Model for the energetics of Si and Ge (111) surfaces

The reconstructions observed on the annealed (111) surfaces of Si and Ge can be explained on the basis of a simple model of the surface energetics. The driving force for the Si 7×7 reconstruction is the formation of dimer-row domain walls, which order in a triangular pattern for topological reasons. Adatoms play an incidental role only. Chemical or strain-induced variations in the parameters of the model can lead to transitions between energetically competitive 7×7, 5×5, $c2\mathrm{}\times 8$, and other structures.