Differential capacitance measurements on passive films

Abstract

The differential admittance (impedance) technique as an in situ electrochemical technique for the characterization of solid-state state properties of semiconductor electrolyte interface is presented, and the validity of the traditional Mott-Schottky approach to the determination of the characteristic energy levels of the junction is discussed. It is shown that this traditional approach presents several pitfalls, when employed outside the rather special conditions underlying its validity, which can bring to erroneous conclusions specially in the case of thin passive films. On the basis of the more general theory of the impedance of amorphous semiconductor (a-SC) Schottky barriers, it is shown that some frequently reported incongruences in the classical M-S equation can be traced out to the amorphous or strongly disordered nature of the semiconducting materials investigated. The role of the film thickness in determining the frequency response of the differential capacitance of passive film/electrolyte junction is investigated and rationalized on the basis of the theory of thin a-SC/electrolyte junction. The possible dependence of the electronic properties from the film thickness is also reviewed on the basis of a test of spatial homogeneity of the electronic density of state distribution recently suggested by the authors.