Versatile Germanium Photodiodes with 3dB Bandwidths from 110 GHz to 265 GHz
Authors: Peczek Anna , Lischke Stefan , Steckler Daniel , Morgan Jesse , Beling Andreas , Zimmermann Lars 
Published: November 2022

We demonstrate waveguide coupled germanium fin photodiodes for C- and O-band applications. By scaling the germanium (Ge) fin widths record 3dB-bandwidths reaching from 110GHz to 265GHz are achieved. The intrinsic, undoped germanium fins of the photodiodes are sandwiched in between two complementary in situ-doped silicon regions, circumventing ion-implantation into Ge. This reduces the impact of minority carrier diffusion, which is beneficial for the bandwidth [1,2].
In silicon photonics, waveguide integrated germanium photodiodes are a key asset. However, until 2020 optoelectrical bandwidth of foundry manufactured germanium photodiodes remained substantially below the 100GHz benchmark, typical values ranging up to approximately 70GHz [3,4,5]. All of these devices rely on ion-implantation into silicon and/or germanium. Discrete devices in InP technology, on the other hand, achieved 170GHz bandwidth and 0.27A/W responsivity already several years ago [6]. Having such performance available in silicon technology has been a major motivation for our work.
The most recent germanium fin photodiodes show a 3dB-bandwidths of 240GHz (with internal responsivity of 0.45A/W) and 265GHz (with internal responsivity of 0.3A/W) both at 1550nm wavelength and 1mA photocurrent at 2V reverse bias [2]. These devices have been fabricated in IHP’s BiCMOS pilot line on 200mm silicon-on-insulator (SOI) wafers. These novel photodiodes do not rely on ion-implantation, but instead the undoped germanium is contacted by in situ-doped silicon offshoot, thus minimizing minority carrier diffusion effects. This novel technology approach allowed us to match or even surpass the state-of-the-art of III-V devices. By scaling the width of the germanium region, different combinations of bandwidths and responsivities can be realized. Certainly, broader fins will yield higher responsivities but on the expense of 3dB-bandwidths.
In this paper, we present for the first time the responsivity and bandwidth at 1310nm and 1550nm, as well as dark current and capacitance of the high-speed germanium fin photodiodes. Several aspects important for high performance photodiodes will be discussed:
(1) Optoelectrical 3dB-bandwidths and responsivities for various germanium fin widths.
(2) Improved power handling capability of the photodiode at 1310nm and 1550nm.
(3) Temperature dependent electrical behaviour for photodiodes with various germanium fin widths.
Figure 1 shows STEM cross-sections (cut perpendicular to the light-incidence direction) of the 70GHz Ge photodiode integrated at IHP EPIC platform (left) and a novel germanium-fin photodiode with 3dB-bandwidth of 265GHz (right). Two major changes, the transition from Ge-stripe to germanium fin and from ion-implantation to in situ-doped silicon offshoot, eventually lead to record 3dB-bandwidths.
[1] Lischke, S. et al., "Ge photodiode with −3-dB OE bandwidth of 110 GHz for PIC and ePIC platforms", In Proc. 2020 IEEE International Electron Devices Meeting (IEDM) 7.3.1–7.3.4, 2020.
[2] Lischke, S., et al., "Ultra-fast germanium photodiode with 3-dB bandwidth of 265 GHz", Nat. Photon. 15, 925–931, 2021.
[3] Chen, H., et al., " −1-V bias 67-GHz bandwidth Si-contacted germanium waveguide p-i-n photodetector for optical links at 56 Gbps and beyond", Opt. Express 24, 4622–4631, 2016.
[4] Boeuf, F., et al., "A silicon photonics technology for 400-Gbit/s applications", In Proc. 2019 IEEE International Electron Devices Meeting (IEDM) 33.1.1–33.1.4, 2019.
[5] Lischke, S., et al., "High bandwidth, high responsivity waveguide-coupled germanium p-i-n photodiode", Optics express, vol. 23, no. 21, pp. 27213–27220, 2015.
[6] Rouvalis, E., et al., "170-GHz uni-traveling carrier photodiodes for InP-based photonic integrated circuits", Opt. Express 20, 20090–20095, 2012.

Figure 1