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.