Geometry and electronic structure of GaAs(001)(2×4) reconstructions

Structural and electronic properties of the As-rich GaAs(001)(2\ifmmode\times\else\texttimes\fi{}4) reconstructions are investigated by means of converged first-principles total-energy calculations. For an As coverage of \ensuremath{\Theta}=3/4, we find the two-dimer \ensuremath{\beta}2 phase to be energetically preferred over the three-dimer \ensuremath{\beta} phase. As the As chemical potential decreases, the \ensuremath{\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 \AA{}, 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 \ensuremath{\beta}2 structure as the surface geometry for an As coverage of \ensuremath{\Theta}=3/4. \textcopyright{} 1996 The American Physical Society.