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Abstract
We experimentally demonstrate a high-speed lateral PIN junction configuration germanium photodetector (Ge-PD) with 4-directional light input. The typical internal responsivity is about 1.23 A/W at 1550 nm with 98% quantum efficiency and dark current 4 nA at 1V reverse-bias voltage. The equivalent circuit model and theoretical 3-dB opto-electrical (OE) bandwidth of Ge-PD are extracted and calculated, respectively. Compared to the conventional lateral PIN Ge-PD with 1-directional light input, our proposed device features uniform optical field distribution in the absorption region, which will be benefit to realize high-power and high-speed operation. In particular, in the condition of 0.8 mA photocurrent, the measured 3-dB OE bandwidth is about 17 GHz at bias voltage of -8 V which is well matched to the theoretical estimated bandwidth. With additional digital pre-compensations provided by the Keysight arbitrary waveform generator (AWG), the root raised cosine (RRC) filter and roll-off factor of 0.65 are employed at transmitter (TX) side without utilizing any offline digital signal processing (DSP) at receiver (RX) side. The 50 Gbit/s, 60 Gbit/s, 70 Gbit/s, and 80 Gbit/s non-return-to-zero (NRZ), and 60 Gbit/s, 70 Gbit/s, 80 Gbit/s, and 90 Gbit/s four-level pulse amplitude modulation (PAM-4) clear opening of eye diagrams are realized. In order to verify the high-power handling performance in high-speed data transmission, we also investigate the 20 Gbit/s NRZ eye diagram variations with the increasing of photocurrent.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Silicon photonics is the technology to realize large-scale electronics-photonics integration on a chip by leveraging the fabrication process of the complementary-metal-oxide semiconductor (CMOS) foundries, thereby enabling low cost, high energy efficiency, large volume manufacturing [1–5]. The active participation of well-established CMOS foundries, such as GlobalFoundries, IME, AIM Photonics, IMEC, CompoundTek, ST Microelectronics, and CEA-Leti, make the silicon photonics technology rapidly evolve from a ‘scientific hot topic’ to an industrially viable platform, largely driven by exponential growth of data communication in long-haul telecommunication, data centers, and high-performance computers [6–8].
An indispensable building block of data communication is photodetector (PD) which requires high sensitivity, large bandwidth, and low dark current [9–11]. However, the intrinsic properties of silicon (Si), an indirect band-gap semiconductor which is transparent at near-infrared (near-IR) wavelengths, make it challenging to realize photodetection [10]. As an alternative, the germanium (Ge) which has strong linear absorption up to 1.55 µm and can be extended up to 2 µm by utilizing tensile-strained Ge-on-Si bandgap shrinkage has appeared as a prime choice for photodetection [10,12–14]. Up to now, Germanium-on-silicon (Ge-on-Si) waveguide p-i-n (PIN) PDs have been widely studied as its fascinating performances and integrating with silicon photonic circuits [15]. In general, Ge-on-Si detectors consist of vertical and lateral PIN junction configuration [10,16–22]. Various types of waveguide-integrated Ge vertical PIN configuration have been extensively investigated and experimentally demonstrated [14,23–33]. However, the lateral PIN junction configuration, where both heavy doping and metal via-contacts are implemented directly on the Ge absorption layer, suffers from detrimental metal absorption losses. Although, the systematical optimization of lateral Ge-PD with one input silicon taper waveguide is proposed, the reported typical responsivity is 0.9 A/W at 1550 nm with 3-dB bandwidth 14 GHz [16]. All of the optical power is transferred into one side of Ge region at once, most of the absorption occurs in the first few micrometers of the detector. Therefore, the optical field distribution in the Ge region will be non-uniform, which will be harmful to carrier transport, especially for high input optical power. The high-power photodiodes have also been comprehensively studied for mm-wave applications [34–40]. However, to the best of the authors knowledge, the lateral Ge-PD with 4-directional light input has not been proposed and demonstrated.
