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Abstract
Novel back-illuminated modified uni-traveling-carrier photodiodes (MUTC-PDs) with wide bandwidth and high saturation power are demonstrated. The effect of cliff layer doping on the electric field distribution is investigated to achieve fast carrier transport. MUTC-PDs with miniaturized device diameter and low contact resistance are fabricated to improve the RC-limited bandwidth. Meanwhile, inductive peaking is implemented to further extend the bandwidth. PDs with 3-µm and 3.6-µm-diameter exhibit a ultrawide bandwidth of 230 GHz and 200 GHz, together with −4.94 dBm and −2.14 dBm saturation power at 220 GHz and 200 GHz, respectively.
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1. Introduction
The fifth-generation (5 G) mobile systems have been deployed and gradually commercialized around the world to meet the needs for intelligent, high-speed and low-latency communications [1,2]. However, the carrier frequency of 5 G technology is mainly concentrated below 6 GHz [3–5]. Bandwidth famine is anticipated in face of high-definition video wireless transmission, virtual and augmented reality (VAR), holographic remote imaging and other applications [6]. In a radio-over-fiber (RoF) communication system, low-loss optical fibers are employed in place of metal cables for long-distance millimeter-wave (MMW) signal transmission [7–9]. Photodiodes (PDs) serve as a key component to convert the modulated optical signal into MMW electrical signal. The bandwidth of the PD directly determines the maximum carrier frequency, while its saturation characteristics affects the MMW carrier power and the transmission distance of the system. Therefore, it is extremely important to improve the bandwidth and saturation characteristics of the PDs.
Uni-traveling carrier photodiodes (UTC-PDs) are particularly attractive for the generation of high frequency and high power MMW signals. For instance, the evanescent-coupled UTC-PDs reported in [10] demonstrate a 3-dB bandwidth of 170 GHz, and an output power about −9 dBm at 200 GHz. Flip-chip bonded 4-µm-diameter modified UTC-PDs (MUTC-PDs) exhibit a 3-dB bandwidth up to 120 GHz as well as an unsaturated RF output power of −8.5 dBm at 160 GHz [11]. By optimizing the load resistance and Au/Sn bumps, near ballistic UTC-PD (NBUTC-PD) of 4.5-µm-diameter achieves 6.12 dBm output power at 170 GHz [12]. In our previous work, 4.5-µm-diameter MUTC-PDs with a 3-dB bandwidth of 156 GHz and a saturation RF power of 0.53 dBm at 170 GHz are demonstrated [13].
In this work, MUTC-PDs with both wide bandwidth and high output power are designed and fabricated. Introducing gradient doped absorption region and additional cliff layer are common ways to optimize the electric field distribution. However, the doping concentration of the cliff layer remains unstudied, which can be used to fine-tune the electric field within the device. By adjusting the cliff layer doping level, the electron transport time can be reduced, leading to improved transit-time limited bandwidth. The increased electron velocity also helps alleviate the space-charge screening, thus beneficial for high saturation performance. PDs with reduced active area exhibit reduced junction capacitance, but suffer from increased ohmic contact resistance and fabrication difficulty. In this work, electron beam lithography (EBL) is employed for the fabrication of 3-µm and 3.6-µm-diameter PDs to ensure low resistivity and improved fabrication consistency. Inductive peaking is implemented by coplanar waveguides (CPWs) with specially designed high-impedance transmission line structure to further improve the bandwidth. Both fabricated PDs exhibit a 3-dB bandwidth over 200 GHz under 2 V reverse bias, together with a responsivity of 0.07 A/W. The 3.6-µm-diameter PD exhibits a 3-dB bandwidth of 200 GHz, as well as high saturation power of −2.14 dBm at 200 GHz, whereas a ultrawide bandwidth of 230 GHz and −4.94 dBm maximum output power at 220 GHz are recorded for the 3 µm-diameter device.
2. Device structure and design
2.1 Epitaxy structure design
The epitaxy layer structure of the proposed MUTC-PD is shown in Fig. 1, together with the band diagram under a reverse bias. A 20-nm thick cliff layer is inserted at absorber/collector interface, whose doping level can be adjusted to tune the internal electric field to ensure an overshoot velocity for electrons [14]. The gradient-doped p-InGaAs absorption region is 80 nm thick. The gradient doping concentration is optimized to create a quasi-electric field to facilitate electron transport [15]. A relatively thick lightly n-doped (1 × 1016 cm−3) InP drift layer of 200 nm is adopted to reduce the junction capacitance.
