Atomic and Electronic Structures of Reconstructed Si(100) Surfaces

New structural models for 21 and 42 reconstructed (100) surfaces of Si determined from energy-minimization claculations are presented. The optimal 21 and 42 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 21 and 42 reconstructed surfaces are discussed.