Dependence of interface state properties of electrolyte-SiO2-Si structures on pH

Two different mechanisms for the pH-response of electrolyte-SiO2-Si (EOS) devices have previously been proposed. To help decide which is dominant, and also to better understand the physical processes involved, a series of experiments was designed to determine the pH dependence of the SiO2-Si interface state densities (Nss) and capture probabilities in the lower half of the bandgap. The experimental procedure makes use of the ac conductance technique and quasi-static measurements. Considerable care was taken to eliminate parasitic effects and to ensure reproducibility. Comparison of EOS results with MOS measurements confirms the validity of the methods and analysis used. Although Nssis found to vary with pH, the variation is not monotonic, Towards the band center, Nssis virtually the same as for MOS measurements made on the same substrate, while closer to the valence band edge, it is greater. The results are discussed in terms of basic mechanisms, and it is concluded that there is strong evidence that the field-effect mechanism at the electrolyte-SiO2interface is dominant.