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Abstract
We report the demonstration of a germanium waveguide p-i-n photodetector (PD) for the C + L band light detection. Tensile strain is transferred into the germanium layer using a SiN stressor on top surface of the germanium. The simulation and experimental results show that the trenches must be formed around the device, so that the strain can be transferred effectively. The device exhibits an almost flat responsivity with respect to the wavelength range from 1510 nm to 1630 nm, and high responsivity of over 1.1 A/W is achieved at 1625 nm. The frequency response measurement reveals that a high 3 dB bandwidth (f3dB) of over 50 GHz can be obtained. The realization of the photonic-integrated circuits (PIC)-integrable waveguide Ge PDs paves the way for future telecom applications in the C + L band.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Silicon photonics technology has been proven to be a very reliable optical interconnect technology in datacom and telecom fields which rely on optical networks at all levels to transport data at very high speed and low cost. As one of the key components in the photonic-integrated circuits, high-speed and high-responsivity germanium (Ge) PDs have been widely reported in various research institutes [1,2]. State-of-the-art Ge PDs typically provide an opto-electric 3 dB bandwidth in the range of 30-70 GHz and a responsivity up to ∼1 A/W in the C-band (1530 nm-1565 nm) [3,4]. Networks with high bandwidth applications such as video on demand, virtual and augmented reality and internet of things, are quickly faced with capacity exhaustion [5]. The L-band is the second lowest-loss wavelength band which is also a popular choice when the use of the C-band is not sufficient to meet the bandwidth demand. However, the responsivity of the traditional Ge PD drops sharply after 1550 nm, which is limited by the band gap of Ge material. Various methods have been investigated to expand the cut-off wavelength of Ge PDs [6–8]. Doping tin (Sn) in Ge to form the GeSn alloy can effectively reduce the Ge bandgap [9–12]. The detection range of GeSn PDs is able to be extended to the whole shortwave infrared range (1-2.5 μm) by tuning the Sn composition [13–18]. However, the fabrication of GeSn PDs is not fully compatible with the existing silicon photonics integrated process, and no GeSn PDs have been reported based on 8-inch Si photonics pilot or production lines. Mechanical strain engineering has been proven to be another effective method to control the band gap of Ge [19–23]. R.W. Millar et.al reported the direct band gap emission can be shifted to 2.3 μm with 2% tensile strain by both theoretical calculations and experiments [24,25]. Y. Lin et.al used the sidewall SiN stressor to introduce tensile strain into Ge, and the normal-incidence metal-semiconductor-metal (MSM) Ge PD was fabricated on Ge on insulator (GeOI) wafers, with responsivity up to 1 A/W at 1625 nm [26].
In this paper, the waveguide Ge PDs working in the C + L band are fabricated on a standard 8-inch silicon photonics platform, which can be easily integrated with other silicon photonics components. The compressive strained SiN is deposited on the top of Ge to introduce the tensile strain into Ge. The dependence of strain transfer from the SiN layer to Ge waveguide is investigated. High responsivity up to 1.1 A/W is realized in the whole C + L band, by introducing the recessed trench around the Si and Ge waveguide. To the authors’ best knowledge, this is the first reported demonstration of the PIC-integrable Ge PDs on a standard 8-inch silicon-on-insulator (SOI) platform working in the C + L band, paving the way for future telecom applications in this wavelength range.
2. Device design and fabrication
The three-dimensional (3D) schematic of the designed Ge PD is shown in Fig. 1(a). Figure 1(b) and Fig. 1(c) show the cross-sectional schematics of the device along A-A’ dash line and B-B’ dash line, respectively. Similar to the well reported lateral PIN (LPIN) Ge PDs [4,27,28], doping or contacting of Ge is not required. The light is coupled from a 220 nm thick single-mode Si waveguide (500 nm width) to the Ge/Si ridge waveguide. The Ge is selective-epitaxial grown in the silicon recess (100 nm) region, which can improve the coupling efficiency between the Si waveguide and Ge/Si ridge waveguide. The compressive strained SiN stressor is deposited on the top surface of the Ge to introduce the tensile strain into the Ge, which reduces the bandgap of Ge, and extends the detection range beyond 1550 nm. The SiN stressor layer can also be formed beside or around the Ge waveguide. However, the whole process flow of the LPIN Ge PD needs more adjustments.
Fig. 1. (a) 3D schematic of the designed LPIN Ge PD with the SiN stressor on top surface. Cross-sectional schematics of the Ge PD along (b) A-A’ dash line and (c) B-B’ dash line shown in (a).
