Corrosion of gallium arsenide: Band bending and decomposition mechanism

Although GaAs has been widely used in industry, such as manufacturing of opto/microelectronic devices, its susceptibility to corrosion in damp or aqueous environment is a major problem hindering its further utilization in solar cell development. The corrosion is directly related to the chemical and (photo-) electrochemical nature of GaAs, which mainly includes the band bending at the interface and the mechanism of corrosion.; In this thesis, the corrosion of GaAs was investigated by several electrochemical methods in combination with surface characterization techniques, including optical microscopy, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), in two solutions: 0.5 M H2SO4 and 2.7 M NH4OH. Open circuit potentials, steady-state polarization curves, direct Mott-Schottky measurements and electrochemical impedance spectroscopy were performed on a series of n- and p-GaAs samples with different doping levels. Morphologies and chemical compositions of the corroded surfaces were then examined.; Open circuit potential studies of GaAs showed that the GaAs/aqueous electrolyte interface was not a simple ideal case. Both the GaAs itself ( i.e., doping type and level) and the solution composition influenced the open circuit potentials, which implies that the Fermi energy level of GaAs is only partially pinned by the redox couples in the solution. From the collected polarization curves, the Tafel slope for n-GaAs was ∼350-400 mV which is unexpectedly large. The breakdown of n-GaAs was explained by an avalanche mechanism, through comparison of theoretical calculations and experimental results. Earlier breakdown than expected, based on theoretical calculations, was explained as being due to surface defects, resulting in a locally high electric field. Flatband potentials from direct Mott-Schottky measurements and impedance spectra were compared and discussed. The frequency dependence in the Mott-Schottky measurements was explained to be due to modeling with an inadequate equivalent circuit. In addition, the surface characterization results suggested that corrosion in 0.5 M H2SO4 was Ga selective, but less so in 2.7 M NH4OH. Based on the impedance results, the corrosion mechanism was identified as being the so-called X-C type, with the corrosion intermediates being the oxidant for further oxidation. The mechanism was simulated mathematically.