GaAs(100): Its spectrum, effective charge, and reconstruction patterns

We have carried out self-consistent wave-mechanical calculations for the Ga-terminated GaAs (100) surface in its unreconstructed form. The calculations were done for an ideal termination geometry, for which the spacing between the Ga and As layers is 2.67, and two additional geometries, one for which the Ga surface layer was relaxed inward by 0.38 a.u., the other for which it was expanded by 0.23 a.u. The most prominent spectral features found were two bands of gap surface states, one having dangling-bond character, the other having $p$ character within the Ga plane. The dangling-bond band is $p_y$ band. These bands are similar to those found for Si(100). The lower band is three-quarters full. Its Fermi surface lies in a region of high electron state density and possesses an almost perfect diamond shape. The connection between the band-gap spectrum for the calculated (1 × 1) geometry and the experimentally realized $c(8\mathrm{}\times 2)$ and (4 × 2) structures is discussed in terms of metal-insulator instabilities. We have also calculated the effective charge on the surface Ga atoms from the change in ionization potential produced by moving the Ga layer. This number, 0.20 electrons, is close to the measured bulk value.