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For comprehensive interpretation of the electrochemical processes occurring at silicon/electrolyte interfaces, the purpose of this work was to use electrochemical impedance measurements (EIS) to characterize silicon/electrolyte interfaces in the dark and to record the physical, electrical, and chemical processes during the different steps of silicon dissolution in hydrofluoric (HF) acid solution. In this study we used two main electrochemical techniques-potentiodynamic polarization and EIS under different bias potentials. The in-situ current-voltage I( V) characteristic clearly identifies the pore formation (porous silicon), transition, and electropolishing regions. Two series of impedance diagrams were recorded in the potential range in which the depletion layer was generated within the semiconductor substrate and in the range of potentials corresponding to the onset of an accumulation layer. Our results show that the impedance of silicon/electrolyte interfaces depends on the surface roughness and porosity of the substrate. At a particular positive value of the bias, an important induction loop was observed in addition to the usual capacitive behavior. It is highly significant that, for finite pore length, mass transfer and pore geometry might lead to different impedance curves. An electrical equivalent circuit model was used to fit the best experimental data to the theoretical data. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, a study on the electrochemical impedance of anodic decomposition of n-GaAs in 0.5 M sulfuric acid, combined with steady-state polarization techniques, is presented. Both dark and daylight conditions have been investigated for comparison of their influence on polarization curves and impedance by hole injection in a potential range from open circuit to breakdown. An inductive loop appears if a high enough positive potential is applied for both conditions, while recombination resistance is only observed for the daylight case. Quantitative simulation of the impedance spectra suggests that the rate-determining step for corrosion/photocorrosion is the generation of mobile intermediates, e.g., AsGa+, which are then replaced by competition between a Faradaic process involving immobile intermediate formation and a parallel chemical step to form AsGa–OH. [Copyright &y& Elsevier]

Abstract: Corrosion of GaAs can readily happen in acidic and alkaline aqueous electrolytes. The instability of GaAs is a major difficulty in implementing GaAs-based photoelectrochemical solar energy-conversion systems. In this paper, the corrosion behavior was investigated by electrochemical methods and surface-analysis approaches, in both H2SO4 and NH3·H2O solutions. Open-circuit testing showed that the GaAs surface was very sensitive to water/air although the process to build a stable interface between GaAs and electrolyte is slow. Comparison of the impedance data for n- and p-GaAs showed that an inductive loop only appeared at a high corrosion rate. This induction loop was due to the Faradaic adsorption process being the rate-determining step. This can only be explained by a mechanism in which a reaction intermediate, instead of holes, after capture of the first hole is the oxidant for further decomposition steps in combination with a chemical reaction with water. The results suggest that the same corrosion/decomposition mechanism occurred on both n- and p-GaAs and also in the two solutions. When the surface hole concentration was low, the hole injection or generation process was the rate-determining step. At large reverse bias on n-GaAs, with high hole concentration or forward biasing of p-GaAs, the corrosion rate was limited by a Faradaic adsorption process, involving an adsorbed intermediate. The different surface compositions after corrosion in the two solutions can be explained by the dissolution of As(0). It is pointed out that the further oxidation of As(0) to As(III) in NH3·H2O solution is still slow, as suggested from X-ray photoelectron spectroscopy (XPS) analysis. [Copyright &y& Elsevier]