Semiconductor analysis using organic-on-inorganic contact barriers. I. Theory of the effects of surface states on diode potential and ac admittance

Abstract

A new method for measuring the density of states at semiconductor surfaces using organic-on-inorganic (OI) semiconductor contact barriers is suggested. This work is an extension of previous models of OI diode behavior which includes the ac admittance characteristics, and which considers the range of validity of approximations to OI diode capacitance used in previous experiments. The theory describes the potential distribution across the device. We consider the case of ideal OI diodes, as well as diodes with significant densities of states at the inorganic semiconductor surface. This analysis leads to a technique whereby the low-frequency conductance and capacitance characteristics can be used to obtain information about the magnitude and the energy distribution of surface states in the inorganic semiconductor bandgap. Also, the carrier concentration profiles of the substrate can be conveniently obtained. Due to the noninvasive nature of the organic/inorganic contact, information about the density of states at relatively undisturbed semiconductor surfaces obtained via this technique may prove useful in determining the processes involved in Schottky barrier formation and metal-insulator-semiconductor diode surface properties. This theory has been applied to the investigation of surfaces of III-V alloy semiconductor-based OI diodes, and the experimental results will be presented in a subsequent paper. In addition, an expression for the OI diode n value obtained from the forward current-voltage characteristics is derived. It is found that the n value depends on surface states which are in equilibrium with the organic as well as the inorganic materials. Estimates of the surface state densities obtained from previously reported n values are consistent with expectations for the semiconductors under test.