ELECTRICAL CHARACTERISTICS AND SURFACE-CHEMISTRY OF P2S5-PASSIVATED GAAS

Sulfur passivations of the GaAs surface using P2S5/(NH4)2Sx and P2S5/NH4OH solutions were investigated by electrical measurements of the Schottky diodes and by X-ray photoelectron spectroscopy (XPS). The GaAs samples lightly washed in deionized water after both P2S5 passivations showed the lowest degree of degradations in the ideality factor n and the Schottky barrier height phi(B) due to exposure to air ambient. XPS data indicated that oxidation of As was drastically suppressed by P2S5 passivation with and without washing in deionized water, while oxidation of Ga was not efficiently suppressed by these passivations without washing. This is probably due to the preferential formation of As-S bonds over Ga-S bonds. However, oxidation of Ga was found to be greatly suppressed in the water-washed samples after the P2S5 passivations. A fair amount of As-S bonds (AsxSy, probably AsS and As2S3) was observed in the P2S5-passivated samples without washing, which increased as these samples were exposed to air. This indicates that As atoms diffuse through the rather thick sulfur film and react with S to create As-S bonds during exposure to air. The increase in the amount of unstable As-S bonds is considered to be a cause of the deteriorations in the electrical haracteristics. The amount of As-S bonds was observed to drastically decrease after lightly washing the P2S5-passivated samples in deionized water, whereas the amount of stable Ga-S bonds did not substantially decrease. This is a possible reason for the more stable passivation effect with washing. From the experimental results obtained in this study, it was concluded that the Ga-S bonds are useful and a large amount of the As-S bonds is probably detrimental to stable passivation, though a certain amount of the As-S bonds is necessary to effectively suppress oxidation of As. It is very important to control the amount of sulfur on the GaAs surface to obtain stable passivation.