Step-like increase of quantum yield of 1.5 μm Er-related emission in SiO₂ doped with Si nanocrystals

We investigate the excitation dependence of the efficiency of the Si nanocrystals-mediated photoluminescence from Er³⁺ ions embedded in a SiO₂ matrix. We show that the quantum yield of this emission increases in a step-like manner with excitation energy. The subsequent thresholds of this characteristic dependence are approximately given by the sum of the Si nanocrystals bandgap energy and multiples of 0.8 eV, corresponding to the energy of the first excited state of Er³⁺ ions. By comparing differently prepared materials, we explicitly demonstrate that the actual values of the threshold energies and the rate of the observed increase of the external quantum yield depend on sample characteristics—the size, the optical activity and the concentration of Si nanocrystals as well Er³⁺ ions to Si nanocrystals concentration ratio. In that way, detailed insights into the efficient excitation of Er³⁺ ions are obtained. In particular, the essential role of the hot-carrier-mediated Er excitation route is established, with a possible application perspective for highly efficient future-generation photovoltaics