In situ investigation of porous anodic films of silica

The electrochemical behaviour of p-type silicon in dilute fluoride electrolyte has been investigated in a potential range extending up to +100 V vs SCE. Beyond the known range where the system is prone to exhibiting an oscillatory behaviour (3-8 V), two marked current rises are observed, at around 10 V and 20 V. Evidence for electronic transport through the oxide is given by the appearance of two distinct electroluminescence bands, a red band above 10 V, and a "blue" band above 20 V. The observation of strong gas evolution above 20 V confirms that in this potential range transport through the oxide is mainly electronic. However, in the range 10-20 V, no gas evolution is seen and electrochemical impedance indicates that the oscillatory behaviour somewhat survives. Furthermore, the electrical thickness of the oxide film appears one order of magnitude lower than its thickness determined by infrared spectroscopy, which proves that the oxide film is largely porous. The appearance of this porosity is associated with the sharp increase in current around 10 V. Direct scanning-electron-microscopy observation of the oxide film indicates a close similarity with porous alumina formed in similar conditions, which suggests that ordered structures may be realisable in this system.