(4x2) and (2x4) reconstructions of GaAs and InP(001) surfaces

We report results of first-principles total-energy calculations. The atomic and electronic structure and the formation energy of the Ga-rich GaAs(001)-beta 2(4 x 2) surface is compared with recent results for the As-rich GaAs(001)-(2 x 4) surface reconstructions. The relaxed beta 2(4 x 2) structure is characterized by an appreciable decrease of the distance between the first and second atomic layer and gives rise to unoccupied Ga-dimer-derived surface states in the upper half of the GaAs bulk band gap. Its stability is limited to extreme Ga-rich conditions. For the InP(001) surface, we investigate a series of (4 x 2) and (2 x 4) reconstructions that were suggested in order to explain recent experiments. Among these models, a structure containing three-and four-fold coordinated In atoms is energetically favourable, independent of the chemical potentials of the surface constituents.