In this paper, we design and fabricate the lateral Ge-PD with 4-directional light input based on a commercial standard silicon-on-insulator (SOI) waveguide platform [41]. The 4-directional light input lateral Ge-PD may be favorable for the more uniform absorption of optical power in the Ge region. This will be significantly reducing the effects of carrier screening [42–45]. To comprehensively characterizing the proposed 4-directional light input lateral Ge-PD, firstly, the static current-voltage (I-V) and photocurrent as a function of input optical power measurements are implemented. Secondly, the small-signal (S21, S11) radio-frequency (RF) measurements and equivalent circuit model are executed. Then, the large-signal measurements, including non-return-to-zero (NRZ) on-off-keying (OOK) and four-level pulse amplitude modulation (PAM-4) eye diagrams are realized. Finally, to verify the high-power handle performance in high-speed data transmission, the 20 Gbit/s NRZ eye diagrams with different photocurrent under 8 V reverse-bias voltage is also measured.
2. Design principle and fabrication of device
Figure 1(a) shows the full schematics structure of the designed device including three 1×2 multi-mode interferometer (MMI) splitters, grating coupler, 4-directional input Ge-PD. Figure 1(b) depicts 3-dimensional (3-D) schematic of the proposed lateral Ge-PD with 4-directional light input silicon taper waveguides. Figure 1(c) and (d) show the simulated optical field distribution in the lateral Ge-PD with 4-directional light input. The uniform optical field distribution in the whole intrinsic region will allow the proposed Ge-PD to handle more optical powers before saturation. Light is firstly coupled from transverse electric (TE) mode grating coupler, then splitting by 1×2 MMI. The two-routing silicon waveguides are further split into four-routing waveguides by another two 1×2 MMIs. Finally, the four-routing waveguides are linearly tapered to Ge absorption region and carefully designed to having equal length. As shown in Fig. 1(b), the thickness and width of the Ge layer are 0.5 µm and 9 µm, the designed intrinsic Ge (i-Ge) region is 2 µm. The Ge-PD is fabricated on a commercial standard silicon-on-insulator (SOI) waveguide platform, with 220-nm-thick Si waveguides and 8-µm-thick buried oxide (BOX) layers. A 500 nm height Ge film is epitaxially grown on the silicon layer with top 100 nm Ge layer implanted by P+ and N+ for ohmic contact. The two positive electrodes and two negative electrodes are put in diagonal location of Ge region. These two pairs of positive and negative electrodes provide strong electrostatic field in the Ge absorption region, ensuring the efficient and fast collection of photo-generated carriers.
Fig. 1. (a) The overall schematic of proposed device. (b) The 3-D schematic of the lateral Ge-PD with 4-directional light input. The Si taper is 0.22 µm thick, with a width varying from 0.45 µm to 2 µm over a length of 45 µm. The thickness and width of the Ge layer is 0.5 µm×9 µm, the intrinsic Ge (i-Ge) region is 2 µm. (c) and (d) Simulated optical field distribution in the lateral Ge-PD with 4-directional light input.
For regular lateral Ge PIN photodetector, the light is input by a taper silicon waveguide from one-directional, causing a large electron-hole pair density at the Ge-Si waveguide interface. The high density of photo-generated electron-hole pair creates a large gradient of charge, which will induce a strong electric field opposing the applied bias voltage. This phenomenon is called carrier screening [42], which degrades the 3-dB opto-electrical (OE) bandwidth at high input optical powers as the photo-carriers cannot be efficiently swept out of the detector. By using the 4-directional light input silicon waveguides, the carrier screening problem may be relieved. The 4-directional light input Ge-PD exploits the Ge 4-sides to receive optical power, benefiting to uniform photo-carriers distribution. Especially, it is very helpful to improve the responsivity and bandwidth with high input optical power.
3. Experimental setups and test results
3.1 Optical micrograph and setups
Figure 2(a) shows optical micrograph of the fabricated lateral Ge-PD with 4-directional light input. The input light was coupled from a single-mode fiber through a grating coupler. The black arrows from 4-directional indicate light input silicon taper waveguides. Schematic of the experimental setup for the measurement of eye diagrams is illustrated in Fig. 2(b). The 3-dB OE bandwidth of lateral Ge-PD is calculated from S-parameter (S21) measured by a 67 GHz Lightwave Component Analyzer (LCA, Keysight N4373D). The AWG (Keysight M9502A) generates pseudo random binary sequence (PRBS) of order 15, then amplified by a driver (SHF S807). The continuous wave (CW) generated by commercial Keysight (8164B) laser with 13 dBm optical power at 1550 nm is injected through a polarization controller (PC1) with 0.6 dB insertion loss. The 6-dB bandwidth 40 GHz lithium niobite (LiNbO3) Mach Zehnder modulator (iXblue MX-LN-40) with insertion loss of 8 dB at 1550 nm is driven by the amplified radio-frequency (RF) data. The modulated optical carrier is amplified and injected into a variable optical attenuator (VOA) with an insertion loss of 1.5 dB. Since the input grating coupler is polarization sensitive, the modulated optical signal is then injected to another PC2, and the designed lateral Ge-PD converts it to photocurrent. The 50 Ω terminated measurement device (Sampling Scope) converts the photocurrent into voltage. The lateral Ge-PD is biased through a 65 GHz bias tee (SHF 49817).
