Saturation of luminescence from Si nanocrystals embedded in SiO2

We present a photoluminescence excitation study of silicon nanocrystals in a SiO2 matrix. We show that although the excitation cross-section is wavelength-dependent and increases for shorter excitation wavelengths, the maximum time-integrated photoluminescence signal for a given sample saturates at the same level independent of excitation wavelength or amount of generated electron-hole pairs per nanocrystal after a laser pulse. We demonstrate explicitly that saturation is achieved when every nanocrystal has absorbed at least one photon. In nanocrystals where several electron-hole pairs have been created during the excitation pulse, fast non-radiative recombinations reduce their number, leading to the situation that only a single electron-hole pair per nanocrystal can recombine radiatively, producing a photon and contributing to the photoluminescence. In this way a natural limit is set for photoluminescence intensity from an ensemble of Si nanocrystals excited with a laser pulse with a short duration in comparison with the radiative recombination time.