Radiative versus nonradiative decay processes in silicon nanocrystals probed by time-resolved photoluminescence spectroscopy

Using time-resolved and continuous wave (cw) photoluminescence (PL) spectroscopy of Si nanocrystals embedded in SiO₂ with varying content of the nanocrystalline silicon phase, we were able to distinguish between microscopic characteristics of the PL decay that are associated with quantum confinement effects, and macroscopic characteristics of the decay that are affected by the environment of the nanocrystals. The PL decay is characterized by a stretched exponential function and two PL lifetimes associated with an upper (allowed) singlet excitonic state and a lower (forbidden) triplet state. In particular, we have found that while the upper radiative lifetime of the singlet state and the singlet-triplet energy splitting originate from quantum confinement, the lower state lifetime and the dispersive nature of the PL decay are connected with characteristics of the crystallites' environment. In addition, we have found that the oscillator strength for radiative transitions is significantly weaker compared to that of direct gap semiconductors and therefore, we concluded that the efficient PL from these nanocrystals should be assigned to the exclusion of nonradiative channels in the medium.