Fiber-based telecommunication systems have sustained a startling increase in capacity demand, due to the exponentially increasing volume of Internet traffic. Tremendous efforts have been made to alleviate this data congestion, such as wave division multiplexing (WDM), space-division multiplexing (SDM), and high-spectral efficiency coding [1,2]. However, traditional optical communication systems with wavelengths around 1.3 and 1.5 μm are still gradually approaching the capacity limit of conventional single-mode fibers (SMFs) [3], which raises the concern of “capacity crunch.” A promising and elegant solution is to exploit a new spectral window around the 2 μm wavelength, owing to the research progress in low-loss hollow-core photonic bandgap fiber (HC-PBGF) and thulium-doped fiber amplifier (TDFA) [4–7]. The optical mode resides largely in the air core of HC-PBGFs, hence minimizing the Rayleigh scattering and enabling a theoretical minimum loss of below 0.1 dB/km, which is lower than that of the best conventional single-mode fiber (SMF) (0.1484 dB/km). TDFAs have the ability to amplify light sources around 1810–2050 nm, which can be used as the equivalent to erbium-doped fiber amplifiers (EDFAs) in a 2 μm communication system. Therefore, the 2 μm optical communication is practicable and has attracted increasing interest.

Research groups have demonstrated several components operating at 2 μm wavelength, including low-loss waveguides, couplers, splitters, multiplexers, and arrayed waveguide gratings (AWGs) [8–11]. However, the current challenges associated with high-speed integrated modulators and photodetectors (PDs) operating at 2 μm still exist [12]. As the core component in the optical receiving end, high-speed PDs at 2 μm wavelength were always realized based on III-V group materials in the past. An InGaAs/GaAsSb PD waveguide with a responsivity of 0.84 A/W and a 3-dB bandwidth of 10 GHz was reported in 2018. The bandwidth record was refreshed by an InGaAs/GaAsSb uni-traveling carrier photodiode (UTC-PD), which has a 3-dB bandwidth of 25 GHz and a responsivity of 0.07 A/W. Another option is to manufacture high-speed PDs at 2 μm wavelength based on IV group materials, which may benefit from mature and low-cost silicon photonics. For the transparency of germanium in the 2 μm band, GeSn is an alternative option as the absorbing material. By adjusting the content of Sn in GeSn alloy, the absorption edge can be extended to a longer wavelength [13]. A high-speed GeSn PD with a bandwidth of 30 GHz was demonstrated in 2021 [14]; however, its responsivity was only 0.014 A/W at 2 μm. Recently, a GeSn PD with a bandwidth of 40 GHz and a responsivity of 0.49 A/W was realized, utilizing a resonance cavity-enhanced (RCE) structure and a higher Sn component. However, the epitaxial growth of GeSn is complicated and the realization of waveguide-coupled GeSn PD is difficult; thus it is difficult to achieve large-scale on-chip applications.

A new CMOS-compatible idea to realize optical response at 2 μm is based on the subbandgap light absorption [15]. For subbandgap light detection, the photocurrent is generated by a combination of two-photon absorption (TPA), surface state absorption (SSA), and photon-assisted tunneling (PAT) effects at the PN junction under high reverse bias voltage [16–19]. Utilizing subbandgap light absorption and avalanche multiplication, Si or Ge PDs with considerable bandwidth and responsivity at 2 μm can be realized. A monolithic Si PD operating at 20 Gbps was demonstrated, with a responsivity of 0.3 A/W. Taking advantage of the stronger subbandgap absorption effect in Ge than in Si, the responsivity was further improved by a waveguide-coupled Ge PD with a separation absorption charge multiplication (SACM) structure. It has a responsivity of 1.05 A/W at 1.95 μm and a 3 dB bandwidth of 6.12 GHz.

For the on-chip application, high-speed and high-responsivity Ge PD with a simple structure and fabrication process is needed. In this work, we demonstrate a germanium photodetector with a remarkable responsivity and data-receiving rate, which is fabricated on a commercial silicon photonics platform. The photodiode is designed with an ingenious and simple asymmetric lateral p-i-n junction structure and assisted by a distributed Bragg reflector (DBR) to enhance optical absorption. It exhibits a responsivity of 3.6 A/W and a maximum bandwidth of 50 GHz. For the first time, to the best of our knowledge, an optical receiving rate of up to 112 Gbps is realized, verifying its feasibility in a high-speed 2-μm-band communication system.

