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The modification of clean GaAs͑100͒ surfaces by in situ deposition of molecular sulfur was investigated by soft x-ray photoemission spectroscopy. Upon S treatment of the clean GaAs͑100͒ sample at 435-455°C in ultrahigh vacuum the formation of a three monolayer thick gallium sulfide-like compound is observed, which exhibits a ͑2ϫ1͒ low-energy electron diffraction pattern. Due to the S modification on n-GaAs a reduction of the band bending by 0.35 eV is achieved, while the band bending on p-GaAs is increased by 0.17 eV. The subsequent Mg evaporation leads to the formation of a metal/semiconductor contact with a reacted magnesium sulfide-like compound at the interface. After 1 nm Mg deposition the Schottky barrier height of the S-modified Mg/n-GaAs(100) contact amounts to 0.44 eV, which is 0.18 eV lower than without S modification, while the Mg/p-GaAs͑100͒ Schottky contact exhibits an increase in the Schottky barrier height by 0.30 eV in comparison to the value of the unmodified Schottky contact ͑0.55 eV͒.

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