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

Effect of large oscillations of electrical potential during anodic polariza tion of silicon in electrolytes composed of phosphoric and hydrofluoric aci ds has been reported. The oscillations last hours without damping if experi mental conditions are optimal. Changes of temperature, anodic current densi ty, intensity of stirring, etc. quench them or convert into less periodic o nes. The oscillations are of very high amplitude (typically 15 V) with a pe riod ranging from 18 to 30 s. Scanning electron microscopy (SEM)-imaging of the samples experiencing the oscillatory kinetic behaviour shows unambiguo usly that the stage of the anodic voltage growth is assisted by the formati on of a thin (50-80 nm) surface film, while the dropping of potential corre sponds 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 triggere d isotropic formation of micropores ai the film/silicon interface. The pres ent data are compared with other known cases of electrochemical oscillation s, and a unified model is suggested. (C) 2000 Elsevier Science S.A. All rig hts reserved.