Geometry and electronic structure of GaAs(001)(2x4) reconstructions

Structural and electronic properties of the As-rich GaAs(001)(2X4) reconstructions are investigated by means of converged first-principles total-energy calculations. For an As coverage of Theta=3/4, we find the two-dimer beta 2 phase to be energetically preferred over the three-dimer beta phase. As the As chemical potential decreases, the alpha phase of GaAs(001) represents the ground state of the surface. All geometries are characterized by similar structural elements as As dimers with a length of about 2.5 Angstrom, dimer vacancies, and a nearly planar configuration of the threefold-coordinated second-layer Ga atoms leading to a steepening of the dimer block. Consequently, the resulting electronic properties also have similar features. The surface band structures are dominated by filled As-dimer states and empty Ga dangling bonds close to the bulk valence- and conduction-band edge, respectively. The measured Fermi-level pinning cannot be related to intrinsic surface states. The calculated surface states and ionization energies support the beta 2 structure as the surface geometry for an As coverage of Theta=3/4.