Photoemission study of the band bending and chemistry of sodium sulfide on GaAs (100)

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Abstract
Recently, there has been a great deal of interest in Na2S⋅9H2O as a passivating chemical treatment for GaAs surfaces. It has been shown that it reduces the high surface recombination velocity characteristic of GaAs surfaces, and may offer hope for ‘‘unpinning’’ the surface Fermi level. We have used photoemission spectroscopy to study the band bending and chemistry of these overlayers on n-type GaAs (100). Identically prepared samples show the characteristic increase in the photoluminescence signal, and have also been characterized using surface conductivity measurements. We find using photoemission that the surface Fermi level of the treated wafer is still near midgap. We also observe the chemistry at the interface, and offer a possible explanation of the photoluminescence and surface conductivity data in terms of it and the advanced unified defect model.