ABSTRACT

In situ ellipsometry was used in order to detect and to characterize surface layers on GaAs electrodes in aqueous and in mixed aqueous-methanolic electrolyte solutions at low pH during anodic current flow as well as under open circuit conditions. During anodic current flow, it was found that surface layer growth takes place in aqueous and in mixed aqueous-methanolic indifferent electrolyte solutions, but not in mixed aqueous-methanolic solutions containing TMPD (N,N,N',N'-tetramethyl paraphenylene diamine), i.e. under circumstances in which the anodic decomposition of GaAs is totally suppressed. A surface layer was also formed on GaAs at open circuit, even in solutions in which no anodic dissolution takes place, indicating that besides anodic dissolution, a parallel chemical decomposition reaction takes place. The ellipsometric data cannot be interpreted by assuming a homogeneous surface layer consisting of oxides of gallium and/or arsenic nor of elementary arsenic. The fact that even at a layer thickness of 50 nm or beyond, linear Tafel behaviour with a 0.06 V slope is observed at p-GaAs anodes, indicates that the surface layer must be porous. Voids in the surface layer with dimensions, smaller than the wavelength of the light, influence the polarization state of the reflected light and are probably responsible for the difference between the measured and the expected ellipsometric data. In order to investigate the microstructure of this layer, ellipsometric results at different wavelengths will be needed.