Characterization of photochemically unpinned GaAs

Abstract

The unpinning of the etched GaAs(100) surface has recently been reported by both photowashing and by Na2S⋅9H2O treatments. Both techniques hold promise of elucidating the mechanism by which GaAs passivation is achieved. We have identified as part of the photowashing process a separate photoactivation step. The activation step did not become apparent until we modified the photowash process in order to minimize light exposure when flowing water was not applied to the substrate. The degree of unpinning produced by this process has been found to be related to the thickness of this oxide, the presence of oxygen and water vapor and the integrated light flux incident on the sample. Photoluminescence experiments clearly show that a photoactivation step involving water vapor is required to achieve the flat-band condition. This process is relatively insensitive to the surface treatment prior to the photowash. We have observed similar photoactivation and insensitivity to surface treatment on GaAs coated with Na2S⋅9H2O. This suggests the possibility of a common passivation process.