The role of the recombination centers on the modulated photocurrent: Determination of the gap state parameters of semiconductors

Abstract

Exact expressions for the out of phase modulated photocurrent (MPC), the so-called Y signal, with a clear physical insight for density of states (DOS) spectroscopy are derived without any approximation. It is found that, apart from the capture rate of the majority carriers into the probed gap states, additional mixed contributions from the recombination processes of the free majority carriers with trapped minority carriers may be important for the Y signal of lower frequencies. These additional contributions prevent the extraction of a reliable DOS. They become important as long as the capture coefficient for the majority carriers of the recombination centers, where the minority carriers are trapped, is comparable to or higher than that of the recombination centers were the majority carriers are trapped. In this case, the recombination rate is relatively high and the mixed contributions from the recombination processes can be detectable in the experimental Y signal and may also induce a phase lead. Taking advantage of this behavior it can be experimentally verified when these recombination processes are negligible in order to safely extract the accurate DOS parameters.