X-Ray Photoelectron Spectroscopy and Electrical Characteristics of Na2S-Passivated GaAs Surface: Comparison with (NH4)2Sx-Passivation

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Abstract
X-ray photoelectron spectroscopic (XPS) analysis was performed for the Na2S-passivated GaAs surface. The XPS data indicated that oxidation of GaAs was not suppressed by Na2S passivation, and fair amounts of As2O3 and Ga2O3 were observed after exposure to air for 3 days in contrast with the case of (NH4)2S x passivation. However, segregation of elemental arsenic was found to be substantially suppressed by this passivation. Both As x S y (probably AsS) and As2S3 were observed at the Na2S-passivated surface. This is due to diffusion of elemental As through the thick Na2S film and its reaction with S to produce As x S y and As2S3, and this is probably the mechanism of suppression of As segregation. After an exposure to air, the XPS signal from As x S y substantially decreased. This is probably due to the reaction of As x S y →As2O3+SO2↑. The XPS measurement for the (NH4)2S x -passivated GaAs samples indicated that oxidation was strongly suppressed, but segregation of elemental As was not effectively suppressed by this passivation, consistent with the previous data. As-S bonds were found to disappear upon annealing after (NH4)2S x passivation. Elemental sulfur was observed after the exposure to air for 3 days for the (NH4)2S x -passivated samples. This may be due to breakdown of both As-S and Ga-S bonds accompanying oxidation of As and Ga. Schottky diodes were fabricated on the substrates treated in various ways, and I-V characteristics were measured. From the point of view of degradation due to air exposure, the best result was obtained for the (NH4)2S x -passivated sample with annealing.