BOUND BIPOLARON AT THE SURFACE - THE NEGATIVE-U BEHAVIOR OF GAAS(110)WITH ADSORBED ALKALI-METALS

Using density-functional theory we investigate the electron-lattice co upling of Ga dangling bonds at GaAs(110)/Na and GaAs(110)/K. We find t hat the electron capture at the bond causes a significant local surfac e unrelaxation. The lattice distortion even overcompensates the electr on-electron repulsion, leading to the formation of a two-electron boun d state (surface bipolaron), which manifests the negative-U behavior o f the Ga orbital. The electron pairing favors the clustering of adatom s. The influence of the pairing on scanning tunneling microscope image s and surface core-level spectra is discussed.