General analysis of the modulated-photocurrent experiment including the contributions of holes and electrons

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Abstract
In this paper we concentrate on an analysis of the modulated photocurrent (MPC) experiment applied to samples of amorphous semiconductors built in coplanar geometry. Taking into account both types of photocarriers, the basic equations describing the modulation of the occupation of the localized states are derived according to the statistics of Simmons and Taylor. Generalized expressions for the phase shift and the modulus of the modulated photocurrent are obtained without any restrictive assumptions and discussed. It is shown that, if one type of carrier is predominant, the modulated photocurrent gives the density as well as the capture cross section of the localized states interacting with these carriers. The precise conditions under which the two-carrier system is reduced to a single-carrier system are given. The main features of the method are illustrated by means of a simulation, where we study the influence of several parameters. We show that the dominant contribution to the modulated photocurrent comes from the carrier type which presents the higher value of μ/(Nσ), where μ is the free-carrier mobility; σ, the capture cross section; and N, the density of trapping states for which the emission rate is equal to the angular frequency ω at which the experiment is performed. Consequently, only the trapping states corresponding to this type of carrier can be probed. Our study underlines some possible experimental misuses of the MPC technique which could lead to erroneous results regarding the inferred density of states.