FDTD analysis of direct optical integration between SiNx waveguides and Ge/SiGe multiple quantum wells on graded buffer for low-voltage optical modulators around the O-band wavelength

Abstract

We focus on investigating the possibility to achieve a direct butt coupling integration of SiNx passive waveguides with germanium/silicon–germanium (Ge/SiGe) multiple quantum wells (MQWs) on a thick graded buffer platform using 3D finite-difference time-domain simulation. Despite the promising potential of a thick graded buffer platform to provide superior crystal quality, its integration with SiNx waveguides has not been discussed so far. We find that in spite of a relatively-low refractive index contrast of the graded buffer platform, it is still possible to attain low-voltage SiNx waveguide-integrated Ge/SiGe MQWs optical modulators around the telecommunication wavelength of 1.31 µm with competitive integrated extinction ratio and insertion loss performance with a compact footprint. Moreover, the fabrication tolerance and the reflection from the butt coupling scheme between the SiNx waveguides and the Ge/SiGe MQWs are also promisingly considered.