energy minimization enhances the migration, as shown in Figure 3c. Likewise, with the migration processing, when the surface area begins to increase, the surface energy minimization turns to reduce the outward migration of the Si atoms, or induce an inward flux, as illustrated in Figure 3d. When the inward flux driven by the surface energy minimization compensates the outward flux by the strain energy gradient, the nanopits with