The three-dimensional and the cross-section schematics of the proposed 2-μm-wavelength-band lateral waveguide Ge PD are depicted in Figs. 1(a) and 1(b), respectively. The incident light of 2 μm propagates along a 600-nm-wide Si rib waveguide, passes through a Si taper, and is eventually coupled into the Ge absorption region via evanescent wave coupling. The detection mechanism for Ge at 2 μm is based on subbandgap light absorption, which is a combination of TPA, SSA, and PAT effects under a large internal electric field. In consideration of this weak absorption effect, a distributed Bragg reflector (DBR) is added to the silicon layer after the Ge absorption region, as shown in Fig. 1(a). The simulated optical reflection spectrum of the DBR is shown in the inset of Fig. 1(a). As can be seen, the DBR structure exhibits a reflectivity of approximately 90% within the wavelength range of 1800–2100 nm. Part of the optical power that is not absorbed at the end of the active region can be reflected back by the DBR for secondary absorption, resulting in a longer effective absorption length and improved quantum efficiency (QE). As can be seen in Fig. 1(b), the cross-section shape of the Ge is isosceles trapezoid or triangle, depending on growth condition and the bottom width of Ge ($W_{\mathrm{Ge}}$). ${\mathrm{N}}^{++}$-Si, ${\mathrm{N}}^{+}$-Si, i-Si, ${\mathrm{P}}^{+}$-Si, and ${\mathrm{P}}^{++}$-Si are defined in the top Si layer of the SOI. A sidewall of the Ge region is lightly doped with ${\mathrm{N}}^{-}$ type and contacted with ${\mathrm{N}}^{+}$-Si. As a result, an asymmetric lateral p-i-n junction is formed in Ge by ${\mathrm{N}}^{-}$-Ge, i-Ge, and ${\mathrm{P}}^{+}$-Si, which is introduced to apply a reverse field in Ge film.

The optical field distribution is simulated by the finite-difference time-domain (FDTD) method. Figure 1(c) exhibits the cross-sectional static optical field distribution. Most of the optical field is confined in the center of the Ge region. Figure 1(d) shows the electrical field distribution at a bias voltage of $-8\mathrm{V}$. The strong electrical field in Ge contributes to obtaining a carrier multiplication and reaching a saturation drift velocity, which is beneficial for the responsivity and bandwidth performance. The doping strategy helps reduce the intrinsic region of Ge with the asymmetric p-i-n junction structure, which contributes to decreasing the carrier transit time. The optical field and electrical field in the Ge region show a good overlap, which is beneficial for high-speed and high-efficiency detection. Figure 1(e) is the optical microscope image of the proposed waveguide Ge PD with an active region length of 150 μm.

The proposed Ge PD is fabricated on a commercial 140 nm SiPH platform with a simple CMOS fabrication process, started on an SOI substrate with a 220-nm-thick Si (001) top layer and a 2-μm-thick buried oxide (BOX) layer. The edge coupler and waveguides are fabricated by deep ultraviolet (DUV) lithography and dry etching. The width of the Si waveguide is 600 nm. A 20-μm-long Si taper is used to linearly widen the width of the Si waveguide from 600 nm to 2 μm. In the active region, the Ge film is epitaxially grown on top Si through a low-pressure chemical-vapor deposition (LPCVD) process, with a bottom length and width of 150 and 1 μm, respectively.

