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Abstract
Novel evanescently coupled waveguide modified uni-traveling carrier photodiodes (MUTC-PDs) employing a thick multi-layer coupling waveguide are reported. To improve the optical-to-electrical (O/E) conversion efficiency, a thick multi-layer coupling waveguide with a gradually increased refractive index from the bottom layer to the absorption layer is utilized. The refractive index profile facilitates the upward transmission of incident light into the absorption region, thereby enhancing the evanescent coupling efficiency. Meanwhile, the coupling waveguide, with a total thickness of 1.75 µm, expands the mode field diameter, thereby reducing the input coupling loss. Additionally, the top layer of the coupling waveguide also serves as the drift layer. This configuration facilitates efficient light absorption within a short PD length, thus ensuring ultrawide bandwidth and high O/E conversion efficiency simultaneously. Without an additional spot size coupler or anti-reflection coating, the measured responsivity is as high as 0.38 A/W for the PD with an active area of 5 × 6 µm2. Meanwhile, an ultrawide 3-dB bandwidth of 153 GHz has been demonstrated.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The exponential growth in data traffic puts significant pressure on existing Ethernet interfaces to provide sufficient throughput within hyperscale data center networks (DCNs) [1]. The attention is now focused on next-generation Ethernet interfaces, such as 400 Gigabit Ethernet (GbE), 800 GbE, and even 1.6 TbE [2,3]. The bandwidth of photodiodes (PDs) directly limits the maximum data transfer rate. The development of PDs with ultra-wide bandwidth, particularly toward 200 GHz, is crucial to the evolution of next-generation networks and advancements in optical transport networks [4–6].
In addition to ultrawide bandwidth, high responsivity is also a key figure of merit for PDs. PDs with high responsivity not only alleviate the tight fiber alignment tolerances [7], but also enable extended link distances, reduce overall power consumption, and contribute to enhancements in dynamic range and a higher signal-to-noise ratio (SNR) [8]. However, for surface-illuminated PDs, there is a trade-off between bandwidth and responsivity. For instance, a record 3-dB bandwidth of 310 GHz with responsivity less than 0.1 A/W has been reported in Ref. [9]. Bandwidth over 110 GHz has been demonstrated but with low responsivity of 0.15 A/W [10]. To overcome this inherent tradeoff, various waveguide photodiodes (WG-PDs) have been developed, where the light propagation is perpendicular to the carrier transportation [11–13]. Among them, evanescently coupled waveguide photodiodes can achieve a relatively uniform light distribution along the absorber, leading to an improved high-power handling capability. However, for WG-PDs, the mode field diameters (MFDs) of the passive waveguide and the single mode fiber (SMF) are often mismatched. To address this issue, PDs integrated with spot-size converters are usually adopted. Double-stage taper mode size converter has been employed in WG-PDs to achieve high responsivity of 0.6 A/W and bandwidth exceeding 40 GHz [14]. By adopting a monolithically integrated taper, 100 GHz photodiodes with a high responsivity of 0.66 A/W and relaxed alignment tolerances of ±2 µm have been demonstrated [15]. WG-PDs based on uncladded rib waveguides, monolithically integrated with a diluted waveguide and taper, have been presented, which could reduce the coupling loss with flat-end fibers and improve alignment tolerances [16]. Nevertheless, achieving precise control over the spot-size converter geometry involves sub-micron lithography and additional etching steps for long tapered mesa, making the fabrication procedure complicated and the device less compact.
In this paper, we propose a novel evanescently coupled waveguide modified uni-traveling carrier photodiodes (MUTC-PDs). Multiple quaternary InGaAsP layers are employed as the coupling waveguide, in which the refractive index of each layer gradually increases from the bottom layer to the absorption layer. The thickness of the coupling waveguide is enhanced to 1.75 µm. This design expands the MFD in the passive waveguide and facilitates optical coupling from the passive waveguide to the absorber, thus ensuring reduced coupling loss with SMF and high evanescent coupling efficiency at the same time. Meanwhile, the top layer of the coupling waveguide also serves as the electron drift layer. The drift-waveguide structure is proposed by B. Tossoun et al., in 2019 [17]. In the drift-waveguide structure, the passive waveguide is situated just beneath the absorption layer. This configuration can further enhance evanescent coupling efficiency, facilitating light transmission from the passive waveguide to the adjacent absorber over a short distance. Consequently, the PD length can be shortened to achieve the same responsivity. The PDs with shorter lengths can reduce the capacitance, relaxing the resistance-capacitance (RC) limited bandwidth. Furthermore, based on Monte Carlo simulation, the electric field within the PD is optimized to minimize the electron transit time. As a result, ultrawide bandwidth and high responsivity can be secured simultaneously. The fabricated 5 µm × 6 µm PDs exhibit a 3-dB bandwidth of 153 GHz, together with a high responsivity of 0.38 A/W. The 3-dB bandwidth and the responsivity for the 5 µm × 10 µm WG-PDs are 119.3 GHz and 0.50 A/W, respectively.
2. Device structure design
The responsivity of a WG-PD is mainly affected by the fiber-waveguide coupling efficiency and waveguide-absorber evanescent coupling efficiency, whereas the bandwidth of the PD depends on both the carrier transport time and the RC constant. To balance the bandwidth and the responsivity of the PD, the epitaxial and waveguide structures should take both the coupling efficiency and the non-equilibrium electron transport into consideration.
