(Invited) Directly Silicon Nitride Waveguide Coupled Ge Photodiode for Non-SOI PIC and Epic Platforms
Authors: Lischke Stefan , Knoll Dieter , Mai Christian , Voigt Karsten , Hesse Anne , Georgieva Galina , Peczek Anna , Zimmermann Lars 
Published: December 2020

We demonstrate a Ge photodiode that is directly coupled to a silicon nitride waveguide and shows more than 67 GHz bandwidth. This device paves the way for utterly new SiN waveguide platform based applications. By light feeding through SiN waveguides, the new photodiode can also be a key enabler for a bulk-Si based, monolithically integrated electronic-photonic integrated circuit platform.
Silicon nitride (SiN) waveguides (WGs) outperform silicon-on-insulator (SOI) based waveguides in various aspects: they offer an enhanced optical bandwidth, allowing for low loss waveguides in the classic communication bands (C- and O- bands), widely addressed by SOI based photonics, as well as in the visible wavelength range, were Si WGs have already become opaque. Moreover, due to the absence of two-photon absorption in SiN, the optical power handling is superior compared to Si WGs [1–3]. Various platforms with outstanding optical loss performance have been demonstrated over the last years and are meanwhile offered for commercial purposes, as summarized in [1]. However, despite all these advantages, so far, commercially offered SiN based photonic integrated circuit (PIC) platforms neither provide monolithically integrated high speed modulators, nor fast SiN WG coupled detectors. In this work we focus on the detector side of a high-speed SiN PIC platform.
Communication applications for upcoming 400 Gbps standards require 56 Gbd symbol rates, which makes photodiodes (PDs) with more than 40 GHz opto-electrical (OE) bandwidth indispensable. Previous works with focus on Ge PDs in SiN platforms have either shown devices directly SiN WG fed or indirectly coupled via multi-layers (SiN on SOI WGs) where the Ge PDs are actually still located on SOI. However, the former ones showed only inferior bandwidth of less than 10 GHz [4] while the latter ones provide at least a bandwidth of about 30 GHz [5]. Here, we demonstrate for the first time a bulk-Si based, directly SiN WG coupled Ge PD with very high OE -3 dB bandwidth of more than 67 GHz which opens new perspectives for opto-electronic platform technologies:
(1) The combination of truly high-speed PDs with SiN waveguides could be the start for a novel active SiN platform e.g. aiming at integrated coherent receivers at wavelengths far below the O-band.
(2) The demonstrated devices are in principle compatible for co-integration with IHP’s photonic BiCMOS process [6], which opens various opportunities for novel sensing or spectroscopic applications as well.
(3) This work could pave the way towards a non-SOI based technology as an alternative to established SOI based PIC or electronic-photonic integrated circuit (ePIC) platforms, e.g. IHP’s photonic BiCMOS technology. Circuit fabrication on pure bulk-Si wafers would significantly relieve process complexity, time and thus costs.
We show that the new devices, fabricated on bulk-Si, provide the same bandwidths as Si waveguide coupled SOI based reference Ge photodiodes. However, their O-band responsivity is 0.3 A/W, which is about three times lower compared to the SOI waveguide coupled devices. We attribute this effect to substrate losses and few specific layout features but see some potential for improvement by design and technological optimizations. We further demonstrate that the diodes can be fabricated with high yield and low metrics tolerances. Figure 1 shows SEM cross-sections (perpendicular to the light incidence direction) of Ge photodiodes fabricated on SOI waveguide (left) as well as on bulk-Si (right).
[1] P. Muñoz et al., “Silicon Nitride Photonic Integration Platforms for Visible, Near-Infrared and Mid-Infrared Applications,” Sensors, vol. 17, no. 9, 2017.
[2] A. Rahim et al., “Expanding the Silicon Photonics Portfolio With Silicon Nitride Photonic Integrated Circuits,” J. Lightwave Technol., vol. 35, no. 4, pp. 639–649, 2017.
[3] K. Ikeda et al, “Thermal and Kerr nonlinear properties of plasma-deposited silicon nitride/ silicon dioxide waveguides,” Optics express, vol. 16, no. 17, pp. 12987–12994, 2008.
[4] D. Ahn et al., “High performance, waveguide integrated Ge photodetectors,” Optics express, vol. 15, no. 7, pp. 3916–3921, 2007.
[5] W. D. Sacher et al., “Monolithically Integrated Multilayer Silicon Nitride-on-Silicon Waveguide Platforms for 3-D Photonic Circuits and Devices,” Proc. IEEE, vol. 106, no. 12, pp. 2232–2245, 2018.
[6] D. Knoll et al. “(Invited) SiGe BiCMOS for Optoelectronics,” ECS Transactions, vol. 75, no. 8, pp. 121–139, 2016.

Figure 1