Excitable Er fraction and quenching phenomena in Er-doped SiO2 layers containing Si nanoclusters

This paper investigates the interaction between Si nanoclusters (Si-nc) and Er in SiO(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 m. Silicon-rich silicon oxide layers with an Er content of 4-6x10(20) at./cm(3) were deposited by reactive magnetron sputtering. The films with Si excess of 6-7 at. %, and postannealed at 900 degrees C showed the best 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 (488 nm) and nonresonant (476 nm) pumping show that an Er effective excitation cross section is similar to that of Si-nc (similar to 10(-17)-10(-16) cm(2)) at low pumping flux (similar to 10(16)-10(17) cm(-2) s(-1)), while it drops at high flux (>10(18) cm(-2) 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 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.