Fig. 2. (a) Micrograph of the fabricated lateral PIN Ge-PD with 4-directional light input. (b) Schematic of the experimental setup for the measurement of eye diagrams. The black and red lines represent the optical and electrical connections, respectively. PD: photodetector, MMI: multi-mode interferometer, AWG: Arbitrary Waveform Generator, EDFA: erbium-doped fiber amplifier, VOA: variable optical attenuator, PC: polarization controller, LN MZM: lithium niobite Mach-Zehnder modulator.
3.2 Static measurements
A typical static current-voltage (I-V) characteristics of proposed lateral Ge PIN photodetector in dark illuminated state is shown in Fig. 3(a). The device exhibits a dark current as low as 4 nA and 7 nA at −1 V and −8 V. To exhibit the optical absorption capability of Ge-PD, the photocurrent was measured as a function of the input power for both four-input and one-input at 1550 nm with −1 V bias voltage, as depicted in the Fig. 3(b). It has to be noted that the one-input based PD is from the same wafer/run with four-input based PD. Here, the loss of grating coupler, silicon waveguide, and 1×2 MMI splitter are about 6 dB/port, 2 dB/cm, 0.3 dB, respectively. In the procedure of estimating the actually power coupled to the PD, the total input optical power subtracts 6 dB grating coupler loss, 0.6 dB MMI splitters loss. And the loss of silicon waveguides is neglected. Then, we can obtain the internal responsivity of Ge-PD. The inset in Fig. 3(b) plots the photocurrent versus the input power with optical power <10 mW. It is obviously that the linearity of the curve starts to deteriorate after the input optical power large than 2 mW for one-input based PD, while four-input based PD can maintain good linearity before the optical power reach 10 mW. In the linear region, the responsivities can be extracted to be 1.23 A/W and 1.15 A/W at 1550 nm for four-input and one-input based PDs, respectively. The responsivity of 1.23 A/W shows an ultra-high quantum efficiency $\eta \textrm{ = }98\textrm{\%}$, defined as follows:$\eta \textrm{ = }(R \times 1.24)/\lambda $, where $R$ and $\lambda $ are the responsivity and operating wavelength. The Fig. 3(b) also shows no obviously saturation effects when the optical power up to an input of 10 mW for four-input. Past 10 mW of input optical power, a slightly saturated absorption effect in the photocurrent was observed, whereas the 4-drectional input Ge-PD continues generating more photocurrent than mode-evolution-based coupler [43]. At 28 mW of input power, the lateral Ge PD with 4-directional light input produces 27 mA of photocurrent, while the mode-evolution-based coupler generates 15.5 mA of photocurrent [43], corresponding to an improvement of 74%. When the input power is 40 mW, the 4-drectional input Ge-PD produces 33 mA of photocurrent and realizes the increasing of the saturation current by more uniformly illuminating the Ge absorption.
Fig. 3. (a) Static current-voltage (I-V) characteristics of 9 µm×9 µm Ge PIN photodetector in dark illuminated state. (b) Measured photocurrent as a function of 1550 nm TE polarized input optical power for both four-input and one-input based PDs with 9 µm×9 µm, at −1V bias voltage.
