Oxide Passivation of Photochemically Unpinned GaAs

Using Auger electron spectroscopy and x-ray photoelectron spectroscopy we show that the unpinned GaAs is covered by a Ga203 layer with only small amounts of species containing arsenic. This oxide is formed by the consumption of GaAs during the unpinn ing treatment. We believe that it is this oxide that passivates the surface, allowing it to remain near flatband for several hours in room air. The near absence of arsenic in the oxide, and the eventual repinning of the surface Fermi level supports the effective work function model of Fermi-level pinning. Offsey et al. (1) reported that the surface of nor p-GaAs could become nearly flatband even in air if the surface was treated with the combination of flowing deionized water and above bandgap laser light. The sharp reduction in surface band bending was detected by a rise in photoluminescence efficiency that was most pronounced at short wavelength excitation (i.e., response to blue laser light) (2). Both nand p-type GaAs responded to this treatment, indicating that the surface Fermi level was actually unpinned, unlike other treatments where low surface recombination velocities are obtained by repinning the Fermi level near the appropriate bandedge (3). The capacitance-voltage characteristic of an Hg/PMMA/n-GaAs structure showed control of * Electrochemical Society Active Member. 1 Present address: Department of Electrical Engineering, Colorado State University, Fort Collins, Colorado 80523. the surface Fermi level. Repinning of the surface under continuous -0.25 W/cm 2 He-Cd laser light occurs on a time scale of hours (1), while others (4) correctly note that the rep inning takes longer in the dark. Although flowing water treatment by Massies and Contour (5) showed a clean GaAs surface, the fact tha t GaAs could remain unp inned in room air for some time suggested that the surface was protected by a passivating layer. In this paper (6) we describe a modified photowashing treatment. We characterize the surface oxide which appears to protect the unpinned GaAs surface.