Atomically resolved study of initial stages of hydrogen etching and adsorption on GaAs(110)

The initial stages of hydrogen adsorption on GaAs(110) surfaces at room temperature are investigated by atomically resolved scanning tunneling microscopy and spectroscopy. Two effects are found to occur simultaneously: On the one hand a surface phase separation occurs, creating 1x1 reconstructed fully hydrogen-covered areas while leaving the surface in between completely hydrogen free. In the fully hydrogen-covered areas, hydrogen bonds equally to As- and Ga-derived dangling bonds, unbuckling and passivating the surface. On the other hand, hydrogen-induced point defects are formed with increasing density. The dominating defects consist of As vacancy-hydrogen defect complexes, formed by preferential hydrogen etching of As. Using a defect-molecule model the Ga-H bridge bonds and double-occupied Ga dangling bonds are suggested to be at the origin of the observed surface Fermi level pinning 0.25 to 0.3 eV above the valence band edge, identical within error margins for p- and n-doped GaAs(110).