Résumé
For a comprehensive interpretation of the electrochemical processes occurring at a semiconductor/electrolyte interface, an accurate value of the flatband potential versus the reference electrode is needed together with the current/voltage response. The present work is mainly devoted to the analysis of impedance diagrams recorded with n- and p-type silicon electrodes in the dark, in a pure diluted HF solution under different bias values, the potential range being chosen so as the dc current is maintained at a low value, e.g. less than 5 mu A cm(-2), and even less than 10 nA cm(-2) in the case of p-type Si under cathodic polarization. Two series of impedance diagrams were recorded. Firstly, the bias value was settled in the potential range where the depletion layer was generated within the semiconductor substrate. In this condition, the high resistance of the space charge region and the resulting low value of the dc current permitted to obtain well-defined semicircles leading to the derivation of both R and C values equivalent to the depletion layer charge distribution. The method based on the analysis of the voltage dependent R-value was compared to the usual Mott-Schottky plot treatment, and proved to be efficient for the determination of a reliable value of the flatband potential versus the reference electrode. In the second series, the analysis of the impedance diagrams was focused on the range of potentials corresponding to the onset of an accumulation layer within the semiconductor. In the case of p-Si, this condition corresponds to the rapid anodic etching of the silicon substrate. At a particular value of the bias, an important induction loop was observed in addition to the usual capacitive behaviour. A Fourier transform treatment demonstrated that the impedance components were interrelated to the time dependent current response under constant bias potential. Then, simultaneous experiments based on chronoamperometry measurements suggested that the electrochemical processes involved in the reaction of Si substrate with HF solution was correlated to a two-step charge transfer mechanism. This interpretation is valid for both the inductive loop obtained in electrochemical impedance spectroscopy and the rise of current versus time observed in chronoamperometry.