Percolation and localization dynamics in silicon nanocrystal films*

We apply time-resolved THz spectroscopy [1] to probe the time progression of the ac-conductivity in optically excited Si nanocrystal (NC) films with varying Si vol {\%}, NC sizes and separations. A percolation transition is observed at 38 $\pm $ 1 vol {\%} Si. Above this threshold, we observe a transition form initial ($<$50 ps) long-range percolative inter-NC transport characterized by a non-zero DC conductivity to eventual localization of carriers at individual NCs. Below percolation threshold, early-time ($<$25 ps) inter-NC tunneling conduction is observed in films with sub-nm separations, followed by the final localization of the photoexcited carriers in the largest NCs. In the films with larger ($>$ 1 nm) inter-NC spacing, long-range transport is suppressed suggesting strong photoexcited carrier localization. Comparison of the observed dynamics to Monte Carlo simulations will be discussed. [1] D. G. Cooke et al, Phys. Rev. B \textbf{73}, 193311~(2006).