SURFACE POLARONS AND BIPOLARONS AT GAAS(110) WITH ADSORBED ALKALI-METALS

We performed density-functional theory calculations for Na or K adsorbates on GaAs(110). The alkali-metal adatoms are found to donate their valence electrons into Ga dangling bonds. This results in a lattice relaxation around the occupied Ga orbital with a very strong polaron shift of about 1 eV. Moreover, it is predicted that two polarons can merge, forming a surface bipolaron. The polaron as well as the bipolaron states are localized, which explains the observed non-metallic character of the surface. The formation of the localized bipolaron can be also described in terms of a ''negative-U'' behavior of the Ga dangling orbital. We discuss how the formation of bipolarons may affect STM images and surface core-level photoemission spectra.