In recent decades, silicon photonics (SiP) has made unprecedented progress, playing a critical role in applications such as high-speed optical communications [1–3], high-performance computing [4], and sensing [5]. One crucial component in SiP is the photodiode (PD), which is responsible for converting the detected optical signal into an electrical one. Unfortunately, due to the low absorption efficiency of pure silicon in the C-band (1530–1565 nm), heterogeneous integration of other materials with the silicon substrate is necessary for effective photodetection in this band. Up to now, high-performance PDs with bandwidths of more than 100 GHz have been achieved using III-V materials or graphene [6–10]. Nevertheless, silicon-based germanium (Ge) PDs are gaining attention due to their complementary metal–oxide–semiconductor (CMOS) process compatibility, promising high speed with low cost and power consumption [11]. A fin-like lateral Ge PD is demonstrated with an impressive bandwidth of 265 GHz [12]. However, its performance in terms of responsivity and dark current is constricted due to a narrow intrinsic region. Conversely, vertical Ge PDs typically exhibit superior performance in the dark current and responsivity but have a slightly smaller bandwidth than lateral ones. The main factors limiting the bandwidth of vertical Ge PDs are the carrier transit time and the resistance–capacitance (R-C) parameters [13]. The former is generally proportional to the thickness of Ge, and it is challenging to decrease the thickness while maintaining high material quality [14]. Currently, engineering the R-C parameters is the primary approach to increase the bandwidth of vertical Ge PDs [15–20]. Previous research achieved a vertical Ge PD with a bandwidth of 103 GHz by utilizing a U-shaped electrode and incorporating inductor gain-peaking technology [20]. Moreover, reducing the device length can also alter the R-C parameters of the device, leading to a larger external bandwidth and smaller dark current, at the cost of decreased responsivity. To address this issue, a distributed Bragg grating (DBR) or a Ge corner reflector has been introduced to achieve secondary light absorption and enhance the device’s responsivity [21–23]. However, DBR structures require high fabrication accuracy, and the etching process of Ge corner reflectors may introduce a large number of defects within the Ge itself, leading to increased dark current. On the other hand, their bandwidths, which are less than 70 GHz, need to be further improved to accommodate high-speed applications.

Here, we present a compact waveguide vertical Ge PD featuring high responsivity, low dark current, and large bandwidth simultaneously, through integrating a silicon corner reflector (SCR). With the stable and mature etching process of silicon, it is easy to achieve an SCR with high perpendicularity and low roughness sidewalls, which can enhance light absorption without deteriorating the dark current performance of a Ge PD. The length of Ge is designed elaborately to ensure efficient absorption taking into account the light oscillation between Ge and silicon layers. Experimental results show a 21% responsivity increase compared to a conventional waveguide vertical Ge PD without SCR, while the length of germanium is only 4.8 μm. A reduced device length simultaneously warrants a remarkably low dark current of 0.76 nA and a large bandwidth of 100 GHz. Such a large bandwidth is the comprehensive result of the reduction in Ge length and the inductor. Clear eye diagrams of 140 Gb/s on–off keying (OOK) and 240 Gb/s four-level pulse amplitude (PAM4) signals are demonstrated. This work provides an insight for achieving high-performance photodiodes while enhancing the level of integration.

The 3D schematic of the proposed SCR-assisted Ge PD is shown in Fig. 1(a). The light is guided by a silicon waveguide with cross-sectional dimensions of 500nm×220nm. A vertical p-i-n structure is used in the active region. Being different from conventional structure, the specially designed SCR integrated at the end of a compact PD enables high levels of light absorption through reflection, while still maintaining the advantages of low dark current and small external R-C product.

