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.