Design of the Waveguide Integrated GeSn PDs on a SiN Platform in $2\,\mathrm{\mu m}$ Wavelength Band

Silicon Nitride (SiN) platform, as an integrated photonics platform compatible with CMOS technology, is increasingly competitive. However, active devices on SiN platform, such as 2 $\,\mathrm{\mu m}$ wavelength band photodetector(PD), remain relatively scarce. In this work, 2 $\,\mathrm{\mu m}$ wavelength band SiN waveguide GeSn PDs based on the SiN process platform were designed, including passive SiN waveguides, tapers, and GeSn PDs. The incident light's optical field propagating in the SiN waveguide couples downward into the GeSn absorption layer in the form of an evanescent wave, achieving efficient light transmission and absorption. The Maxwell's equations are solved using the finite difference method to obtain the field distribution of the electromagnetic components on the cross-section of the waveguide, determining the dimensions of the SiN waveguide and taper for single-mode transmission. Additionally, a taper structure gradually narrowing from the input end to the output end is employed to connect the waveguide above the active layer. This structure achieves a bandwidth of 75 GHz and a responsivity of 1 A/W at 2 $\,\mathrm{\mu m}$ for the Ge ${_{0.86}}$ Sn ${_{0.14}}$ PD by simulation. The design of waveguide integrated GeSn PD on SiN platform provides meaningful guidance for the preparation of 2 $\,\mathrm{\mu m}$ wavelength band photonic integrated circuits (PIC).