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In this paper we present a complete theoretical analysis of the oscillating photocarrier grating (OPG) method, starting from the generalized equations that describe charge transport and recombination under oscillating grating illumination conditions. The solution of these equations allows us to implement a calculation reproducing the experimental OPG curves. We study both experimentally and from our calculations the dependence of the OPG curves on different external parameters, such as the applied electric field, grating period and illumination intensity. We find that the response of the sample is linked to a characteristic time of the material, which could be the dielectric relaxation time or the small signal lifetime depending on the regime at which the experiment is performed. Therefore, the OPG technique provides a simple method to estimate these parameters. In addition, we demonstrate that the small signal lifetime provides information on the density of states of the material.

In this paper we apply various photoconductivity techniques to study different types of semiconductors. These methods are the modulated photocurrent, the steady-state photocarrier grating, and the steady-state photoconductivity techniques, and they are used to investigate a chromium-doped gallium arsenide crystal and different hydrogenated amorphous silicon thin films. First, we briefly recall what information on the material transport parameters can be extracted from the results of these various techniques. Second, we experimentally put into evidence the links existing between these apparently very dissimilar techniques by applying them first to a GaAs:Cr crystal and finally to three hydrogenated amorphous silicon samples prepared under different conditions. For this latter material, we show that the density of states distribution, the electron capture cross sections of the states-even that of the valence band tail-and the electron extended-states mobility can be obtained from the comparison of the results of these techniques. We conclude by showing that, by introducing these parameters into a numerical simulation, we can reproduce the behaviors experimentally observed for all the photoconductivity techniques.

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In this paper we present a complete theoretical analysis of the steady-state photocarrier grating ͑SSPG͒ method, starting from the generalized equations that describe charge transport and recombination under grating conditions. The analytical solution of these equations and the application of simplifying assumptions leads to a very simple formula relating the density of states ͑DOS͒ at the quasi-Fermi level for trapped electrons to the SSPG signal at large grating periods. By means of numerical calculations reproducing the experimental SSPG curves we test our method for DOS determination. We examine previous theoretical descriptions of the SSPG experiment, illustrating the case when measurements are performed at different illumination intensities. We propose a procedure to estimate the minority-carriers mobility-lifetime product from SSPG curves, introducing a correction to the commonly applied formula. We illustrate the usefulness of our technique for determining the DOS in the gap of intrinsic semiconductors, and we underline its limitations when applied to hydrogenated amorphous silicon. We propose an experimental procedure that improves the accuracy of the SSPG-DOS reconstruction. Finally, we test experimentally this new method by comparing the DOS obtained from SSPG and modulated photocurrent measurements performed on the same samples. The experimental DOS obtained from both methods are in very good agreement.

An experimental technique to study the energy profile of localized states in the gap of amorphous semiconductors is proposed. The method is based on the relationship between the recombination lifetime and the density of states (DOS) at the quasi-Fermi level for trapped carriers. We use the modulated photocurrent experiment in the recombination-limited regime as a convenient method to measure the recombination lifetime. Measurements performed as a function of temperature allow the DOS above the Fermi energy to be determined. The accuracy and limitations of the method are studied by means of computer simulations. The experimental technique is applied to obtain the density of defect states of a hydrogenated amorphous silicon sample.

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