The dark current and photocurrent measurements are performed using an Agilent B1500A semiconductor parameter analyzer at room temperature. For the photocurrent test, the wavelength of the laser is fixed at 2 μm and the bias voltage varies from 0 to $-9\mathrm{V}$. Figure 2(a) shows the measured I-V curves of the CMOS-process-fabricated waveguide Ge PD when operating at 2 μm with different optical powers varying from $-3.1$ to $-11.7\mathrm{dBm}$. As shown in Fig. 2(a), the measured dark current is about 7 nA (corresponding to a dark current density of $0.006\mathrm{A}/{\mathrm{cm}}^2$) at $-3\mathrm{V}$, begins to increase rapidly at $-5\mathrm{V}$ (35.7 nA, $0.032\mathrm{A}/{\mathrm{cm}}^2$), and rises to $\sim 100\mathrm{\mu A}$ at 8.4 V, indicating that the avalanche multiplication occurs at around $-5\mathrm{V}$. The optical power is coupled from fiber to chip through an edge coupler, with the measured coupling loss of about 5 dB. The responsivity of the device at different input optical power ($P_{\mathrm{in}}$) is calculated and shown in Fig. 2(b) after deducing the edge coupling loss. It can be seen that the responsivity at 2 μm increases as the reverse bias voltage increases and also increases as the $P_{\mathrm{in}}$ decreases. With the $P_{\mathrm{in}}$ of $-11.7\mathrm{dBm}$, the manufactured PD shows a responsivity of $\sim 0.17\mathrm{A}/\mathrm{W}$ at $-5\mathrm{V}$, which begins to increase rapidly at $-7\mathrm{V}$ (0.78 A/W) and reaches 3.64 A/W at $-9\mathrm{V}$. We also test the responsivity of the Ge PDs with and without the DBR under different voltages. The calculated results of the ratio of the responsivity, as shown in the inset of Fig. 2(b), indicate that the DBR structure enhanced the responsivity by approximately 60%–70%. This I-V measurement shows the device with a competitive advantage in optical responsivity in detection of 2 μm for mid-infrared (MIR) photonics with a simple CMOS fabrication process. To evaluate the avalanche effect, the responsivity at $-5\mathrm{V}$ is defined as the reference responsivity value and the gain is calculated by the ratio between the responsivity at a bias voltage and the reference responsivity. Figure 2(c) shows the multiplication gain of the proposed photodetector under different bias voltages ranging from $-5$ to $-9\mathrm{V}$, as the input optical power varies.

High-frequency characteristics measurements are implemented to experimentally verify the $S_{22}$ and $S_{21}$ response of our proposed 2 μm Ge PD, utilizing a Keysight vector network analyzer (VNA, N5247B). The measurement setup is depicted in Fig. 3(a). The modulated optical signal is generated by a commercial lithium niobate Mach–Zehnder modulator (LN MZM) and then amplified by a thulium-doped fiber amplifier (TDFA). After passing through a variable optical attenuator (VOA), the 2 μm optical signal is injected into the Ge PD and converted to an electrical radio frequency (RF) signal. Finally, the electrical signal is fed to the VNA, where the S-parameters of the Ge PD are shown. The calibration of the high-speed RF system is carefully conducted to consider the contributions of coaxial cables, LN MZM, and the GSG probe.

Figure 5(a) shows the normalized optical-electro frequency response ($S_{21}$) of the Ge PD at 2 μm under bias voltages of $-4$ to $-9\mathrm{V}$. Here the input optical power is set to be $-7.6\mathrm{dBm}$. The 3-dB bandwidth ($f_{3\mathrm{dB}}$) and responsivity at 2 μm from $-2$ to $-9\mathrm{V}$ are depicted in Fig. 5(b). It can be seen that the bandwidth of the fabricated Ge PD is $\sim 33.3\mathrm{GHz}$ when operating at a bias voltage of $-2\mathrm{V}$ and increases to a maximum bandwidth of $\sim 51\mathrm{GHz}$ at $-4\mathrm{V}$. When the bias voltage is higher than $-6\mathrm{V}$, the bandwidth begins to decrease with reverse voltage. The decrease in bandwidth at high bias voltage can be explained by the increased avalanche build-up time (also known as the multiplication time), resulting in a declining avalanche buildup time bandwidth. The corresponding GBP is also shown in Fig. 5(c). It shows a maximum GBP of 590, benefiting from the strong electrical field in Ge and Si.

The Ge PD exhibits high bandwidths of $>39.4\mathrm{GHz}$ (39.4–47.7 GHz) at the operating voltages from $-7$ to $-9\mathrm{V}$, where it has a good responsivity of $>0.53\mathrm{A}/\mathrm{W}$ (0.53–2.41 A/W). In practical applications, a tradeoff must be made among responsivity, bandwidth, and dark current (or noise) to select the appropriate operating voltage. High bias voltage can lead to higher responsivity, but it also suffers from increased dark current and reduced bandwidth due to the extended avalanche buildup time [20,21]. Overall, the high-bandwidth and responsivity characteristics of the Ge PD in avalanche operating mode are essential for its application in the 2-μm-band high-speed optical communication system, indicating its considerable potential in the MIR integrated photonics.

