Probing localized states distributions in semiconductors by Laplace transform transient photocurrent spectroscopy

We report on the effectiveness of two methods for recovering the density of electronic states from transient photocurrent data, one employing an exact solution of the Laplace transformed multiple-trapping rate equations and one a Tikhonov regularization technique. In order to evaluate these methods for recovery of energetically broad and narrower distributions of states we have applied each to data obtained from plasma-enhanced chemical vapor deposition (PECVD) a-Si:H films subjected to progressive light soaking and also to a single crystal sample of tin-doped crystalline CdTe. Both methods are found to perform equally well in terms of accuracy and resolution but the exact method is more sensitive to noise on the input data. A featureless increase in defect density in the PECVD a-Si:H film of a factor of 5–10 is observed on light soaking. Preliminary analysis of the CdTe:Sn data indicates the presence of two narrow bands of states, approximately 0.15 and 0.36 eV below the conduction band edge.