IONIC PROCESSES THROUGH THE INTERFACIAL OXIDE IN THE ANODIC-DISSOLUTION OF SILICON

The interfacial oxide involved in the anodic dissolution of silicon in fluoride electrolytes has been investigated by using coulometry, transient current and transient capacitance measurements upon removing the anodic polarization. The transient capacitance measurements allow one to determine a transient flat-band potential, providing direct information on the charges stored in the oxide and on their dynamics as well. It is found that negative and positive charges are both present. Upon removing the anodic polarization, the positive charges decay by backward migration and hole injection into the electrode, and the negative charges stay until the oxide layer is partly dissolved. A detailed model is inferred, where the oxide is composed of an inner part, mostly dry, substoichiometric, and positively charged, existing only in the potential region of the second electropolishing plateau, and an outer part, hydrated, porous, and negatively charged. The positive charge in the inner oxide is suggested to be associated with threefold coordinated silicon atoms.