Electrodissolution of p-Si in Acidic Fluoride Media. Modeling of the Steady State

With the aim of explaining the dependence of the steady-state electrodissolution current density (I) of Si in acidic fluoride media on total fluoride concentration (CF), pH, and mass transfer rate, a kinetic model is proposed. The Si electrode is assumed to be covered by a thin SiO2 layer, formed by an electrochemical reaction at the Si/oxide interface, which undergoes dissolution by three parallel chemical reactions. These reactions convert SiO2 to SiF62-, under the action of HF and HF2-, and occur under mixed control; their kinetics is assumed to be of the second order. Fast equilibria between fluoride species and H+ are assumed. The model is compared with experimental results obtained by ourselves and with those in the literature. The CF, pH, and ? dependencies of the current density measured at a fixed potential in the electropolishing domain are well reproduced with an appropriate choice of the rate constants of the dissolution reactions, the only adjustable parameters. It is shown that I depends on the angular speed of the electrode (?) in such a way that I-1 vs. ?-1/2 plots are almost linear, resembling traditional Koutecky-Levich behavior.