Thermal and photochemical pathways of H2S on GaAs(100)

The thermal and photo-induced dissociation of H2S on the Ga-rich GaAs(100) surface has been studied by synchrotron radiation soft X-ray photoelectron spectroscopy (SXPS), in conjunction with thermal desorption spectroscopy (TDS) and low-energy electron diffraction (LEED). H2S adsorbs in both molecular and dissociative forms at a surface temperature of 90 K, with a saturation coverage of 0.57 ML. The molecularly adsorbed H2S is unstable and desorbs/dissociates below 200 K; above 200 K, only SH and atomic S species are left on the surface. The dissociation of surface SH species is accompanied by the recombinative desorption of H2S and H-2. This process is completed by similar to 450 K, above which temperature surface sulfur atoms in two different chemical environments are observed. These sulfur atoms are attributed to Ga-S monomer and dimer surface species, respectively. The former is converted to the latter upon annealing al 700-750 K. The annealed and dimerized GaAs(100)/S surface shows a (2 x 1) LEED pattern in a broad sulfur coverage region. Both surface H2S and SH species dissociate readily upon UV laser or white light synchrotron radiation. At 193 nm, the initial photodissociation cross-sections for adsorbed H2S and SH species were estimated to be 6 x 10(-18) and 6 x 10(-19) cm(2), respectively. As a result of photodissociation, the amount of sulfur deposition can be significantly increased. At sulfur coverages above 0.5 ML, annealing at 750 K results in two distinctively different sulfur states, which are attributed to surface and subsurface S, respectively. (C) 1997 Elsevier Science B.V.