The multi-physics simulation by COMSOL is performed to explore the efficiency of the strain transfer from the SiN layer to the Ge waveguide with various device structures. The width and the thickness of Ge are set to be 0.55 μm and 350 nm, respectively. The pre-stress of the compressive SiN material is set to be 2 GPa, which can be achieved by the conventional plasma enhanced chemical vaper deposition (PECVD) method. It can be observed that the strain magnitude becomes higher toward the top of the waveguide, indicating a larger strain introduction on top of the Ge as compared to the region near the Ge/Si interface (Fig. 2(a)–2(c)). The trench formation besides the Ge/Si waveguide has a crucial importance for the introduction of strain. Almost no strain can be transferred into Ge without trench as shown in Fig. 2(a). The transferred strain becomes larger when the depth of trench increases (Fig. 2(d)). With the trench depth of 720 nm, the values of introduced tensile strain in center (region 1 (0.275, 0.175)) and upper (region 2 (0.275, 0.3)) layers of the Ge waveguide are 0.142% and 0.171%, respectively. With the depth increases to 1220 nm, the values of tensile strain increase to 0.177% and 0.213%, respectively. Further increasing the depth to 2720 nm, the values of tensile strain slightly increase to 0.197% and 0.235% and become saturate. It is found that the strain transfer also depends on the width of SiN between two trenches, the strain in the Ge is getting larger when the width of SiN is decreased. Here, in order to balance the effectiveness of the strain transfer and the compatibility of process, the widths of Si and SiN between two trenches are fixed at 3 μm and 4 μm, respectively.
Fig. 2. Two-dimensional (2D) strain profile in Ge region along B-B’ in Fig. 1 of the LPIN PD with the SiN stressor with (a) 0 nm (b) 720 nm- and (c) 1220 nm-depth trench formation beside Ge and Si waveguide. (d) The values of introduced strain in region 1 and 2 with various depths of trench.
3D simulation is carried out to further analyze the strain components in longitudinal (x) and transverse direction (y) in Ge waveguide. The length of the Ge waveguide is set to 5 μm. Figure 3(a) shows the strain εxx profile in the longitudinal direction of the Ge waveguide. Higher strain can be obtained in the edge regions (region 3 (1, 0.175) and 4 (1, 0.3)) compared with center regions (region 1 (2.5, 0.175) and 2 (2.5, 0.3)). The value of εxx in region 1-4 is 0.177%, 0.194%, 0.186% and 0.241%, respectively. Figure 3(b) shows the strain εyy profile in the transverse direction, which is close to that shown in Fig. 2(c). The value of εyy in region 5 (0.275, 0.175) and 6 (0.275, 0.3) is 0.162% and 0.184%, respectively. Figure 3(c) plots the simulated strain εxx as a function of the length of the device in regions 1-4. The strain in region 1 and 2 drops to 0.084% and 0.085% when the length increases from 5 μm to 10 μm. Only 0.016% strain remains when further increasing the length to 20 μm. However, in region 3 and 4, the introduced strains just slightly decrease to ∼0.2% with the length increasing from 5 μm to 20 μm. Figure 3(d) plots the simulated strain εyy as a function of the length of Ge waveguide in region 5 and 6. When the length of the device increases to 10 μm, the strains slightly increase to 0.178% and 0.206%, and further increase to 0.193% and 0.226% with the length of 20 μm. As the length of the device increases, the length of trenches on both sides of the device also increases, which is beneficial for the strain transfer in the εyy direction. For the strain in the εxx direction, however, the central region is further away from the trench at both ends, resulting in a significant decrease in strain.
Fig. 3. The strain profiles in (a) longitudinal and (b) transverse direction in Ge waveguide by 3D simulation. The length of the PD is 5 μm. The values of introduced strain (c) εxx in regions 1-4 and (d) εyy in regions 5-6 with various lengths of the device.
The Ge LPIN PDs are fabricated on 200 mm SOI wafers with 2 μm buried oxide (BOX) and 220 nm upper Si layer, in the fully integrated Silicon Photonics Platform at Shanghai Industrial μTechnology Research Institute (SITRI). The strip Si waveguides as well as the grating couplers are fabricated by 248 nm deep-UV lithography followed by dry etching of the Si layer. The Si P-type and N-type regions are then obtained by the ion implantation of Boron (B) and Phosphorous (P), respectively. The 1.1 μm-thick Ge is selectively grown on Si by reduce pressure chemical vapor deposition (RPCVD), followed by a Ge chemical-mechanical polishing (CMP) process step. Due to the different thermal expansion coefficient between Si and Ge, the as-grown Ge is under slight tensile strain [6,29–31], which is beneficial to expand the detection range. Subsequently, by tuning the deposition parameters carefully, the SiN stressor is deposited by PECVD with a compressive stress of 2 GPa. After that, the SiN is patterned and etched to form the trenches around the Si and Ge waveguide as shown in Fig. 2(c). Finally, after silicon dioxide (SiO2) deposition and CMP planarization, the waveguide Ge PDs are fabricated with the contact formation and metallization processes. Figure 4(a) shows the top view scanning electron microscope (SEM) image of the fabricated PD. The aluminum (Al) electrodes of the PD are isolated by SiO2. The cross-sectional schematic of the device along the dash line C-C’ is shown in Fig. 4(b). The light can be coupled from the Si waveguide to Ge through butt-coupling. The cross-sectional schematic along the dash line D-D’ is shown in Fig. 4(c). The height and width of the Ge region are 350 and 550 nm, respectively. The metal, the SiN stressor, the Ge and Si waveguide regions can be clearly identified by the Energy Dispersive X-Ray Spectroscopy (EDX) mapping (Fig. 4(d)) of the device. The SiN stressor is on top of the Ge. The size of the contact hole is 1×1 μm2, which has little effect on the strain introduction into Ge.