3.3 Equivalent circuit model
An extracted equivalent circuit model can explain the influence of various parameters on Ge-PD performances [46]. Furthermore, such a model is very useful for optimal designing of high-speed and high-sensitivity simultaneously. Figure 4(a) shows the extracted equivalent circuit model of our proposed Ge-PD with 4-directional light input, where ${C_j}$ is the total junction capacitance and ${C_p}$ is the parasitic capacitance, and ${R_s}$ is the series resistance. In order to extract ${C_j}$,${C_p}$,${R_s}$, the S11 parameters are measured from 100 MHz to 40 GHz using LCA under two bias voltages. Figure 4(b) shows measured and simulated reflection coefficients at two different bias voltages. The fitting result exhibited good agreement with the measured S11 curve. It resulted in a junction capacitance ${C_j}$ of 29.2 fF, series resistance ${R_s}$ of 52 Ω, and parasitic capacitance ${C_p}$ of 10.4 fF at −8 V bias voltage. The load resistance ${R_{load}}$ is 50 Ω.
Fig. 4. (a) The equivalent circuit model of Ge-PD with 4-directional light input. (b) Measured and simulated reflection coefficients for Ge-PD with 4-directional light input from 100 MHZ to 40 GHz at −1 and −8 V.
3.4 Small-signal measurements
To understand and analyze the high frequency response of the fabricated 4-drectional input lateral Ge-PD, we first give the theoretical calculation of the 3-dB OE bandwidth. Then the small-signal measurements are implemented to verify it. The RF response of a Ge-PD is mainly controlled by the carrier transit-time-limited bandwidth (${f_{tr}}$) and resistor-capacitor ($RC$)-limited bandwidth (${f_{RC}}$) in the active PIN regions [47,48]. The ${f_{tr}}$ can be written as [47,48]:
where ${\nu _b}$ is the saturation drift velocity (Ge: ${\nu _b}\textrm{ = }6 \times {10^6}cm/s$), and d is the thickness of the Ge intrinsic layer. For the proposed Ge-PD with 4-directional light input, the designed intrinsic Ge (i-Ge) region is 2 µm. However, considering the diffusion of P+ and N+ implanted ion and fabrication error, the fabricated i-Ge region is about 1.5 µm. Therefore, the theoretical ${f_{tr}}$ is estimated to be 18 GHz.The limit of the $RC$ bandwidth can be described by [47,48]
where ${R_{}}$ is the resistance, which consists of the series resistance ${R_s}$ and the load resistance ${R_{load}}$, and ${C_{}}$ is the capacitance, including the junction capacitance ${C_j}$ and the parasitic capacitance ${C_p}$. The PIN junction capacitance ${C_j}$ can be calculated by ${C_j} = \frac{{\varepsilon {\varepsilon _0}WL}}{d}$, where $\varepsilon$ and ${\varepsilon _0}$ are the relative and vacuum permittivity, W and L are the width and length of the PD, respectively. Based on the calculated $RC$ value above, the ${f_{RC}}$ is about 40 GHz. So, the major limitation factor of 3-dB OE bandwidth is ${f_{tr}}$.The 3-dB OE bandwidth, determined by both carrier transit-time-limited bandwidth ${f_{tr}}$ and $RC$ limited bandwidth ${f_{RC}}$, can be estimated by [48]
Therefore, the theoretical 3-dB OE bandwidth ${f_{3dB}}$ of Ge-PD with 4-drectional input is estimated to be approximately 16.4 GHz.In order to experimental verify the 3-dB OE bandwidth of the lateral Ge-PD with 4-directional input, we implemented small-signal RF measurements. Prior to testing, the calibrations of the high-speed RF path were carried out to consider the contributions from GSG probes and coaxial cables. In the condition of 0.8 mA photocurrent, the 3-dB OE bandwidth test experiments were achieved by collecting the response of the S21 transmission parameter in the LCA tool as a function of frequency under different reverse-bias voltages, as depicted in the Fig. 5. At −1 V bias voltage, the 3-dB OE bandwidth is about 3.5 GHz, which means that the electric field in i-Ge region is not high enough to realize maximum drift velocity. It is obviously that the 3-dB bandwidth increases with reverse-bias voltages before −8 V, which is attributed to the higher electric field benefiting to reduce the carriers transient time, especially for 2 µm i-Ge region. At −8 V bias voltage, the 3-dB OE bandwidth is about 17 GHz, which is well matched to the theoretical estimated bandwidth. However, further increasing the reverse-bias voltage, the 3-dB OE bandwidth remains nearly unchanged. The limit of the bandwidth is mainly attributed to the carrier transit-time ${f_{tr}}$ as analyzed above. The strong bias-dependence of the 3-dB bandwidth is actually related to the i-Ge region of lateral junction. Reducing the i-Ge region should alleviate the bias-dependence of the bandwidth.
