An experimental evaluation of transient and modulated photocurrent density-of-states spectroscopies

An evaluation of transient photocurrent (TPC) and modulated photocurrent (MPC) spectroscopies as a means of studying the density and capture properties of localized states in amorphous semiconductors is presented. Frequency-domain analysis of TPC data via the discrete Fourier transform (TPC FT) permits a direct comparison with MPC data obtained using conventional lock-in techniques to be made. Results obtained from undoped hydrogenated amorphous silicon over a wide range of temperatures and optical excitations are used to explore the limits of resolution and applicability, and to highlight the relative merits, of each approach. It is shown that TPC spectroscopy offers significant practical advantages over MPC spectroscopy from the viewpoint of signal-to-noise performance. Discrepancies between TPC FT and MPC data obtained from the same sample under equivalent conditions suggest that the Fourier transform pairing of these methods is not exact, even when the requirements of low excitation are met. Capture coefficients of defect states calculated from the temperature dependence of the TPC data and the optical excitation dependence of the MPC data disagree, having values of 7 x 10(-9) and 5 x 10(-7) cm(3) s(-1) respectively. Density-of-states profiles agree well for tail states (measured at low temperatures) but are less satisfactory at deeper energies.