Fig. 4. (a) Top view SEM image of the fabricated Ge PD with the SiN stressor. Cross-sectional schematics of the Ge PD along (b) the C-C’ and (c) D-D’ dash line shown in (a). (d) EDX mapping image of the same device.
3. Result and discussion
Table 1 lists the fabricated Ge PDs with various structures. Type 1 PDs are the control devices without the SiN stressor. For type 2, 3, 4 PDs, a 200 nm SiN stressor is deposited on the Ge waveguide, with various values of trench depths around the Ge/Si waveguide. As compared to type 4 PDs, the thickness of SiN stressor is increased to 250 nm for type 5 PDs.
Table 1. Overview of the fabricated Ge LPIN PDs with various structures
Figure 5 shows the typical current I versus bias voltage Vbias (I–Vbias) characteristics of the type 4 PDs with and without illumination, all the measurements are carried out at room temperature. The detectors show a typical rectifying characteristic without illumination. The relatively low forward current at 1 V indicates a high series resistance. A tunable laser is used to generate light with the wavelength λ from 1510 to 1630 nm. The light is coupled into a Si waveguide through the grating coupler, and finally coupled into the Ge waveguide. The incident light power Pin is fixed at 1 mW. Clear optical response can be observed at 1625 nm. At low reverse bias, Itotal increases with Vbias, and saturates at ∼ -2V.
Fig. 5. I-V characteristics of the type 4 Ge PD with and without illumination. Itotal is defined as the sum of Iphoto and Idark under the light illumination.
The coupling loss between the fiber and grating coupler is about 4.2 dB. Therefore, after removing the input coupling loss from the grating coupler and the measurement link loss (∼ 0.7 dB, which includes waveguides, polarization control device and power meter) from 0 dBm, the optical power coupled into the Si waveguide is about -4.9 dBm. The responsivities in Fig. 6 are calculated using the loss test results at different wavelengths. Figure 6(a) shows the responsivities of the Ge PDs under illumination with λ ranging from 1510 to 1630 nm at -3V. Five devices with the same structure were measured from 5 different dies across the wafer. For the control PDs (type 1), the responsivity drops obviously beyond 1570 nm, which has been normally observed for the Ge/Si photodetectors. The absorption edge slightly extended beyond 1550 nm, which is due to the introduction of tensile strain caused by the different thermal expansion coefficient between Ge and Si. The type 2 PDs show similar response spectrum with the control PDs at the whole measurement range. This indicates that almost no strain is transferred from the SiN stressor to the Ge waveguide without the trench formation, which is consistent with the simulation results. For the type 3 PDs, the responsivity is ∼0.8 A/W over the entire measurement range. The absorption edge is extended beyond 1630 nm, which is due to the narrowing of the energy band caused by tensile strain. The SiN stressor layer can expand after the trench formation and help transfer the strain into Ge. With the trench depth increasing to 1090 nm (type 4), the responsivities further increase to 0.91 A/W and 0.92 A/W at 1600 nm and 1625 nm, respectively. From simulation results, the transferred strain will saturate with the trench depth of ∼2620 nm. However, it should be noted that the larger depth will bring higher process challenges from the perspective of process integration. Type 5 PD shows the high responsivities of beyond 1 A/W at 1625 nm. The strain energy increases with increasing thickness of the SiN stressor, so a higher strain can be transferred into Ge with 250 nm thick SiN. The thickness of the SiN stressor should be carefully evaluated, because thick SiN may lead to high wafer warpage and film cracking [32,33].
Fig. 6. (a) Response spectra of the fabricated Ge PDs at different light wavelengths (λ) ranging from 1510 to 1630 nm. The length of the device is 9.7 μm. (b) Response spectra of the type 5 PDs with various lengths.
