Modulated photocurrent to characterize the density of states of thin film semiconductors in the recombination regime

It has been demonstrated that the Modulated Photo Current (MPC) technique is a very good tool to probe the density of states (DOS) in the band gap of semiconductors. In this technique two regimes have been underlined: the low frequency or recombination regime and the high frequency or trapping-and-release regime. It is the latter case that has been mainly used up to now. In this paper we concentrate on the recombination regime. Calculations are made for a single species of trapping states characterized by the respective capture coefficients of electrons and holes. From the general continuity equations, a very simple relation is found between the density of states around the pseudo Fermi level and the slope of the tangent of the phase shift as function of the angular frequency of the excitation. A simulation illustrates the validity of this relation by comparing the DOS reconstructed from both the high frequency and the low frequency regimes of the MPC technique with that introduced in the simulation. The approximations used to obtain this very simple relation are underlined. Finally, it is suggested that by combining the results of the MPC technique obtained in the low and high frequency regimes, good orders of magnitude of the capture coefficient of the majority carriers and of their mobility can be obtained.