Band-tail photoluminescence in nanocrystalline Si

The characteristic subgap photoluminescence (PL) observed in nanocrystalline Si films was found to shift to higher energies with increasing optical gap and decreasing crystallite size. This behavior, along with the temperature dependence of the PL, is consistent with transitions between band-tail states in the energy gap of the crystallites. The PL bandwidth is too broad to be explained by the tail width as deduced from the temperature behavior. Hence, the broadening is ascribed to electron–phonon coupling, while the related Stokes shift was found to increase with decreasing crystallite size, possibly due to decreasing exciton size.