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

Abstract

We have investigated the surface chemistry of (${\mathrm{NH}}_{4}$${)}_{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\ifmmode^\circ\else\textdegree\fi{}. 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 \ifmmode^\circ\else\textdegree\fi{}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 \ifmmode^\circ\else\textdegree\fi{}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. \textcopyright{} 1996 The American Physical Society.