Fig. 5. (a) Small-signal S21 transmission parameter as a function of frequency of the fabricated lateral PIN Ge-PD with 4-directional light input under different reverse-bias voltages, in the condition of 0.8 mA photocurrent. (b) 3-dB OE bandwidth as a function of reverse-bias voltage.
Figure 6 shows the normalized RF response of the fabricated four-input and one-input based PDs under different photocurrent levels, the bias voltage is fixed at −8 V. Under a small photocurrent of 0.3 mA, the 3-dB bandwidths are about 16.5 GHz and 16.8 GHz for four-input based PD and one-input based PD, respectively. It means that both types of PD have similar frequency response at a low input optical power. However, with the increasing of photocurrents, the bandwidth of the one-input based PD degrade rapidly. It is because under high input optical power condition, the photo-generated carriers cannot be effectively swept out of the Ge absorption region, which will result in the saturation effect. In 7 mA photocurrent, the 3-dB bandwidths are about 9.3 GHz and 4.9 GHz for four-input and one-input based PDs, respectively. Comparing to the reported Ge-PD assisted by light field manipulation with 3-dB bandwidth of 9 GHz at 7 mA photocurrent [44], our proposed structure possesses 9.3 GHz.
Fig. 6. (a) Normalized RF response of the fabricated lateral PIN Ge-PD with 4-directional light input, the bias voltage is fixed at −8 V. (b) Measured 3-dB OE bandwidths with different photocurrent levels. (c) Normalized RF response of the fabricated lateral PIN Ge-PD with one-input tapered silicon waveguide, the bias voltage is fixed at −8 V. (d) Measured 3-dB OE bandwidths with different photocurrent levels.
3.5 Large-signal measurements
The viability of the performances of this 4-directional light input Ge-PD was checked by measuring the data transmission with OOK and PAM-4 under photocurrent 0.8 mA. A (215−1) long optical NRZ PRBS data pattern at 50 Gbit/s, 60 Gbit/s, 70 Gbit/s, and 80 Gbit/s, generated by a commercial LiNbO3 modulator at 1550 nm was launched into the Ge-PD with 4-directional light input. A −8 V bias voltage was applied to the photodetector using a 50 GHz RF probe (Cascade Infinity Probe GSG-150) connected to a 67 GHz bias-tee. The output electrical data was measured with a Keysight DCA-X series wide-bandwidth sampling oscilloscope (N1000A). The electrical open eye diagrams from 4-directional light input Ge-PD at −8 V are obtained with the use of digital pre-compensations provided by the Keysight AWG, as shown in Fig. 7. The performances of PAM-4 data reception for 4-directional light input Ge-PD are also investigated. As shown in Fig. 8, the clear open electrical eye diagrams of 60 Gbit/s, 70 Gbit/s, 80 Gbit/s, and 90 Gbit/s PAM-4 are again realized. It indicates the high-quality data reception performance of the fabricated Ge-PD for high speed OOK and PAM-4.
Fig. 7. Measured 50 Gbit/s, 60 Gbit/s, 70 Gbit/s, 80 Gbit/s NRZ eye diagrams under 8 V reverse-bias voltage.
Fig. 8. Measured 60 Gbit/s, 70 Gbit/s, 80 Gbit/s, 90 Gbit/s PAM-4 eye diagrams under 8 V reverse-bias voltage.
Finally, to confirm the high-power performances in high-speed data transmission, the 20 Gbit/s NRZ modulated signals at different photocurrent levels are used, as shown in Fig. 9. As shown in Fig. 6(b), the 3-dB bandwidth is about 12.5 GHz under 4 mA photocurrent. For 20 Gbit/s NRZ signals, the eye diagram degrades obviously but clear open when the photocurrent is 4 mA, which indicates the device appears large-signal saturation effects. Our proposed PD with 4-directional light input still can support 20 Gbit/s clear open eye diagram with 7 mA photocurrent. The eye diagram saturation effect is similar to the 10 Gbit/s with 12 mA photocurrent as demonstrated in previous work [44]. These validated results indicate that the proposed Ge-PD with 4-directional light input silicon waveguides also possesses the high-power and high-speed handling capability.
