Excitable Er fraction and quenching phenomena in Er-doped Si O2 layers containing Si nanoclusters

Abstract

This paper investigates the interaction between Si nanoclusters (Si-nc) and Er in Si O2, 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 μm. Silicon-rich silicon oxide layers with an Er content of 4-6× 1020 at. cm3 were deposited by reactive magnetron sputtering. The films with Si excess of 6-7 at. %, and postannealed at 900°C showed the best Er3+ 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 (∼ 10-17 - 10-16 cm2) at low pumping flux (∼ 1016 - 1017 cm-2 s-1), while it drops at high flux (> 1018 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. © 2007 The American Physical Society.