The responsivity, dark current, and external bandwidth characteristics of waveguide vertical Ge PDs are closely associated with the length of Ge. Unfortunately, a trade-off exists among these figures of merit. Specifically, when the width and height of the Ge are fixed, both the dark current and the product of R-C parameters (which considers the external load typically at 50Ω) demonstrate a negative relationship with the length of the Ge, while light absorption shows a positive relationship. The absorption of light by Ge can be determined by integrating the radiative flux in the cross section across its length, as demonstrated in Eq. (1): Pabs=−ω·Im(ϵ)·η·∫0LGeΦ(x)dx,(1)where ϵ is the permittivity of germanium, and Φ(x)=0.5η·∫0HGe∫0WGe|E(x,y,z)|2dydz is the radiant flux of light with a cross section HGe×WGe in the direction of propagation, while η is a constant denoting the wave impedance. Therefore, it is generally challenging to reduce dark current and increase bandwidth simultaneously while maintaining a high level of responsivity. Here, we introduce a corner reflector integrated into the silicon layer at the end of the PD to address this issue, as depicted in Fig. 1(b). The fabrication process for SCRs is simple and mature, offering notable advantages in terms of reproducibility and the prevention of dark current degradation. The SCR is an isosceles triangle with an angle of 90°, and its two sides are made of Si/SiO2 interfaces that ensure the total internal reflection of light. The unabsorbed light strikes the sides of the SCR at a 45° angle and then undergoes two consecutive total internal reflections at both sides, before returning to the Ge region for secondary absorption. Figure 1(c) presents the simulated optical field distribution, illustrating the process of evanescent coupling between the Si and Ge layers. During this process, the light oscillates back and forth between the Si and Ge layers as it propagates forward, ultimately entering the SCR. The details of the process of light being reflected within the SCR are demonstrated in the inset. The reflection characteristics of the SCR are explored over the wavelength spectrum ranging from 1.2 to 1.7 μm, as illustrated in Fig. 1(d). The SCR demonstrates impressive reflectivity surpassing 80% within this specific wavelength range, underlining its potential for achieving high reflectivity over a broad wavelength range. Meanwhile, since the process of evanescent coupling is closely related to the length of the Ge layer, it is essential to precisely select the appropriate Ge length to ensure that the light accurately enters the SCR. As shown in Fig. 1(e), we simulate scenarios with and without the SCR for various Ge lengths, using an absorption coefficient of 2400cm−1 for Ge. The enhancement of light absorption through the SCR demonstrates a fluctuating downward relationship with the length of Ge. After passing through an effective absorption length of 9.6 μm, nearly all of the light is absorbed or scattered, with only 1%–2% reflected back into the waveguide, where it will subsequently be scattered. Taking into account the enhancement and the level of integration, a final design length of 4.8 μm is comprehensively determined.

To investigate the influence of SCR, we design two PDs with and without SCR, respectively. The dimensions of their germanium regions are uniform, with a length of 4.8 μm and a width of 4.3 μm. The devices are fabricated on silicon-on-insulator (SOI) with a silicon top layer thickness of 220 nm and a buried oxide layer thickness of 2 μm. To enhance light confinement, the silicon top layer is etched to form a ridge waveguide with 90 nm silicon slabs. Then, the specific region of the silicon layer is P++ doped using high doses of boron. Following this, selective low-temperature epitaxy is performed at 550°C to deposit Ge with appropriate dimensions on the ridge waveguide, and the device is then annealed at 750°C for 30 min to form a pyramid structure with a thickness of 500 nm. The top 100 nm of Ge is then N++ doped with phosphorus to form a vertical n-i-p structure and an ohmic contact with the metal electrode [24]. A meander inductor is formed using a top metal layer with a thickness of 2 μm. The optimal inductance length and width of the metal line are determined to be 360 and 3 μm, respectively. Figure 2(a) shows the microscope image of the fabricated device, where the light is coupled into the Ge PD through a silicon grating coupler and a silicon waveguide. The details of the Ge region and the SCR are shown in the inset.

The static characteristics of the two PDs are measured, as shown in Figs. 2(b)–2(d). The PDs are biased using a source meter (Keithley 2401), and the corresponding current values are recorded. A tunable laser is utilized as the light source, and the measured dark currents and photocurrents are shown in Fig. 2(b). When the SCR exists, the dark current of the PD at −1 V is 0.76 nA, while it is 0.49 nA without the SCR. The etching process for the SCR might be the cause of the marginal increase. Nevertheless, both values are extremely low, and the Ge length reduction is responsible for the excellent dark current performance. The grating loss is measured using a reference device and the actual power entering the PD is calculated by subtracting the grating loss. With a 5 dBm optical input at 1550 nm and a 5.9 dB grating loss, the photocurrent results show a higher output in the device incorporated with SCR compared to the one without it, while both remain stable within the bias voltage range of 0 to −3V. Figure 2(c) demonstrates the corresponding photocurrents at −1V while varying the incident optical power. In the measured optical power range of 0 to 0.9 mW, the photocurrents exhibit a linear relationship with the incident optical powers, and the slopes of the curves indicate the responsivity of each PD. The incorporation of SCR shows an enhancement in the responsivity of approximately 21%, advancing from 0.79 to 0.96 A/W. Figure 2(d) depicts the responsivity of the SCR-assisted PD across the 1520–1600 nm range, highlighting a notable responsivity around 1 A/W particularly in the 1545–1570 nm range. Due to the presence of tensile stress and dislocations, the Ge thin film retains a higher absorption coefficient at longer wavelengths [25,26]. As a result, it maintains a responsivity of approximately 0.5 A/W at 1600 nm. These results indicate that the compact PD assisted by SCR enables excellent responsivity over a broad wavelength range.

