Size dependence of photoluminescence quantum efficiency of Si nanocrystals

The radiative recombination rate of excitons confined in Si nanocrystals was modified by placing a Au layer nearby. Oscillation of the rate was observed when the distance between the active layer and the Au layer was changed. By comparing the experimentally obtained oscillation behavior with a calculated one, the radiative and nonradiative recombination rates, and also the internal quantum efficiency of excitons in Si nanocrystals were estimated. The relation between the radiative rate and the luminescence wavelength was on a single curve for all the samples studied. On the other hand, the nonradiative rate depended strongly on samples. For the samples annealed at 1250 degrees C, the estimated quantum efficiency was close to 100% at longer wavelength side of the luminescence bands, while the maximum quantum efficiency was 70% for the sample annealed at 1200 degrees C. The present results provide evidence that in Si nanocrystal assemblies, the majority of nanocrystals in samples do not contribute to photoluminescence and a small part of nanocrystals luminesce with high quantum efficiencies, and thus the total quantum efficiency is mainly determined by the number ratio of bright and dark Si nanocrystals in the assembly.