The silicon/silica electrode

Electron flow between silicon and an aqueous solution through a thermally grown SiO2 layer is observed after a suitable activation process. The activation is either a heat treatment in vacuum or a negative bias applied to the silicon. The duration required for the cathodic bias is consistent with the time needed to provide a high positive ion density in the SiO2. As the SiO2 becomes more electronically conducting, copious hydrogen evolution begins. If metal ions such as zinc are present, they can be reduced giving metal deposits. If sufficiently high voltage is applied, cathodic luminescence, characteristic of the ions in solution, is observed. When the electronic conductivity is obtained, the system SiSiO2 behaves like a metal electrode, in that current is passed to reducing or oxidizing agents in solution when the electrode potential reaches the Eº for the redox couple. The results are explained in terms of field-aided ion diffusion into the oxide under the cathodic bias, occurring particularly at flaws in the oxide. The potential importance of the results in MOSFET quality control, in ISFET technology, and in basic models of anodic oxidation is discussed.