Spectral and Injection Level Dependence of Recombination Lifetimes in Silicon Measured by Impedance Spectroscopy

The effect of photo-generated carrier concentration on recombination lifetime is studied on silicon wafers using the impedance spectroscopy technique. The induced p⁺-p-n structures are created by depositing semitransparent layers of high (Pd) and low (Al) work function (w.r.t silicon) metal layers on crystalline silicon (c-Si) wafers. The measurements are carried out by illuminating the device from the p⁺-p side. The impedance data are analyzed by assuming a network consisting of a number of RC circuits, which, in turn, give recombination times (τ) controlled by defects in bulk and at the interfaces of the device. The maximum value of the effective recombination lifetime (τ) is found to be 12.3 ± 0.4 μs at an intensity of ~100 mW/cm², which matches closely with the maximum value 11.4 ± 1.0 μs obtained under forward bias (+ 0.4 V) conditions in the same sample in the dark. These values match well with the effective minority carrier lifetime obtained using the microwave photoconductive decay technique on the same silicon wafer prior to device fabrication.