Figure 6 reviews the responsivity and 3-dB bandwidth of high-speed photodetectors operating at the 2 μm wavelength in recent years. The proposed Ge photodetector shows the best responsivity and 3-dB bandwidth characteristics among all reported photodetectors in the III-V and IV groups. We owe its outstanding performance to the design of the asymmetric lateral p-i-n junction, which maximizes the overlap between the mode field and the electrical field. The introduction of DBR almost doubles the effective absorption length of the device without degrading the RC time of the device, allowing it to have high responsivity and high bandwidth simultaneously.

To further verify the feasibility of this Ge PD in a 2-μm-band high-speed optical communication system, the eye diagram large-signal acquisitions are measured. As shown in Fig. 3(b), the schematic setup of high-speed NRZ and PAM-4 eye diagram measurement is depicted. High-speed modulated optical signals are generated by adding a pseudo-random bit sequence (PRBS) of $2^{31}-1$ to the commercial LN MZM. After that, the high-speed optical signals are coupled into the Ge PDs. The converted RF signals are amplified by an external electrical amplifier (SHF 804B) and then injected into a Keysight DCA-X series wideband width sampling oscilloscope (N1000A). The input optical power is set to be $-3.1\mathrm{dBm}$. Clear open-eye diagrams of the NRZ modulation format up to 56 and 70 Gbps at 7.5, 8.0, 8.5, and 9.0 V are obtained, as can be seen in Fig. 7(a). It is known that photodetectors working in avalanche state and at the optimized bias voltage enable improved signal-to-noise ratios (SNRs). For the NRZ eye diagrams of 56 Gbps, the Ge PD shows the SNRs of 6.69, 6.85, 6.97, and 6.99 at 7.5, 8.0, 8.5, and 9.0 V, respectively. For the 70 Gbps NRZ eye diagrams, the Ge PD shows the SNRs of 4.63, 4.70, 4.68, and 4.59 at 7.5, 8.0, 8.5, and 9.0 V, respectively. The eye diagram acquisitions of the PAM-4 modulation format are also carried out to check its potential for MIR communications application. The measured 90 and 112 Gbps PAM-4 eye diagrams are shown in Fig. 7(b). This is the first demonstration of a photodetector operating at 2 μm wavelength with a data-receiving speed of more than 100 Gbps. The eye-diagram measurement results indicate that the proposed Ge PD is qualified as the receiver of a 100G communication system in the 2 μm wavelength band.

Table 2 presents the literature overview of the state-of-the-art high-speed photodetectors operating at 2 μm wavelength in different material groups. For the high-speed application in the 2 μm wavelength band, an ideal photodetector should have high bandwidth and high responsivity. More importantly, its fabrication process needs to be as simple as possible, which determines the yields, costs, and practicability. Compared with their competitors in the III-V group and IV group materials, the proposed Ge PD shows an overall leading performance in responsivity, data reception speed, and fabrication complexity.Table 2.

Literature Overview of the State-of-the-Art High-Speed Photodetectors Operating at 2 μm Wavelength in Different Material Groups^a

In this work, we have reported a high-responsivity and high-speed waveguide-coupled Ge photodiode working at the 2 μm wavelength based on a commercial standard CMOS process. The proposed Ge PD has a recorded-high responsivity of 3.6 A/W at 2 μm wavelength at the bias voltage of 9 V. Working at the avalanche state, it shows a 3-dB bandwidth of 39.4–47.7 GHz from 7 to 9 V, breaking the bandwidth record created by a GeSn-based photodetector at 2 μm. High-speed optical reception based on the proposed PD is demonstrated with 70 Gbps OOK and 112 Gbps PAM4 signals, which is the first optical receiving demonstration at 112 Gbps per lane in a 2-μm-wavelength optical-electrical system, to the best of our knowledge. The remarkably high responsivity, high bandwidth, and low fabrication complexity of the Ge PD may pave the way for chip-based optical receivers in high-speed optical interconnects and communication systems in the 2 μm wavelength band.