New adatom model for Si(111) 7×7 and Si(111)-Ge 5×5 reconstructed surfaces

A new adatom structure with significantly less angular strain than the simple adatom geometry is proposed. The model involves a rebonding of ⋍⅛ of surface atoms to the substrate in a manner similar to that occurring in the $\pi$-bonded-chain structure for the Si(111) 2×1 surface. The interference between adatom and substrate reconstructions forces the smallest threefold-symmetric unit cells to be 5×5 and 7×7 in size. The proposed reconstructed-adatom model gives rise to structural features in good agreement with experiment. In particular, the inhomogeneous corrugation of the two halves of the 7×7 unit cell seen in vacuum-tunneling microscopy and the apparent need for stacking fault sequences in ion-channeling experiments are accounted for. The results of surface energy and structure calculations on 2×2, 3×3, 5×5, and 7×7 adatom models are presented.