The small-signal RF measurement is conducted using a vector network analyzer (Keysight N5227B) in the 10 MHz to 110 GHz frequency range, and the measured S21 parameters are depicted in Fig. 3. Notably, the 3 dB bandwidth of the SCR-assisted PD can achieve 100 GHz at a −2V bias. However, the gray solid line for the response beyond 100 GHz is unreliable, which is influenced by the baseline noise from S-parameter de-embedding. A more accurate response in this segment, represented by a red dashed line, is obtained through fitting. We attribute this surprisingly large bandwidth to a comprehensive effect of the reduced length of Ge and the appropriate series inductor. Upon reducing the PD length from the conventional 8 μm to 4.8 μm, the junction capacitance decreases to 8.9 fF and the series resistance rises to 84Ω, resulting in a decrease in the external R-C product. According to the equivalent circuit model of PD [19], the bandwidth increases from 46 GHz to 53 GHz due to the reduction in length, and it is further boosted to around 100 GHz with the contribution of the inductor gain-peaking effect. In contrast, a PD with conventional length can only achieve a bandwidth of ∼80GHz when assisted by an inductor [19]. The results indicate that using an SCR-assisted compact Ge PD in combination with an inductor is a straightforward and efficient method for ensuring high absorption and low dark current and maintaining a large bandwidth.

The high-speed optical transmission performance of the SCR-assisted PD is characterized by eye diagrams. Various bit rates of OOK or PAM4 signals with a pattern length of 215−1 are generated by an arbitrary waveform generator (AWG) with a sampling rate of 256 GSa/s (Keysight M8199A). The output signal of the AWG is amplified by a 60 GHz high-speed driver and then connected to a 90 GHz Mach–Zehnder modulator. The modulated optical signal is detected by our SCR-assisted PD, and the converted electrical signal is ultimately captured by a real-time oscilloscope whose sampling rate is 256 GSa/s (Keysight UXR0704A). The received signals are optimized by post-compensation and offline digital signal processing for better transmission performance. Figure 4 shows the eye diagrams of the OOK or PAM4 signals at the input optical power of −1dBm. In OOK signal transmission at rates of 120, 130, and 140 Gb/s, the eye diagrams maintain excellent clarity, leading to signal-to-noise ratios (SNRs) of 19.09, 14.86, and 13.71 dB, respectively. Similarly, in PAM4 signal transmission at 200, 220, and 240 Gb/s, our PD effectively receives signals, resulting in SNRs of 20.51, 19.36, and 18.30 dB, respectively. These results demonstrate that our PD possesses a capability for high-speed data transmission.

Table 1 presents the recent developments of Ge-Si PDs. Lateral PIN Ge PDs have achieved the largest bandwidth of 265 GHz [12]. Nevertheless, the fabrication process is complicated, requiring strict control of the diffusion of dopant ions, and the etching of Ge leads to large dark currents. Additionally, the limited Ge width results in insufficiently high responsivity. Vertical PIN Ge PDs generally have higher responsivity (>0.7A/W), smaller dark current (a few nA), and relatively lower bandwidth (currently up to 103 GHz [20]). Achieving comprehensive performance improvement in terms of responsivity, dark current, and bandwidth poses a challenge in the current work. Since the sources of dark current are mainly surface leakage current and body dark current [28], reducing the device length is a favorable way to decrease the dark current. However, this will lead to lower responsivity. Thanks to the compact design of the PDs, this work has achieved a dark current below 1 nA. This is the lowest dark current density, 3.68mA/cm2, in the field of Ge PDs to date. Meanwhile, the introduction of SCR enables the compact Ge PD to achieve a responsivity close to 1 A/W.Table 1.

Performance Comparison between Proposed PDs and Literature^a

To assess the comprehensive performance of the Ge PDs, we introduce the concept of noise equivalent power (NEP), which is commonly employed to evaluate the noise level and sensitivity of a detector, taking into account both dark current and responsivity. The demonstrated NEP of 1.62×10−14W/Hz1/2 is the lowest value among waveguide vertical Ge PDs, to the best of our knowledge. In addition, a large bandwidth of 100 GHz has been achieved through the comprehensive regulation of the Ge length and the inductor. In comparison, our proposed PD is superior to the DBR-assisted one in all aspects [21]. Moreover, it has a similar responsivity and bandwidth to Ref. [20] and a smaller dark current due to the reduced dimensions of Ge. Summarily, this work offers an innovative and straightforward way to maintain the device’s high sensitivity and high speed while further miniaturizing it.

In conclusion, we have proposed and demonstrated a Ge PD with outstanding comprehensive performance by utilizing reduced Ge length in conjunction with SCR. The compact PD with a Ge length of 4.8 μm allows for a high responsivity of 0.96 A/W at 1550 nm, with an extremely low dark current of ∼0.7nA and a large bandwidth of 100 GHz. Furthermore, 140 Gb/s OOK and 240 Gb/s PAM4 signal transmissions are achieved based on the proposed PD. Our work provides an improved option for Ge PDs in the applications of high integration density, high sensitivity, and high-speed optical interconnects.