Passivation effect of (NH₄)₂S_x treatment on GaAs surface before photo-resist and O₂ processes

Surface properties of GaAs passivated with (NH₄)₂S_x solution have been analyzed in GaAs wafer and photoresist-treated GaAs using X-ray photoelectron spectroscopy. Sulfur treatment on GaAs surface results in the formation of S-Ga and S-As bonds, which remain after successive rinsing for 1 min in DI water. After oxygen plasma treatment with the HCl-treated sample, the GaAs surface is converted to an As-rich surface, namely, arsenic composition is higher than gallium composition. Meanwhile, the compositions of gallium and arsenic keep almost constant, even after oxygen plasma treatment of the sulfidation-treated sample. This indicates that As-S and Ga-S bonds are revealed to resist the oxidation of the surface. Through in-situ annealing under UHV condition, it is found that the Ga-O bond is thermally stable, but the As-S bond unstable.