To investigate its influence on the transit time performance, the doping level of the cliff layer is varied from 1 × 1017 cm−3 to 4 × 1017 cm−3. The simulated electric field in the absorber and the depletion layer is plotted in Fig. 2(a). When the n-doping of the cliff layer is about 3 × 1017 cm−3, the electric field in the InP-depletion region is 20-40 kV/cm, corresponding to the range for electron velocity overshoot [14], and the calculated average drift velocity of electrons is about 3.5 × 107 cm/s, as shown in Fig. 2(b). The extracted transit time of electrons and the corresponding transit-time-limited bandwidth under different doping concentrations are shown in Fig. 2(c). It is evident that a minimum transit time around 0.75 ps can be secured with a cliff layer doping level of 3 × 1017 cm−3.
Fig. 2. (a) Electric field and (b) electron velocity within the depletion region. The layers are indicated by the same colors as in Fig. 1. (c) Transit time and the corresponding transit-time-limited bandwidth for different cliff layer doping levels.
The doping concentration of the cliff layer also significantly affects the saturation performance of the device. Figures 3(a) and 3(b) show the simulated electric field distribution in a 3-µm-diameter MUTC-PDs with n-doped cliff layer of 1 × 1017 cm-3 and 3 × 1017 cm−3, respectively. As plotted in Fig. 3(a), for the PD with a cliff layer doping level of 1 × 1017 cm−3, as in our previous work [13], the electric field in the depleted absorber collapses at a photocurrent of 4.3 mA, as indicated by the blue shade. Increasing the cliff layer doping level to 3 × 1017 cm−3, the electric field in the depleted absorption region is significantly enhanced, which accelerate the electron transport and alleviate the space-charge screening effect. The saturation position is shifted to the absorber/collector interface and screening only occurs at a photocurrent of 5.4 mA, as shown in the red region in Fig. 3(b). Figure 3(c) shows the simulated 1-dB compression point of 5.6 mA at 2 V reverse bias, corresponding to an output RF power of −2.5 dBm.
Fig. 3. (a) Electric field distributions in the depletion region of a 3-µm-diameter PD with different cliff layer doping concentrations: (a) 1 × 1017 cm−3 and (b) 3 × 1017 cm−3. (c) Simulated RF power versus photocurrent.
2.2 Extending the RC-limited bandwidth
2.2.1 Series resistance and parasitic capacitance reduction
It is straightforward to extend the RC-limited bandwidth by scaling down the device area so as to reduce the junction capacitance. However, to achieve a substantial bandwidth improvement, it is crucial to maintain a low series resistance while shrinking the device area. As shown in Fig. 4(a), a 3-dB bandwidth over 200 GHz is expected for the 3-µm-diameter device if the series resistance can be kept to less than 18 Ω.
Fig. 4. Variation of the 3-µm-diameter PD bandwidth with (a) the series resistance and (b) the parasitic capacitance.
The parasitic capacitance also has a significant effect on the device bandwidth [16]. As illustrated in Fig. 5(a), PDs are designed with triple mesa structure for structural stability after wet-etching, as reported in our previous work [13]. The parasitic capacitance mainly consists of two parts. Cpn is the capacitance between the PN electrodes, which can be effectively reduced by increasing the distance between the electrodes. On the other hand, Csn is the capacitance between the signal line of the CPW and the underlying n-type ohmic contact layer, which is mainly determined by the dielectric constant and thickness of the passivation layer. To improve heat dissipation of the PD, SiNx and SiO2 are used instead of benzocyclobutene (BCB) as the passivation material [17,18]. As shown in Figs. 5(b) and 5(c), a parasitic capacitance less than 4 fF can be secured by using SiO2 as the passivation layer, together with n-mesa radius Rn = 4 µm and passivation layer thickness d ≥ 400 nm. According to Fig. 4(b), the 3-µm-diameter PD is expected to exhibit a 3-dB bandwidth exceeding 230 GHz with such a small parasitic capacitance.
