Sulfur adsorption at GaAs(1 0 0) from solution: role of the solvent in surface chemistry

The chemistry of GaAs(1 0 0) oxide-free surfaces treated with solutions of ammonium sulfide in different amphiprotic solvents (water, alcohols) was studied by photoemission spectroscopy in order to clarify the role, which the solvent plays in adsorbate interaction with the surface atoms at the semiconductor/solution interface. Adsorption of sulfur from solution results in the appearance of As–S bonds with solvent-dependent chemical shifts vs. the As–Ga bulk emission. Bonds with larger chemical shifts are formed in low dielectric constant alcohols. Annealing of the surfaces results in the decomposition of As–S bonds and the appearance of Ga–S bonds. At the surfaces treated with high dielectric constant solutions (water- or methanol-based) elemental arsenic appears after As–S decomposition. At the surfaces formed in the solutions with low dielectric constant (isopropanol- or tert-butanol-based) the formation of new Ga–S species with annealing is accompanied by a decrease in the As–S chemical shift. Further annealing of the surface causes the disappearance of As–S bonds and the appearance of As–As surface dimers. Valence band spectra of GaAs(1 0 0) surfaces after annealing contain a solvent-dependent Ga–S related valence state. The observed variation of surface chemistry with change of sulfide solution is related to the variation of the adatom reactivity by different solvation shells.