Avalanche photodetectors (APDs) can offer high gain and high sensitivity, making them widely used in optical sensing, quantum communication, LiDAR, and autonomous driving applications [1–3]. Compared to the detectors based on III-V materials, silicon-based photodiodes exhibit the unique advantages of CMOS compatibility and low power consumption, serving as a competitive candidate for optical interconnection [4,5]. A silicon (Si) detector with epitaxial growth of germanium (Ge) with an absorption coefficient of ∼5000cm−1 can be used for light detection at 1550 nm [6], which makes Ge/Si APD highly suitable for applications in the infrared band. Due to the small size and long absorption length, the waveguide-integrated APD exhibits higher responsivity and bandwidth than the surface illumination APD [7–9]. However, a high bias voltage limits the application of Ge/Si APDs in computing architecture. The typical Ge/Si APD operating voltage is usually above 20 V [10–12]. It limits the application in data transmission, since the current computer architecture requires power rails below 12 V [13]. A Ge/Si APD with 10 V breakdown voltage has been demonstrated [14,15]. Furthermore, a three-terminal Ge/Si APD is demonstrated to show independent control of electric fields in the absorption and multiplication regions, giving rise to an extremely low breakdown voltage of 6 V [16]. However, these devices feature a vertical structure that needs the epitaxial growth of silicon, thereby increasing process complexity. Germanium detectors are of interest due to their low avalanche voltage. Virot et al. designed a germanium avalanche photodetector (Ge APD) that achieves a gain of more than 20 at −7V [17]. Chen et al. reported a p-i-n Ge APD obtaining a gain bandwidth product (GBP) of 140 GHz at −5V by utilizing a 185 nm thick Ge layer [18]. Hu et al. reported a waveguide-integrated vertical Ge APD with doping optimization, which obtains a bandwidth of 30 GHz at −7V bias [19]. However, due to the ∼4.18% lattice constant mismatch between Ge and Si, a large number of dislocations are often induced when a Ge epitaxial layer is grown on Si [20,21]. These defects serve as carrier recombination centers and significantly increase the dark current of the Ge/Si APDs. In addition, electrons and holes equally contribute to the noise in Ge (that is, the effective k ratio of the ionization coefficient of holes and electrons is close to unity) [17], which means that when the avalanche process occurs in the germanium region, more noise will be generated, which is further manifested as a higher dark current.

In this research, we introduce a p-i-n-i-n type Ge APD that incorporates an intrinsic Ge layer for both avalanche and absorption functions, along with an intrinsic silicon region to minimize the dark current. The experimental results reveal that the Ge APD achieves a high primary responsivity of 1.1 A/W at a bias voltage of 2 V, with an impressively low dark current density of 6.1×10−11A/μm2. The avalanche voltage of the Ge APD is −8.4V, with 3 dB bandwidth up to 25 GHz. Additionally, we successfully showcased optical signal reception of non-return-to-zero (NRZ) data at 32 Gbps, highlighting the potential as an essential component in silicon photonic data links.

Generally, the epitaxial Ge layer is thin, and germanium has a relatively low avalanche electric field. As a result, the avalanche voltage required for Ge APD is lower compared to a separate absorption-charge-multiplication (SACM) Ge/Si APD. We designed a waveguide Ge APD with an n-type doped charge region, ensuring that the avalanche region is confined within the germanium layer. Both the avalanche region and the absorption region are in the germanium layer.

The structure is constructed on an SOI wafer with a 220 nm thick top-silicon layer and a 3 μm thick silica buried box. This design fully meets the design rules of the foundry. Figure 1(a) shows the cross section of the proposed Ge APD. Intrinsic Ge is grown on a shallowly doped n-type region. Due to the angled growth of the germanium epitaxy, the final structure approximates to a trapezoidal shape. To mitigate the tip effect, the window area for the germanium epitaxy is larger than that of the n-type doped region on the silicon substrate. Boron (B) ions and phosphorus (P) ions are heavily doped on the top of Ge and the edge of silicon to form P++ and N++ regions for ohmic contact, respectively. The silicon intrinsic region is situated between the n-type doped region and the N++ doped region. This region assists the n-type doped region in regulating the electric field in the germanium. Another function of the intrinsic silicon region is to directly connect a resistor in series with the device on the chip, thereby reducing its dark current. In this work, the dimensions of the germanium region are as follows: the width WGe=5μm, the height HGe=500nm, and the length LGe=24μm, while the width of the n-type doped region Wn=4.8μm, which is slightly narrower than the width of the germanium strip WGe. The doping concentration of the n-type region in silicon is 5×1017cm−3. The extended length of the germanium region is designed to enhance the light absorption rate, thereby improving the light responsivity. However, it also increases the capacitance, giving rise to bandwidth reduction.