Fig. 5. (a) The schematic view for parasitic capacitance simulation. (b) The parasitic capacitance varies with different (b) n-mesa radius and (c) passivation layer thickness.
2.2.2 CPW with inductive peaking
The equivalent circuit model [13] shown in Fig. 6(a) is employed to estimate the frequency response of the MUTC-PD, in which the CPW electrodes is modeled by an LC circuit. The equivalent inductance Lc can be adjusted to compensate the capacitance of the PD, thus extending the device bandwidth. For a 3-µm-diameter PD, the maximum 3-dB bandwidth is obtained with Lc around 40 pH, as shown in Fig. 6(b).
Fig. 6. (a) Equivalent circuit model for frequency response simulation. (b) Frequency responses of 3-µm-diameter PD with different CPW inductances.
Inductive peaking can be implemented with a high-impedance CPW section [19]. The inductance and the impedance of the CPW transmission line can be tuned by adjusting the gap G. As shown in Fig. 7(a), an equivalent inductance of 40 pH is secured by choosing G = 60 µm, and the corresponding impedance is 114 Ω. As shown in Fig. 7(b), the 3-dB bandwidth of the PD reaches 238 GHz when the length of the high-impedance CPW section is L = 20 µm, which is about 86 GHz improvement over a PD with a standard 50 Ω impedance CPW.
Fig. 7. (a) Characteristics of the high impedance line with different gaps. (b) Variation of the PD frequency response with the high impedance line length.
3. Device fabrication and characterization
3.1 Fabrication of the p-mesa and p-electrode
Backside-illuminated PDs are fabricated following a fabrication procedure similar to that described in [13]. The triple-mesa is formed by a combination of inductive coupled plasma (ICP) dry-etching and HCl wet-etching process. As the p-electrode diameters for the 3-µm and 3.6-µm-diameter PDs are only 2.4 µm and 3 µm, respectively, electron beam lithography (EBL) is employed in place of contact photolithography to define the p-electrode and the p-mesa to ensure accurate and consistent dimensions. The schematic process flow is illustrated in Fig. 8. Firstly, 800-nm-thick PMMA and MMA are used to define the p-electrode (step I) by EBL. With the same dose of electron beam exposure, the MMA bottom resist is slightly overexposed, resulting in a double layer photoresist with undercutting after development (step III). Such a double-layer resist facilitates the formation of Ti/Pt/Au p-electrode by magnetron sputtering and lift off (steps IV & V). The p-mesa is then defined by an EBL overlay with HSQ resist. The HSQ pattern is used as the mask for subsequent dry etching of the p-mesa.
3.2 Low contact resistance and parasitic capacitance fabrication
To reduce the ohmic contact resistance, we investigate the contact characteristics of Ti/Pt/Au electrode on p + -InGaAs. It is found that increasing the Pt interlayer thickness from 20 nm to 90 nm helps reduce the contact resistance. Figure 9 reveals the influence of annealing temperature on the specific contact resistivity measured with the circular transmission line model (CTLM) [20]. Low contact resistivity is achieved by annealing the Ti(20 nm)/Pt(60 nm)/Au(200 nm) electrode at 350°C for 1 min. The contact resistance for a 3-µm-diameter PD is expected to be around 9-11 Ω.
In order to reduce the parasitic capacitance of the PDs, the diameter of the n-mesa is only 2 µm larger than that of the 2-mesa. Moreover, a 600-nm-thick SiO2 are deposited on the n-mesa for passivation, as shown in Fig. 10.
3.3 Device performance
Figure 11(a) shows the microscopic photo and the scanning electron microscope (SEM) image of the fabricated back-illuminated MUTC-PD. The measured dark currents of PDs with different diameters are plotted in Fig. 11(b). At 2 V reverse bias, the dark currents for the 3.6- and 3-µm-diameter PDs are 10 and 11.5 nA, respectively. The inset shows the typical I-V curves under forward bias.
Fig. 11. (a) Optical microscope picture and SEM image of the fabricated 3-µm-diameter device and (b) I-V curves of the 3.6- and 3-µm-diameter PDs.
