Generation rate dependence of carrier lifetime measurements in nanocrystalline silicon using transmission modulated photoconductive decay

We investigate the decay of photoexcited carriers in nanocrystalline silicon as a function of generation rate. We use a non-contact method based on the conduction of a 500 MHz RF field called transmission modulated photoconductive decay (TMPCD). In order to interpret the results we employ a combination of theories based on the direct recombination of free-carriers and the effects of multiple trapping of carriers which exist in disordered materials. Our results show that free-carrier recombination can, to some extent, describe the decay dynamics, which occur at times shorter than our nanosecond pulse length. We establish this interpretation through its measured generation rate dependence. Also, it is shown that the theory of multiple trapping can accurately describe the behavior of the photoconductive decay that exists at times following the pulse even though this is not preceded by an establishment of a steady-state photoconductivity.