Investigation of bandgap states using the modulated photocurrent technique in both high and low frequency regimes

The modulated photocurrent (MPC) technique is reviewed, emphasizing both the widely used high frequency regime and the newly developed low frequency regime in order to characterize localized bandgap states. The matching of the density of states reconstructed from both regimes in principle allows a good determination of the ratio of the capture coefficient to the extended states mobility of the type of carriers giving the predominant contribution to the MPC. Experimental discrepancies are discussed in the framework of non-homogeneous generation rates and the existence of states having quite different capture coefficients.