Electron-induced carrier dynamics in semiconductors

Abstract

In the recent years, there has been a lot of research focused on new photovoltaic materials, complementary to Si-based technologies. In order to improve their efficiency, a fundamental understanding of the excitation and recombination mechanisms of charge carriers in materials is needed. In this work, we investigate carrier dynamics of Cu2ZnSnS4 (CZTS) and GaAs using fast electrons as the excitation source, in contrast to previous experiments in which carriers are optically induced. Our results are largely based on time resolved single photon counting measurements. The fast electrons are generated by a femtosecond laser-driven scanning electron microscope which generates pulses consisting of (statistically) less than one electron up to 1200 electrons per pulse at voltages between 5 and 30 kV. We analyse our results both qualitatively and quantitatively by considering Auger, radiative, Shockley-Read-Hall and Surface recombination. For CZTS, we have found a value of B = (4.8+-0.2)10^-10cm^3s^-1 for the bimolecular coefficient in optical experiments. We observe that this value becomes variable as a function of number of electrons per pulse in fast electron experiments: B / N_e^-1.1 . In the case of GaAs, the model does not show a good ?t with the data. However, we distinguish two main decay mechanisms, one in a short timescale (up to 0.5 ns) and a second one in the longer timescale (tens of ns), which we attribute to Shockley-Read-Hall recombination.