Photoluminescence excitation spectroscopy of Si nanocrystals in SiO2

We present results on time-resolved spectroscopy and quantum yield of photolu-minescence from Si nanocrystals (NCs) embedded in a SiO2-matrix. The materials are prepared by co-sputtering. Transmission electron microscopy reveals formation of Si NC with diameters in the 3-5 nm range. We identify 3 different emission bands, corresponding to radiative recombination of excitons, hot carriers and electron-hole pairs trapped at oxygen-related defects, and give their spectral and temporal characteristics. We evaluate external quantum yield (EQY) of these recombination channels and conclude that the overall emission is determined by the excitonic channel, with the other contribution not exceeding 1‰. Subsequently, we investigate EQY of the excitonic emission as a function of excitation photon energy and find a significant increase for shorter wavelengths. This is explained in terms in exciton multiplication taking place for sufficiently large energies of the incoming photons.