IN-SITU INFRARED CHARACTERIZATION OF THE ELECTROCHEMICAL DISSOLUTION OF SILICON IN A FLUORIDE ELECTROLYTE

The anodic dissolution of p-type silicon has been investigated by using electromodulated infrared spectroscopy. Infrared spectra give evidence that during porous silicon formation, the surface remains hydrogenated, whereas an insulating interfacial oxide layer is formed during electropolishing. The interfacial oxide is heavily hydrated at moderate positive potential (''wet'' oxide or hydroxide). Electrolyte species readily adsorb on this wet oxide when the potential is made more positive. At large positive potential the wet oxide coexists with an inner oxide, more stoichiometric and non hydrated (dry oxide). The formation of this dry oxide coincides with the onset of free-carrier accumulation at the silicon/oxide interface. This regime is also characterized by the observation of current oscillations in suitable conditions. A significant change in the dry oxide thickness, locked to the current oscillation, is evidenced; a non-trivial behavior is also detected in the spectral range of the hydroxide absorption, which should help in making clear the microscopic origin of the oscillation.