Figure 6 (b) shows the responsivities of type 5 PDs with various lengths. For the control PDs, even for the 39.7 μm long device, the responsivity at 1625 nm is still very low. In contrast, the type 5 PDs show high responsivity of 0.83 A/W at 1625 nm with the length of 4.7 μm. When the length is increased to 14.7 μm, high responsivities of 1.19 A/W and 1.11 A/W are obtained at 1550 nm and 1625 nm, respectively. At the wavelength range of 1510 nm to1630 nm, the type 5 PDs show an almost flat responsivity with respect to λ rather than a roll-off of the responsivity. This indicates that the strain of SiN stressor can be effectively transferred from the SiN stressor to Ge waveguide, and the absorption edge can be extended beyond L band.
Low capacitance is one of the main advantages of the LPIN Ge PDs. It is possible to realize the LPIN Ge PDs with high responsivity and high speed at the same time. The optoelectronic frequency responses of the Ge PDs are measured by an Agilent light-wave component analyzer (LCA) N4373D with a 50 Ω-terminated GS RF probe. Figure 7 (a) and (b) show the normalized frequency response of the type 5 PDs at 1550 nm with the length of 4.7 μm and 9.7 μm, respectively. For the Ge PDs with the length of 4.7 μm, 3 dB bandwidth f3dB is 15 GHz and 52.5 GHz at a Vbias of -2 V and -6 V, respectively (Fig. 7 (a)). In general, f3dB of a PIN PD is mainly limited by two factors, carrier transit time and resistance-capacitance (RC) delay. It can be found that increasing the reverse bias from -2 V to -6 V leads to an obvious f3dB increasing. Therefore, at low bias, f3dB of the PDs should be limited by the carrier transit time of the photocarriers generated in the top part of the Ge. At bias -6 V voltage, f3dB of the PDs becomes saturated. Then f3dB should be limited by the RC delay, suffering from high contact resistance confirmed by the cross Kelvin resistor test. For the PD with the length of 9.7 μm, high responsivities of over 1 A/W can be obtained in the whole C + L band. However, f3dB of the device decreases to 36 GHz at a Vbias of -6 V. Longer devices have a higher capacitance, leading to a lower saturation 3 dB bandwidth. After optimizing the design and the metal-semiconductor contact process, high responsivity of over 1 A/W and high speed of over 50 GHz can be expected in the same device by increasing the length of the device to 9.7 μm or even longer. Table 2 presents the benchmarks of performance of the C + L band Ge (Sn) photodiodes as compared to other reported values. GeSn photodetectors were reported to extend the detection range to L band (Ref. 34–37). However, the compatibility of fabrication process and the high dark current limit their practical application. Ref. 26 reported the vertical illuminated MSM Ge PD with responsivity of 1 A/W at 1625 nm. However, the device cannot be integrated with other Si photonics devices, and dark current is high. Ref. 38 realized a PD with high responsivity of 1.3 A/W at 1611 nm with the help of the Franz-Keldysh (F-K) effect. However, a high reverse bias voltage of -8.8 V is needed, so that the absorptivity of Ge at long wavelengths can be enhanced by high electric fields. 1.1 A/W and 35 GHz were reported in process design kit (PDK) of AIM Photonics, but only 1600 nm wavelength can be covered. In this work, the PIC-integrable waveguide Ge PDs only need 4.7 μm can reach the responsivity of 0.83 A/W at 1625 nm. 80 nA of dark current and beyond 50 GHz 3 dB bandwidth are also reported at the same time, which can be further improved by optimizing the fabrication process.
Table 2. Summary of the reported L Band PDs
Fig. 7. Measured (dots) and curve-fitted (solid curves) frequency response of the type 5 PDs at 1550 nm with the lengths of (a) 4.7 μm (b) 9.7 μm.
4. Conclusion
The PIC-integrable LPIN Ge waveguide PDs are demonstrated for the C + L band light detection. The SiN stressor is deposited on top surface of the Ge to introduce the tensile strain in the Ge layer, extending the absorption range to the whole C + L band. The dependence of strain transfer from SiN layer to Ge waveguide on device structure is investigated. It is found that the strain transfer significantly depends on the depth of the trenches formed around the device, that the strain becomes larger when the depth is increased. The spectral response of the device covers wavelengths ranging from 1510 nm to 1630 nm. High responsivities of 1.19 A/W and 1.11 A/W can be obtained at 1550 nm and 1625 nm, respectively. The device exhibits a f3dB of 52.5 GHz at 1550 nm, which can be further improved by optimizing the design and process.
Funding
National Key Research and Development Program of China (2019YFB2203502); Strategic Pioneer Research Projects of Defense Science and Technology (XDB43020500); National Natural Science Foundation of China (61935003); National Outstanding Youth Foundation of China (61904185); Shanghai Sailing Program (20YF1456900, 21YF1446300).
Disclosures
The authors declare no conflicts of interest
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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