The frequency responses and output RF power are measured with a two-laser heterodyne system. Three different MMW power sensor heads are employed to cover the frequency range from dc to 50 GHz, V-band (50-75 GHz) and W-band (75-110 GHz), respectively, and the RF output power is measured with a Keysight E4419B power meter. The insertion losses of a bias tee, a 2.4 mm coaxial cable and probes are carefully calibrated out. For frequency greater than 110 GHz, a VDI-Erickson power meter (PM5B) is used to record the output power. The maximum measurement bandwidth is up to 325 GHz, which is limited by the WR-3.4 waveguide probe. Similarly, the loss of the GGB probes, WR10-WR8, WR10-WR5.1 and WR10-WR3.4 waveguide transitions are carefully de-embedded.
The frequency response tested under a fixed reverse bias of 2 V are plotted in Fig. 12. The 3.6-µm-diameter PD demonstrates a 3-dB bandwidth of 200 GHz at 4 mA photocurrent, and the 3-µm-diameter PD exhibits an ultrafast performance with a 3-dB bandwidth of 230 GHz at 3 mA. The saturation characteristics at 2 V reverse bias are plotted in Fig. 13. The photocurrent at 1-dB compression point of reaches 8.4 and 5.85 mA for the 3.6- and 3-µm-diameter PDs, corresponding to an output power of −2.14 dBm at 200 GHz and −4.94 dBm at 220 GHz, respectively. These results are in good agreement with our simulations.
Fig. 12. Frequency response of (a) 3.6-µm-diameter and (b) 3-µm-diameter PDs under 2 V reverse bias.
Fig. 13. Output RF power versus output photocurrent of (a) the 3.6-µm-diameter PD at 200 GHz and (b) the 3-µm-diameter PD at 220 GHz under 2 V bias voltage.
4. Discussion
In order to further confirm the validity of RC delay time optimization and the epitaxy structure design, S22 parameters of both PDs are also measured with a vector network analyzer up to 170 GHz. The same circuit model shown in Fig. 6(a) is used to fit the experimental results and Table 1 summarizes the extracted values of circuit elements for the 3-µm-diameter PD. Figure 14(a) shows the fitted and measured S22 parameters of the 3-µm-diameter PD at −2 V. The ohmic contact resistance and the parasitic capacitance are extracted to be R2 = 9.5 Ω and Cp of 3.4 fF, respectively, in good agreement with our design. By fitting the measured frequency response as shown in Fig. 14(b), the extracted capacitance Ct and series resistance Rt are 8.3 fF and 31.6 Ω, respectively. The carrier transit time-limited bandwidth of 607 GHz is obtained accordingly, which is consistent with the simulation results shown in Fig. 2(c).
Fig. 14. (a) Smith chart: The measured and simulated S22 parameters, and (b) S21 parameters for PD with diameter of 3 µm under 2 V reverse bias.
Table 1. Key parameters in circuit model
5. Conclusion
In this paper, we have demonstrated high performance back-illuminated MUTC photodiodes with wide bandwidth and high saturation power. The doping level of the cliff layer is carefully investigated for enhanced electron transport and improved RF output power. The EBL based fabrication procedure are optimized to enhance the RC-limited bandwidth. Meanwhile, CPW with high impedance line is designed to further extend the bandwidth. The fabricated 3- and 3.6-µm-diameter PDs exhibit 3-dB bandwidth of 230 GHz and 200 GHz and deliver saturated RF power of −4.94 dBm and −2.14 dBm, respectively.
Acknowledgments
This work was supported in part by National Key R&D Program of China (2018YFB2201701); National Natural Science Foundation of China (62235005, 62127814, 62225405, 61975093, 61927811, 61991443, and 61974080); and Collaborative Innovation Centre of Solid-State Lighting and Energy-Saving Electronics.
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.
References
1. M. Series, “IMT Vision–Framework and overall objectives of the future development of IMT for 2020 and beyond,” Recommendation ITU, 2083-0 (2015).
