RESONANT AND NONRESONANT BEHAVIOR OF THE ANODIC-DISSOLUTION OF SILICON IN FLUORIDE MEDIA - AN IMPEDANCE STUDY

The anodic dissolution of silicon in fluoride media has been investigated by impedance measurements. In nonresonant conditions, behavior typical of metal corrosion is observed: at low positive potentials, up to a first current maximum (regime of porous silicon generation), the main feature of the impedance is an inductive component, accounted for in terms of roughening of the surface; at more positive potentials, corresponding to a first current plateau (electropolishing regime), the main contributions to the impedance arise from the oxide layer formed onto the silicon surface. At potentials beyond a second current maximum, a steady-state current is observed, but the interface presents a resonant behavior, i.e., there is no spontaneous oscillation, but the current is prone to oscillate upon applying a small perturbation. When the perturbation is turned off, the oscillations decay, and the steady-state current is recovered. In this potential range, the impedance exhibits very peculiar characteristics: though the response is linear, a resonant response is found for an excitation at the natural oscillation frequency as well as at its overtones. This can be accounted for in a picture of an electrode surface partitioned into small self-oscillating domains, uncorrelated in the steady state, but synchronized by the potential excitation. The corresponding "synchronization" contribution to the Faradaic impedance can be computed in the framework of a specific model. This model accounts semiquantitatively for the presence of multiple resonances and for the main features of the impedance results, with a very limited set of parameters.