Versatile Germanium Photodiodes with 3dB Bandwidths from 110 GHz to 265 GHz

Abstract

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