4. Discussions
In this work, the series resistance of Ge-PD with 4-directional light input is 52 Ω, which is relatively high. In the device fabrication process, a 500 nm height Ge film is epitaxially grown on the silicon layer with top 100 nm Ge layer implanted by P+ and N+ for ohmic contact. The doping concentration of Ge P+ and N+ region is estimated to be 2×1018 cm−3 to avoid large optical absorption loss. This will lead to high series resistance. Additionally, the designed i-Ge region is about 2 µm, which may also result to large series resistance. In the future work, after Ge growth, an amorphous-Si layer can be deposited and heavily implanted to form ohmic contact [49,50]. This will simultaneously reduce series resistance and free carrier absorption. Furthermore, decreasing the i-Ge region to 1 µm would also increase the operation speed and reduce the series resistance. Therefore, the relatively high series resistance of this proposed lateral junction PD with 4-directional light input can be further optimized.
As shown in Figs. 1(c) and 1(d) which is the simulated optical field distribution in the lateral Ge-PD with 4-directional light input, there may have a standing wave pattern in the center of Ge region, but its effect on optical signal reception can be nearly ignored. The reason is briefly summarized as follows. As shown in Figs. 6(a)–6(d), the 3-dB bandwidth of lateral Ge-PD with 4-directional light input is superior to lateral Ge-PD with one-input tapered silicon waveguide as the increasing of photocurrent. It implies that the impact of standing wave pattern on the RF response is very small. And the clear open electrical eye diagrams in Figs. 7–9 also exhibit the high performances of our proposed device.
For bandwidth-limited modulators, the combination of the electrical and optical equalization is an efficient way to realize ultra-high speed transmission [51–53]. Based on the silicon photonic MZM with a 3-dB bandwidth of 22.5 GHz, the single lane bit rate of 200 Gb/s (80 Gbaud) PAM-6 is demonstrated by using Nyquist shaped PAM signal [47]. In this work, for the first time, with the use of digital pre-compensations, we demonstrate 80 Gbit/s NRZ signal reception based on the propsoed 3-dB bandwidth of 17 GHz PD. The root raised cosine (RRC) filter and roll-off factor of 0.65 are employed at transmitter (TX) side without utilizing any offline digital signal processing (DSP) at receiver (RX) side. It reveals the powerful of DSP pre-compensation to deal with bandwidth limited PDs.
Finaly, in order to reduce the bias voltage and improve the device frequency response, the intrinsic Ge region should be decreased. In this proposal, for the lateral PIN junction configuration Ge-PD with 4-directional light input, the two positive electrodes and two negative electrodes are put in diagonal location of Ge region. For the future design, the Ge film can be epitaxially grown on the silicon slab waveguide with 4-input silicon waveguides [18]. The four electrodes are put in diagonal location of 150 nm silicon slab with P++ and N++ implanted for ohmic contact. The width of Ge and intrinsic Si may be designed to 600 nm and 200 nm. This way will reduce the total Ge area and yet keep very nice level of quantum efficiency. We are doing the simulations and fabricating the strcuture.
5. Conclusions
A lateral Ge-PD with 4-directional light input is proposed and demonstrated with responsivity about 1.23 A/W at 1550 nm and dark current 4 nA at 1 V reverse-bias voltage. Based on the equivalent circuit model, the theoretical 3-dB OE bandwidth of Ge-PD is estimated to be approximately 16.4 GHz, which is well matched to the experimental demonstrated 17 GHz. The RF response of the fabricated four-input and one-input based Ge-PDs under different photocurrent levels are also investigated, the bias voltage is fixed at −8 V. With additional digital pre-compensations provided by the Keysight AWG, the 50 Gbit/s, 60 Gbit/s, 70 Gbit/s, and 80 Gbit/s NRZ, and 60 Gbit/s, 70 Gbit/s, 80 Gbit/s, and 90 Gbit/s PAM-4 clear open eye diagrams are realized. The measured clear open eye diagrams of 20 Gbit/s NRZ with different photocurrent demonstrate the high-power handling capability of fabricated lateral Ge-PD with 4-directional light input.
Funding
National Key Research and Development Program of China (2019YFB2205201, 2019YFB2205203); Hubei Technological Innovation Project (2019AAA054); Natural Science Foundation of Hubei Province (2019CFB216).
Disclosures
The authors declare no conflicts of interest.
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