Sulfide-passivated GaAs(001) .1. Chemistry analysis by photoemission and reflectance anisotropy spectroscopies

We have investigated the surface chemistry of (NH4)(2)S-passivated GaAs(001) by studying this surface after passivation and after several subsequent annealings. We have used ultraviolet photoemission spectroscopy (UPS) at a near-normal takeoff angle and at a takeoff angle of 70 degrees. We have also used, in the same ultrahigh-vacuum chamber, reflectance anisotropy spectroscopy, which probes the optical transitions due to surface dimers, and therefore provides useful information for the interpretation of core-level spectra. We find that gallium dimers appear after annealing to 520 degrees C due to sulfur desorption. At the same stage, there appears in the corresponding 3d core-level spectrum a surface component lying at a relative binding energy of -0.32 eV with respect to the volume contribution. Arsenic dimers appear after annealing to 360 degrees C, following the desorption of excess arsenic. In the same way, there appears in the arsenic 3d core-level spectrum a component at a relative binding energy of -0.28 eV. After desorption of the passivating overlayer the gallium-terminated part of the surface is essentially covered with sulfur. These sulfur atoms are dimerized, and contribute to a line in the reflectance anisotropy spectrum. At this same stage, the arsenic-terminated one is covered with excess arsenic. Unlike what was believed previously from UPS analysis, As-S chemical bonds are situated only in the passivating overlayer, which contains no gallium atoms. Comparison of the core-level spectra as a function of takeoff angle shows that the progressive desorption of this overlayer produces a change of its morphology.