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

Abstract

X-ray photoelectron spectroscopic (XPS) analysis was performed for the Na 2 S-passivated GaAs surface. The XPS data indicated that oxidation of GaAs was not suppressed by Na 2 S passivation, and fair amounts of As 2 O 3 and Ga 2 O 3 were observed after exposure to air for 3 days in contrast with the case of (NH 4 ) 2 S x passivation. However, segregation of elemental arsenic was found to be substantially suppressed by this passivation. Both As x S y (probably AsS) and As 2 S 3 were observed at the Na 2 S-passivated surface. This is due to diffusion of elemental As through the thick Na 2 S film and its reaction with S to produce As x S y and As 2 S 3 , 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 →As 2 O 3 +SO 2 ↑. The XPS measurement for the (NH 4 ) 2 S 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 (NH 4 ) 2 S x passivation. Elemental sulfur was observed after the exposure to air for 3 days for the (NH 4 ) 2 S 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 (NH 4 ) 2 S x -passivated sample with annealing.