The etch rates of anodic oxides in dilute fluoride solutions have been determined by using a new approach to the analysis of anodic current oscillations. The variation of the thickness of the anodic oxide with time has been measured by in situ Fourier transform infrared spectroscopy. The change in oxide thickness can be simulated by considering the time-dependent current and the etch rate of the oxide in the HF-containing solution. The pH-dependent etch rates can be well fitted for different concentrations of NH 4 F by applying Judge's model ͓J. Electrochem. Soc., 118, 1772 ͑1971͔͒. The coefficients in Judge's model are obtained for the etching process of the respective anodic oxides. The electronic properties of the Si/anodic oxide interface, which have been investigated by in situ photovoltage and photoluminescence measurements, correlate with the anodic current.

In situ photovoltage ͑PV͒ and photoluminescence ͑PL͒ are used to obtain information about the band bending of Si and the PL quenching by nonradiative ͑nr͒ surface recombination during electrochemical treatments of silicon in aqueous fluoride solutions. The onset of the porous silicon formation in diluted NH 4 F solution is accompanied by a decrease of nr surface recombination. Intensive electrochemical etching leads to an increase of nr surface recombination, to positively charged surface states, and to a strong increase of the current density. The increase of the current density is higher than expected for an ideality factor of 1, which gives evidence for a decrease of the effective barrier height during porous silicon formation. The evolution of hydrogen and its penetration into near surface regions lead to the formation of nr surface defects. The lowest nr surface recombination in the regime of porous silicon formation has been observed in higher concentrated HF solution even at high current densities. Therefore, the intermediates of the reactions which lead to pore formation are more of an ionic nature than like Si dangling bonds. Negatively polarized intermediate states ͑charged species or dipoles͒ arise when the electrolyte reaches the Si surfaces during the etchback of oxide layers.