Atomic and Electronic Structures of Reconstructed Si(100) Surfaces

New structural models for 2×{}1 and 4×{}2 reconstructed (100) surfaces of Si determined from energy-minimization claculations are presented. The optimal 2×{}1 and 4×{}2 structures are found to correspond to asymmetric dimer geometries with partially ionic bonds between surface atoms, resulting in semiconducting surface electronic bands. The atomic and electronic structures for the 2×{}1 and 4×{}2 reconstructed surfaces are discussed.