In addition, the electric characteristic of the device is simulated using the Technical Computer Aided Design (TCAD) simulation tool. Figure 1(b) shows the simulated results for the distribution of electric field in the cross section of the device when the bias voltage sets to −8V. The electric field in the germanium layer is approximately 3×107V/m, which is sufficiently strong to cause ionization internally. However, the electric field at the bottom edge of the germanium is 1×106V/m, which effectively prevents the tip effect at the edge.

A three-dimensional image of the APD is shown in Fig. 2. An incident light (Pin) enters the input waveguide through a grating coupler, and then passes through a beam splitter. One portion of the light is directed into the APD, while the other portion passes through the same combiner and is emitted by the grating coupler. The emitted light (Pout) is coupled into the optical power meter through an optical fiber. The light entering the APD can be calculated using the formula P(dB)=(Pin+Pout)/2 in a log scale. Therefore, the power mentioned in the performance parameters of this article corresponds to the optical power entering the device, and all measurements are conducted under constant room temperature conditions.

To characterize the DC performance of the APD, we have measured the I-V curve and responsivity. The relation between the generated current and the bias voltage at the room temperature is shown in Fig. 3(a). The photocurrents are measured in the case of the incident light centered at 1550 nm, with optical power ranging from −25dBm to 0 dBm. As a reference, the dark current changes as the bias voltage is also measured. An initial dark current is as low as ∼7.42nA at −2V bias voltage, and then exhibits a sharp increase at −7V. Finally, it reaches 100 μA at −8.4V, as indicated by black line in Fig. 3(a). Here, the breakdown voltage of APD is defined as the dark current reaches 100 μA referenced in Ref. [15]. This indicates the avalanche voltage is −8.4V. Figure 3(b) shows the simulated electric field in germanium at the bias voltage from 0 V to −8.5V. When the bias voltage is −7V, the electric field strength reaches 1.79×107V/m, which is slightly higher than the electric field conditions required for ionization in germanium (∼1.7×107V/m [22]). The simulation results match well with the experimental measurement, confirming that the Ge APD undergoes avalanche breakdown at a voltage of −8.4V.

Figure 4(a) shows the relation between the responsivity of the Ge APD and the bias voltage. It is shown that the primary responsivity of the APD is about 1.1 A/W at the voltage of −2V. The responsivity increases as the reverse bias increases and reaches a maximum value of 4.11 A/W when the voltage is around −8.4V. At this voltage, the dark current reaches 100 μA so the Ge APD has entered avalanche breakdown. When the reverse bias further increases, the responsivity decreases since the dark current increases more quickly than the total current. The primary responsivity at the bias voltage of −2V and −8V varying with the optical power is shown in Fig. 4(b). It shows that the device responsivity decreases as the incident light increases. It can also be seen that with the increase in optical power, the primary responsivity does not change significantly in the case of the bias at −2V. Figure 4(c) shows the photocurrent varying with the optical power at the bias voltage of −2V and −8V. When the bias voltage is −2V, the photocurrent increases linearly as the optical power. When the optical power increases to −5dBm, the photocurrent growth rate becomes slow and the responsivity decreases. This is due to the nonlinear optical effect of the material [23], which leads to effects like saturable absorption and nonlinear refraction. The saturated optical power of the device is −5dBm.

The opto-electric (O-E) bandwidth was measured by using a light-wave component analyzer (LCA, Keysight). The sweeping frequency is from 10 MHz to 30 GHz. The modulated optical signal was injected to the APD through a polarization controller (PC). Figure 5(a) shows the measured O-E frequency responses for the APD under different bias voltages. Here, the optical power is set to −17dBm, and the data is normalized. When the bias voltage is −5V, the Ge APD achieves a 3 dB bandwidth of 1.55 GHz. Subsequently, the bandwidth can be increased from 1.7 GHz to 25 GHz by increasing the bias voltage from −5V to −11V. When the bias voltage is greater than −8V, the low frequency response decreases with further increase in bias voltage, while the high frequency range is enhanced. This enhancement becomes more pronounced as the bias voltage increases. This is caused by the space charge effect from high current density reported by previous works [8,24,25]. The space charge effect might introduce an inductive-peaking-like behavior, resulting in the bandwidth enhancement [24]. The bandwidth limitation at low voltage is due to the insufficient electric field in silicon, which prevents carriers from reaching their saturation drift velocity. To achieve saturated drift velocity for carriers in germanium or silicon, the electric field should reach 1×107V/m [26]. Figure 5(b) shows the simulated electric field in germanium at different voltages. It shows that the electric field in silicon can surpass 1×107V/m at the bias of −8V, boosting up the drift velocity of carriers in silicon to the saturation level.

