Si(111)-7×7 surface: Energy-minimization calculation for the dimer–adatom–stacking-fault model

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Abstract
An energy-minimization calculation was performed on the Si(111)-7×7 surface reconstruction model proposed by Takayanagi et al. The structure contains dimers, adatoms, and stacking faults (DAS) in the surface layer. The DAS model has 19 dangling bonds and 12 ‘‘top-site’’ adatoms per 7×7 unit cell. Hellmann-Feynman forces were used to relax adatom and three layers of a five-layer slab consisting of 249 atoms. The surface energy is found to be 0.40 eV per 1×1 cell lower than that of an ideal and unrelaxed Si(111) surface, as compared to 0.36 eV for the π-bonded chain structure for the 2×1 cleaved surface. This is the lowest surface energy calculated for the Si(111) surface. Atomic configurations were obtained for the completely relaxed system. The calculated surface electronic density of states and the atomic origin of the surface electronic structure are in good agreement with experimental data. The results of similar calculations for Si(111)-5×5 DAS and Si(111)-c(2×8) dimer-chain models are compared and discussed.