Excitable Er fraction and quenching phenomena in Er-dopedSiO2layers containing Si nanoclusters

Abstract

This paper investigates the interaction between Si nanoclusters (Si-nc) and Er in $\mathrm{Si}{\mathrm{O}}_2$, reports on the optical characterization and modeling of this system, and attempts to clarify its effectiveness as a gain material for optical waveguide amplifiers at $1.54\mu \mathrm{m}$. Silicon-rich silicon oxide layers with an Er content of $4–6\times {10}^{20}\mathrm{at.}∕{\mathrm{cm}}^3$ were deposited by reactive magnetron sputtering. The films with Si excess of $6–7\mathrm{at.}\%$, and postannealed at $900°\mathrm{C}$ showed the best ${\mathrm{Er}}^{3+}$ photoluminescence (PL) intensity and lifetime, and were used for the study. The annealing duration was varied up to $60\min$ to engineer the size and density of Si-nc and optimize Si-nc and Er coupling. PL investigations under resonant $\left(488\mathrm{nm}\right)$ and nonresonant $\left(476\mathrm{nm}\right)$ pumping show that an Er effective excitation cross section is similar to that of Si-nc $(\sim {10}^{-17}–{10}^{-16}{\mathrm{cm}}^2)$ at low pumping flux $(\sim {10}^{16}–{10}^{17}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1})$, while it drops at high flux $(>{10}^{18}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1})$. We found a maximum fraction of excited Er of about 2% of the total Er content. This is far from the 50% needed for optical transparency and achievement of population inversion and gain. Detrimental phenomena that cause depletion of Er inversion, such as cooperative up conversion, excited-stated absorption, and Auger deexcitations are modeled, and their impact in lowering the amount of excitable Er is found to be relatively small. Instead, Auger-type short-range energy transfer from Si-nc to Er is found, with a characteristic interaction length of $0.4\mathrm{nm}$. Based on such results, numerical and analytical (Er as a quasi-two-level system) coupled rate equations have been developed to determine the optimum conditions for Er inversion. The modeling predicts that interaction is quenched for high photon flux and that only a small fraction of Er (0.2–2 %) is excitable through Si-nc. Hence, the low density of sensitizers (Si-nc) and the short range of the interaction are the explanation of the low fraction of Er coupled. Efficient ways to improve Er-doped Si-nc thin films for the realization of practical optical amplifiers are also discussed.