Improvement in the luminescence properties of Si/CaF2 nanostructures

We report on the photoluminescence quantum efficiency and the lifetime of two Si/CaF2 heterostructures grown by molecular beam epitaxy. The first is a nanocrystalline Si/CaF2 multiquantum well which consists of interacting Si nanocrystallites within Si layers; the second is an annealed Si/CaF2 multiquantum well which can be described as a collection of non-interacting Si nanocrystallites embedded in the CaF2 matrix. While a photoluminescence efficiency of 0.01% has been found in the former, it reaches 1% in the latter. The energy dependence of the PL lifetime differs drastically between both sets of samples. These results are explained in terms of carrier localization.