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 (C-F), 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 C-F, pH, and Ohm 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 (Ohm) in such a way that I-1 vs. Ohm(-1/2) plots are almost linear, resembling traditional Koutecky-Levich behavior. (C) 2000 The Electrochemical Society. S0013-4651(99)10-025-9. All rights reserved.