SYNCHROTRON RADIATION PHOTOEMISSION ANALYSIS FOR (NH4)2SX-TREATED GAAS

The chemistry of the (NH4)2S(x)-treated n-GaAs (100) surfaces has been studied using synchrotron radiation photoemission spectroscopy. Ga 3d, As 3d, and S 2p photoemission spectra are measured before and after annealing in vacuum with a photon energy of about 210 eV, where S 2p core level spectra can be sensitively detected. It is found that Ga-S, As-S, and S-S bonds are formed on the as-treated GaAs surfaces, and that stable Ga-S bonds become dominant after annealing at 360-degrees-C for 10 min in vacuum. The thickness of the surface sulfide layer is reduced from about 0.5 to 0.3 nm by annealing. The surface Fermi-level position of the as-treated surfaces is determined to be about 0.8 eV below the conduction band minimum, which is about 0.1 eV closer to the valence band maximum than that of the untreated surfaces. A Fermi-level shift of 0.3 eV toward a flat band condition is also observed after annealing. It is found that the Ga-S bonding plays an important role in passivating GaAs surfaces.