Charge transport in silicon nanocrystal superlattices in the terahertz regime

Silicon nanocrystals prepared by thermal decomposition of silicon-rich 2--5-nm-thick SiOₓ layers (0.640.16em0ex≤0.16em0exx0.16em0ex≤0.16em0ex1) are investigated using time-resolved terahertz spectroscopy. The samples consist of a superlattice of isolated monolayers composed of Si nanocrystals with controlled variable size and filling fraction. Experiments with variable optical pump fluence over almost two orders of magnitude allow us to determine the depolarization fields in the structure. Careful consideration of the local fields along with Monte Carlo calculations of the microscopic conductivity of Si nanocrystals supported by structural characterization of the samples provide detailed information about the electrical connectivity of nanocrystals and about the charge transport among them. Well below the percolation threshold, nanocrystals grow mostly isolated from each other. In thicker or in more Si-enriched layers, nanocrystals merge during their growth and form tens-of-nanometer-sized photoconducting Si structures with a good electrical connection. In addition, in thick SiOₓ layers, imperfectly connected clusters of Si nanocrystals are observed which develop probably at the end of the growth process and allow only limited charge transport due to energy barriers.