We have also verified the data reception performance of the Ge APD by detecting NRZ optical signal at data rates varied from 20 Gbps to 32 Gbps. The eye diagrams are measured without using a trans-impedance amplifier (TIA). The experiment setup is shown in Fig. 7. A continuous wave laser centered at 1550 nm is directed to a Mach–Zehnder modulator (MZM) driven by an arbitrary waveform generator (AWG, Keysight M8196A). The generated NRZ signal is amplified prior to the MZM. The modulated optical signal is then coupled to the APD through a grating coupler. Here a bias-tee is used to apply the reverse bias to the APD and transmit the RF electrical signal simultaneously. The RF signal is sampled by a digital sampling oscilloscope (Keysight N1000A) for eye diagram observation. The AWG and the oscilloscope are synchronized via an internal clock connection. We have recorded the eye diagram of the on-off keying (OOK) modulation mode at data rates of 20 Gbps, 25 Gbps, and 32 Gbps under the bias voltages of −11V and −12V, as shown in Fig. 8. The input optical power for the device is −11dBm. The linear feed forward equalizer is added when the oscilloscope displays the eye diagram to overcome the influence of channel attenuation. A clear eye diagram of OOK signal at 32 Gbps with 4.18 signal-to-noise ratio (SNR) is observed in the case of voltage biased at −12V.

Table 2 provides a summary of reported Ge/Si APDs and Ge APDs. These Ge/Si APDs are designed for the 1550 nm or 1310 nm bands. The waveguide APDs have shown significant potential in achieving both high 3 dB bandwidth and high responsivity [33,34]. For the application of APD in optoelectronic links, it is necessary to pay attention to the noise and avalanche voltage of the device. Due to the low ionization coefficient of silicon (k=0.02 for bulk silicon [9]) and the high absorption coefficient of germanium in the 1550 nm band, SACM APD has attracted much attention for obtaining high bandwidths with low noise. However, the avalanche voltage may exceed 12 V, if a lateral PN junction is used in the device structure [31,32]. On the other hand, a vertical PN junction requires silicon epitaxy increasing the process complexity [15,16]. The responsivity of reported APDs is limited to around 1 A/W. For Ge APDs where all PN junctions are in germanium, the responsivity is only 0.4 A/W due to the loss of generation efficiency caused by the absorption in nearby doped regions and insufficient absorption length [17]. For Ge APDs containing a germanium silicon heterojunction, the primary responsivity is less than 1 A/W [18,19]. The present waveguide Ge APD was fabricated with a standard process by a silicon photonics foundry. The primary responsivity of this APD is 1.1 A/W, which is the best in the summary of Table 2. The dark current density of the device discussed in this article is 0.124A/mm3, which is significantly lower than that of the devices in Ref. [19], about one tenth of it (the dark current density of the device in Ref. [19] is approximately 1.3A/mm3). We have also demonstrated the capability of data reception on 32 Gbps NRZ optical signal. In conclusion, the Ge APD has the potential to be used in silicon photonic data links.Table 2.

Summary of the Reported Ge/Si APDs and Ge APDs

We also observed that the bandwidth of the Ge APD is limited within 25 GHz. It is well established that the 3 dB bandwidth of a germanium photodetector is primarily governed by two factors: the carrier transit-time-limited bandwidth (fT) and the RC-limited bandwidth (fRC) in the active region: f3dB=1fT−2+fRC−2.(1)The carrier transit-time-limited bandwidth (fT) can be expressed as follows [35–37]: fT=0.45vd,(2)where v is the saturation drift velocity, and d is the thickness of the intrinsic layer. The RC-limited bandwidth fRC can be calculated by fRC=12πRC,(3)where R is the resistance, including the series resistance and the load resistance, and C is the capacitance, mainly affected by a cross-sectional area. Compared to the reported Ge APDs [19], the 3 dB BW of the proposed APD is only 5 GHz. It is smaller than that of devices with a larger thickness and three times the junction area. To further enhance bandwidth, the relationship between the absorption rate of 1550 nm wavelength light and the length of the Ge layer needs to be investigated. By appropriately reducing the length of the Ge layer and designing the width according to process limitations, we can effectively reduce the device’s size. The reduction in size decreases the capacitance, thereby increasing the bandwidth. This is our next step of improvement.

In this paper, a high performance waveguide Ge APD is proposed, whose avalanche region is in germanium and fully meets the silicon photonics platform process. It shows an excellent primary responsivity of 1.1 A/W at the bias voltage of −2V and the wavelength of 1550 nm, which is among the top results reported to date. At this voltage, the dark current is as low as 7.42 nA. The device’s avalanche voltage is −8.4V, which is defined as the point where the dark current reaches 100 μA. Finally, the device bandwidth can reach 25 GHz, and the 20/25/32 Gbps NRZ data receiving has been demonstrated successfully with open eye diagrams by using the present Ge APD with the input power of −11dBm. In conclusion, the device demonstrates excellent performance at low voltages, laying a foundation for its application in current computer architecture. Future work should focus on further reducing the dark current and improving the bandwidth.