Evaluation of the Radiative Recombination Mechanism in Si Nanocrystals Embedded in Silica Matrix

We report measurements of the temperature dependence of photoluminescence (PL) life-time and efficiency of Si nanocrystals (Si-Nc) embedded in silica matrix. We use a practical technique based on lock-in acquisition that allows us to simultaneously evaluate, at each emission-energy, intensity and decay-time of the detected signal. Samples are prepared by Silicon-ion implantation in a SiO₂ layer followed by thermal annealing. The implantation dose of Si ions ranges between 2 × 10¹⁶ cm⁻² and 2 × 10¹⁷ cm⁻². Intensity of Si-Nc PL shows the characteristic rising by increasing the temperature up to ∼100 K followed by a flattening or a weak reduction up to room temperature. This behaviour reveals a population of radiative states built up by a thermally activated process. Similarly, the measured PL decay-rate is not constant with temperature but shows evidence of a thermal activation. By measuring on different samples the activation energies E _a involved in the temperature dependence of PL intensity and decay time we verify that in all these processes E _a is a decreasing function of implantation dose (i.e., of crystallite size). This result is consistent with models connecting radiative recombination to excitons confined inside Si-Nc, in seeming contrast with the common attribution of PL of non-passivated Si-Nc to the recombination from surface/interface states. To verify the consistency of this statement, we have compared our experimental data with the predictions of quantum confinement theory obtaining an excellent agreement.