Analysis of field-effect and capacitance–voltage measurements in amorphous semiconductors

Abstract

We describe the details of an iterative computer method for deriving N(E), the density of gap states, from field-effect or capacitance–voltage measurements without using simplifying approximations. The method of analysis has been tested by fitting artificially constructed data derived from an arbitrarily chosen N(E). It shows that, although gross features of N(E) can be determined, the presence of detailed structure or narrow peaks cannot be verified. Several different N(E) can yield good fits to the same set of data. The determination of experimental parameters needed for this analysis is discussed. Field-effect and capacitance–voltage measurements are complementary but each is particularly useful in a different range of field voltage values. The field effect probes a layer less than 100 Å thick at the interface. This has significant implications for the interpretation of the results. We question whether N(E) deduced from field-effect measurements represents the density of localized gap states in the bulk.