Silicon nitride stoichiometry tuning for visible photonic integrated components

Abstract

In integrated photonics, silicon nitride-based devices operating in the visible range of light may experience auto-fluorescence, an undesired effect that can interfere with the propagating signal. In this article, a reduction in auto-fluorescence has been obtained by studying stoichiometric and silicon-rich silicon nitride, subjected to different post-thermal annealings in different atmospheres. Stoichiometric silicon nitride treated with rapid thermal annealing at 1100 °C in an argon atmosphere reduces the photoluminescence intensity of the material by 95%. Silicon-rich nitride shows a more stable photoluminescence response to different annealings and atmospheres than the stoichiometric. Compared to the stoichiometric material, the emission peaks experienced by the silicon-rich silicon nitride are red shifted between 140 and 190 nm, and the refractive index value is increased by 7% at 633 nm. Also, the interface effects have been studied, showing a remarkable contribution when the annealing is performed in an argon atmosphere, while no contribution from these effects is observed in a nitrogen atmosphere. Finally, taking advantage of the refractive index variation between nitrides, a vertical directional coupler using two asymmetric waveguides, one of each type of silicon nitride, has been designed and simulated, obtaining a coupling length of 9.8 μm with a coupling power of 95.8%, demonstrating the 3D integration capabilities of combining silicon nitride layers of variable composition.