Mechanism of large oscillations of anodic potential during anodization of silicon in H3PO4/HF solutions

Effect of large oscillations of electrical potential during anodic polarization of silicon in electrolytes composed of phosphoric and hydrofluoric acids has been reported. The oscillations last hours without damping if experimental conditions are optimal. Changes of temperature, anodic current density, intensity of stirring, etc. quench them or convert into less periodic ones. The oscillations are of very high amplitude (typically 15 V) with a period ranging from 18 to 30 s. Scanning electron microscopy (SEM)-imaging of the samples experiencing the oscillatory kinetic behaviour shows unambiguously that the stage of the anodic voltage growth is assisted by the formation of a thin (50–80 nm) surface film, while the dropping of potential corresponds to its lifting-off. A mechanism responsible for the successive built-up and lifting-off of the surface passive film is assumed to be a triggered isotropic formation of micropores at the film/silicon interface. The present data are compared with other known cases of electrochemical oscillations, and a unified model is suggested.