2. M. Sung, J. Kim, E. S. Kim, S. H. Cho, Y. J. Won, B. C. Lim, and J. H. Lee, “RoF-based radio access network for 5 G mobile communication systems in 28 GHz millimeter-wave,” J. Lightwave Technol. 38(2), 409–420 (2019). [CrossRef]
3. Y. Wang, J. Li, L. Huang, Y. Jing, A. Georgakopoulos, and P. Demestichas, “5 G mobile: Spectrum broadening to higher-frequency bands to support high data rates,” IEEE Veh. Technol. Mag. 9(3), 39–46 (2014). [CrossRef]
4. N. Al-Falahy and O. Y. Alani, “Technologies for 5 G networks: Challenges and opportunities,” IT Prof. 19(1), 12–20 (2017). [CrossRef]
5. S. Z. Chen and S. L. Kang, “A tutorial on 5 G and the progress in China,” Frontiers Inf. Technol. Electronic Eng. 19(3), 309–321 (2018). [CrossRef]
6. I. F. Akyildiz, A. Kak, and S. Nie, “6 G and beyond: The future of wireless communications systems,” IEEE access 8, 133995 (2020). [CrossRef]
7. J. Capmany and D. Novak, “Microwave photonics combines two worlds,” Nat. Photonics 1(6), 319–330 (2007). [CrossRef]
8. J. Yao, “Microwave photonics,” J. Lightwave Technol. 27(3), 314–335 (2009). [CrossRef]
9. D. Wake, A. Nkansah, and N. J. Gomes, “Radio over fiber link design for next generation wireless systems,” J. Lightwave Technol. 28(16), 2456–2464 (2010). [CrossRef]
10. E. Rouvalis, M. Chtioui, C. C. Renaud, and A. J. Seeds, “170 GHz uni-traveling carrier photodiodes for InP-based photonic integrated circuits,” Opt. Express 20(18), 20090–20095 (2012). [CrossRef]
11. J. S. Morgan, K. Sun, Q. Li, S. Estrella, and A. Beling, “High-power flip-chip bonded modified uni-traveling carrier photodiodes with− 2.6 dBm RF output power at 160 GHz,” 2018 IEEE Photonics Conference, 1–2 (2018).
12. J. W. Shi, M. F. Kuo, and J. E. Bowers, “Design and analysis of ultra-high-speed near-ballistic uni-traveling-carrier photodiodes under a 50 Ω load for high-power performance,” IEEE Photonics Technol. Lett. 24(7), 533–535 (2011). [CrossRef]
13. E. Chao, B. Xiong, C. Sun, and Y. Luo, “D-Band MUTC photodiodes with flat frequency response,” IEEE J. Sel. Top. Quantum Electron. 28(2), 1–8 (2021). [CrossRef]
14. T. J. Maloney and J. Frey, “Transient and steady-state electron transport properties of GaAs and InP,” J. Appl. Phys. 48(2), 781–787 (1997). [CrossRef]
15. H. Ito, S. Kodama, T. Nagatsuma, and T. Ishibashi, “High-speed and high-output InP-InGaAs unitraveling-carrier photodiodes,” IEEE J. Sel. Top. Quantum Electron. 10(4), 709–727 (2004). [CrossRef]
16. X. Sun, S. Ye, J. Seddon, C. C. Renaud, L. Hou, and J. H. Marsh, “Modeling and Measurement of a HSQ Passivated UTC-PD with a 68.9 GHz Bandwidth,” In 2021 IEEE Photonics Conference, 1–2 (2021).
17. W. Cai, A. L. Moore, Y. Zhu, X. Li, S. Chen, L. Shi, and R. S. Ruoff, “Thermal transport in suspended and supported monolayer graphene grown by chemical vapor deposition,” Nano Lett. 10(5), 1645–1651 (2010). [CrossRef]
18. W. Luo, J. Luo, Y. Shuai, K. Zhang, T. Wang, C. Wu, and W. Zhang, “Infrared detector based on crystal ion sliced LiNbO3 single-crystal film with BCB bonding and thermal insulating layer,” Microelectron. Eng. 213, 1–5 (2019). [CrossRef]
19. Q. Li, K. Li, A. Beling, and J. C. Campbell, “High-power flip-chip bonded photodiode with 110 GHz bandwidth,” J. Lightwave Technol. 34(9), 2139–2144 (2016). [CrossRef]
20. G. S. Marlow and M. B. Das, “The effects of contact size and non-zero metal resistance on the determination of specific contact resistance,” Solid-State Electron. 25(2), 91–94 (1982). [CrossRef]
