Dimer-chain model for the 7×7 and the 2×8 reconstructed surfaces of reconstructed surfaces of Si(111) and Ge(111)

A dimer adatom stacking-fault model for the Si(111)-7×{}7 reconstructed surface structure is described as a network of walls and domains. The wall consists of a chain of dimers and the domain of adatom clusters and adatoms arranged locally in a 2×{}2 structure on a ``1×{}1'' surface lattice. It is shown that with a proper wall configuration (dimer chain), a new structure model for the Ge(111)-2×{}8 reconstructed surface can be made. The structure, which has four adatom clusters and three dimers in a unit cell with only four dangling bonds, is consistent with the scanning tunneling-microscopy image and electron-diffraction data. A dimer-chain structure which describes reconstructed structures of n×{}n with odd periodicity and 2×{}m with even periodicity is presented.