Stratified structure of anodically formed mesoporous silica

Films of mesoporous silica are formed on silicon single crystal by high-potential anodisation in neutral fluoride electrolyte. The films formed at 7-15 V potential exhibit a bright coloured aspect, characteristic of thin films of submicrometer thickness with flat parallel surfaces. SEM examination of cleaved profiles and X-ray reflectivity curves point to a stratified structure of these films on the nanometric scale. The stacking period, on the order of 10 nm, is an increasing function of the preparation potential. This structure is clearly related to the well-known oscillatory electrochemical behaviour observed above 3 V potential in the silicon/acidic-fluorideelectrolyte system. These observations confirm the idea that this oscillatory behaviour is accompanied by significant changes in the oxide morphology, namely a modulation of the porosity. The surviving of this oscillatory behaviour even though the interface is buried below a stack of several tens of oxide layers is a challenging observation for the theoretical modelling of the oscillation mechanism. We propose that periodic cracking of the inner oxide film may originate from a tensile stress due to electrochemically-induced desiccation near the silicon/oxide interface.