Carrier dynamics in Si nanocrystals in an SiO₂ matrix investigated by transient light absorption

We report on investigations of optical carrier generation in silicon nanocrystals embedded in an SiO₂ matrix. Carrier relaxation and recombination processes are monitored by means of time-resolved induced absorption, using a conventional femtosecond pump-probe setup for samples containing different average sizes of nanocrystals (d_NC = 2.5-5.5 nm). The electron-hole pairs generated by the pump pulse are probed by a second pulse over a broad spectral range (E_probe = 0.95-1.35 or 1.6-3.25 eV), by which information on excited states is obtained. Under the same excitation conditions, we observe that the induced absorption intensity in the near-infrared range is a factor of ∼10 higher than in the visible range. To account for these observations, we model the spectral dependence of the induced absorption signal using an empirical sp3d5s^* tight-binding technique, by which the spectrum can be well reproduced up to a certain threshold. For probe photon energies above this threshold (dependent on nanocrystal size), the induced absorption signal is found to feature a long-standing component, whereas the induced absorption signal for probe photon energies below this value vanishes within 0.5 ns. We explain this by self-trapping of excitons on surface-related states.