Time-Domain Impedance Analysis of Carrier Recombination in Thin Films
A. Nadtochiy, A. Podolian, O. Korotchenkov
Faculty of Physics, Taras Shevchenko National University of Kyiv, Kyiv 01601, Ukraine
The use of the frequency- and time-domain impedance measurements for the parametrization of equivalent circuit models is widely applicable. The transmission line model (TLM) and the full wave model (FWM) are the most commonly used methods for quantifying transient impedance. In this work, we approximate the surface photovoltage (SPV) decays in nm-sized ZnO films by the equivalent RC circuit model. The SPV rises in time during approximately 100 microseconds after the exciting light pulse at 275 nm is off at different pulse widths ranging from 1.2 to 12 microseconds. The key to this observation is a considerable amount of defects in the films, which form a trap capacitance in the equivalent circuit. The photogeneration of nonequilibrium electrons and holes near the film surface is described by charging of the capacitance C1 by the current source I1. The rate of spatial separation of the electrons and holes is determined by the R1 resistance, which reflects the obstacle in the spatial separation of charge carriers. The capacitance C2 determines the carrier separation distance. The R2 resistance is introduced into the equivalent circuit to account for the discharge of C2 due to electron-hole recombination processes. Faster recombination rates make R2 smaller. The resulting modeled SPV decay allows to reproduce the experimental decay curve rather well.