In-situ monitoring of electrochemical oscillations during the transition between dense and porous anodic silica formation

This study investigates the mechanism responsible for the electrochemical oscillations during silicon galvanostatic anodizing. Two oscillatory regimes of anodic silica formation in dilute fluoride electrolyte are monitored by combined in-situ curvature measurement and ellipsometry. At lower applied current density, a dense silica film is formed and the oscillations features are similar to those observed in fluoride-free electrolytes. At higher applied current density, a porous silica film is formed and the oscillations progressively reappear after a transition regime without oscillations. The disappearance and the reappearance of the oscillations are associated to variations of the degree of synchronization between the self-oscillating domains. In addition, the similarities between the oscillatory regimes and the persistence of sustained oscillations during the formation of thicker silica indicate that all oscillations arise from a same mechanism that survives the oxide accumulation at the silicon surface.