Hydrogen adsorption on the GaAs(001)-(2×4) surface: A scanning-tunneling-microscopy study

Using scanning tunneling microscopy (STM) we have investigated the process of hydrogen adsorption on the GaAs(001)-(2×{}4) surface. Stepwise hydrogen-induced changes in the atomic structure of the surface are revealed in the STM images. In a first step, one or two hydrogen atoms bond per As atom, giving rise to localized protrusions within the As dimer rows. In a second step, after adsorption of a third H atom per As atom, AsH₃ forms and desorbs, resulting in depressions and thus a degradation of the As dimer rows. Finally, at high exposures a disordered surface develops.