Cui et al. Nanoscale Res Lett 2011, 6:59 http://www.nanoscalereslett.com/content/6/1/59 grow QDs. The substrates were chemically cleaned by Shiraki [21] method before put into load-lock chamber. After transferred into growth chamber, the substrates were heated to 980°C for 10 min to remove the protect- ing oxide, then clear 2 × 1 reconstruction pattern could be observed by reflection high energy electron diffrac- tion. The chamber pressure was below 1 × 10-9 Torr. Then a 50 nm thick Si buffer layer was deposited at 650°C with a growth rate of 0.36 Å/s. At 640°C, by two- step growth method [22], two layers of Ge with a total thickness of 1.0 nm were deposited in sequence to form uniform QDs. The deposition rate was 0.11 Å/s. Then the sample was cooled down to 300°C to grow Si cap- ping layers at a growth rate of 0.54 Å/s. The in-situ annealing process was performed at a high temperature of 640°C. After growth or annealing processes, the tem- perature was decreased to room temperature immedi- ately. In order to unveil the detailed kinetics for the nanopit formation, in-situ annealing at a low tempera- ture of 540°C was also carried out. Atomic force micro- scopy (AFM, Solver P47-SPM-MDT) was used to measure surface morphology. Cross-sectional transmis- sion electron microscopy (TEM) was used to character- ize microstructures. Results and discussion Figure 1a shows the surface morphology of the as-grown QDs. Two types of QDs, i. e. with shape of dome or hut, can be observed. The density of the dome-shaped QDs Figure 1 AFM image (2 × 2 μm 2) of a the as-grown SiGe QDs, b the Si capping layer grown at 300°C with a thickness of 50 nm. c the Si capping layer after in-situ annealing at 640°C for 10 min. d Cross-sectional TEM image of the sample shown in (c). Page 2 of 5 is 3 × 109cm-2. The Si capping layer grown at a low temperature of 300°C on QDs shows similar surface morphologies [5] with a slight change in shape from dome to mound, as shown in Figure 1a and 1b. It is much different from the surface morphology of Si cap- ping layer grown at a high temperature, i. e. the QRs with a thin Si capping layer and flat surface with a thick Si capping layer [3], since the surface migration of Si atoms and the interdiffusion between Si and Ge at this low temperature were inhibited greatly. After in-situ annealing at 640°C for 10 min, square nanopits were formed, as shown in Fig.1c. The surface morphology keeps unchanged with increasing annealing time. The nanopit density is almost the same as that of the as grown dome-shaped QDs, which indicate that the for- mation of the nanopits highly correlates to the QDs. Cross-sectional TEM image (Figure 1d) indeed shows that below each nanopit there exists one QD, confirm- ing that the formation of the nanopits is associated with the buried SiGe QDs. In order to unveil the kinetic process of the nanopit formation, in-situ annealing was carried out at a low temperature of 540°C for a shorter period of 7 min (see Figure 2), which slows down all the atomic processes for the nanopit formation. Several typical profiles in transi- tional states from a mound to a nanopit are extracted from Figure 2a and plotted in Figure 2b. A large portion of the transitional states can be observed, which is due to the non-uniformity of the QDs, as well as the statisti- cal fluctuation in the atomic processes. It is reasonable to take these profiles as the snaps at different evolution- ary states, which are shown by lines 1–6 in Figure 2. From the evolutionary states, several features or conclu- sions can be drawn. (1) The Si atoms at the rims of the mounds together with the outer shell of the mounds may first migrate outwards, which is concluded by com- parison of the profiles before annealing (dotted line in Figure 2b) and the ones in transitional states after annealing (solid lines 1–6 in Figure 2b). (2) After the initial migration, the migration of Si atoms at the rims tends to stop, and trenches around each mound are formed with a nearly constant depth of 2.5 nm. The deepest positions of the trenches are labeled by two ver- tical blue lines in Figure 2b and two white lines in Fig- ure 2c–e. The distances between the lines are consistent with each other to be about 50 nm. (3) The final shapes of the nanopits keep nearly unchanged after further annealing. Strain energy relaxation and surface energy minimiza- tion are considered as the two dominating factors for the nanopit formation. Firstly, atomic intermixing or interdiffusion between Si and Ge, and Ge surface segre- gation could be excluded during the process for the nanopit formation because the mounds consist purely of