Charge transport and localization in nanocrystalline CdS films: A time-resolved terahertz spectroscopy study
Abstract
Assessment of characteristic length and time scales of the charge localization in nanostructured semiconductors is a key point for understanding the initial stage of carrier transport after photoexcitation. A use of time-resolved terahertz spectroscopy and Monte Carlo simulations of the electron motion allow us to obtain this information and develop a microscopic model of the electron transport in a nanocrystalline CdS film.
A weak localization is observed inside individual nanocrystals (NCs) while much stronger localization stems from the existence of NC clusters. The efficiency of the short-range transport is controlled by the excess energy of electrons: Its increase enhances the conductive coupling between adjacent NCs and clusters.
Relaxation of electrons with high excess energy then leads to a decrease of their mobility on a sub-ps time scale. Filling of conduction-band states by increasing the optical pump fluence allows us to maintain a high mobility even at later times.