2.1 Fiber coupling and evanescent coupling
Traditional WG-PDs commonly employ a thin waveguide core, e.g., 300 nm, to ensure high evanescent coupling efficiency [18]. However, the mode mismatch between the SMF and the input waveguide can be significant, making it difficult to achieve low fiber coupling loss and high evanescent coupling efficiency simultaneously without additional spot-size converters. In this work, we employ multiple InGaAsP layers as the coupling waveguide, in which the refractive index of each layer gradually increases from the bottom layer to the absorption layer. This multi-layer coupling waveguide can direct the input light upward into the absorption layer. Additionally, according to the coupled mode theory [19], the power coupling coefficients between two closely spaced slab waveguides can be enhanced by decreasing the effective refractive index difference. The effective refractive index of the coupling waveguide is calculated to be 3.41 by the finite difference eigenmode (FDE) method, which is closer to the refractive index of the InGaAs absorber (n = 3.56) compared with the traditional cladding-core-cladding waveguide structure.
To achieve high responsivity, it is straightforward to increase the absorber area. However, this leads to increased junction capacitance, thus limiting the bandwidth of the PD. To achieve ultrawide bandwidth, a small active area is commonly employed. For small-sized PDs, it is a challenge to secure high optical absorption within a relatively short absorption region. In addition to adopting the multi-layer coupling waveguide, decreasing the distance between the coupling waveguide and the absorption layer can significantly decrease the coupling length [20]. In a traditional waveguide MUTC-PD, an InP drift layer (refractive index ∼ 3.167) is positioned between the coupling waveguide and the absorption layer [21]. In Refs. [17,22,23], certain layers within the coupling waveguide are employed simultaneously as the drift layer, thereby eliminating the need for an additional InP drift layer. According to Ref. [19], a weakly guiding condition can be achieved by slightly increasing the cladding refractive index, resulting in a significant rise in the average overlap integral between the coupled waveguides. In our WG-PD, the top layer of the coupling waveguide, which has a high refractive index of 3.53, serves as the electron drift layer, performing both optical and electrical functions simultaneously. As a result, the passive waveguide layer with a high refractive index is situated just beneath the absorption layer, greatly boosting the evanescent coupling efficiency. Meanwhile, the drift layer is processed into a rib waveguide structure to achieve lateral optical confinement. The two n-doped coupling waveguide layers below the drift layers also serve as the n-contact layer. The structure of our proposed WG-PD is shown in Fig. 1.
Optical transmission within PDs with different active areas is depicted in Fig. 2, which is simulated using the finite difference time domain (FDTD) method. The absorption of the InGaAs layer has been taken into account by setting the imaginary part of its refractive index as 0.075, corresponding to an absorption coefficient of 8200 cm−1 [24]. For PDs with an active area of 5 × 6 µm2, the input light is efficiently coupled from the waveguide to the absorption layer within a short length of 6 µm, corresponding to an internal responsivity of 0.58 A/W. For the 10-µm-long PD shown in Fig. 2(b), almost all input power can be coupled into the absorption layer, resulting in a high internal responsivity of 0.87 A/W. Only a marginal increase in the absorbed optical power is expected when the PD length is further extended to 15 µm, as depicted in Fig. 2(c). From simulation results, we can see that the coupling length is reduced to about 8 µm, making it particularly attractive for compact PDs.
Fig. 2. Optical power distribution with optical absorption along the y direction of the PDs, where the active areas are (a) 5 × 6, (b) 5 × 10, and (c) 5 × 15 µm2.
The multi-layer structure raises the overall thickness of the coupling waveguide to 1.75 µm, resulting in a wider MFD and consequently decreasing the fiber coupling loss. A Gaussian beam with a diameter of 2.5 µm is employed as the input power source, which is believed to provide a close representation of the actual scenario. When the incident light diverges spatially, it excites multimode field distribution at the waveguide facet. Considering spatial divergence of the light, multimode propagation, and reflection loss, the external responsivity can be calculated.
The relationship between external responsivity and fiber misalignment is depicted in Fig. 3(a). In the horizontal direction, the 7 µm wide waveguide provides a relaxed alignment tolerance. The misalignment of ±1 µm results in only 0.04 A/W decrease in responsivity. In the vertical direction, our structure exhibits a -1 dB alignment tolerance of approximately ±0.6 µm. Additionally, the coupling efficiency is calculated for various states of polarization of the input beam, considering our proposed thick coupling waveguide with a width of 7 µm. The coupling efficiency is determined by calculating the overlap integral between the multimode field distribution at the input end of the coupling waveguide and the Gaussian beam profile. As shown in Fig. 3(b), the coupling efficiency changes from 79.3% to 72.6% as the polarization angle of the Gaussian beam transitions from 0 degrees to 90 degrees, indicating low polarization dependence loss of 0.38 dB.
Fig. 3. (a) The simulated relationship between the external responsivity and the misalignment. (b) The simulated relationship between the coupling efficiency and the polarization angle.
To provide a quantitative estimation of the O/E conversion efficiency, the simulated responsivity and efficiency are summarized in Table 1. The internal responsivity is calculated using the fundamental mode of the coupling waveguide as the input source. The external responsivity is calculated using the Gaussian beam.
Table 1. Simulated quantum efficiency and responsivity
2.2 Photogenerated carrier transport
For high-speed UTC-PD, the electron velocity overshoot effect is commonly utilized [25]. J.-W. Shi et al. proposed a near-ballistic UTC-PD, taking advantage of the electron overshoot effect to reduce the electron transport time in the drift layer [26]. In our device, the electrical field profile across the entire electron transition region is optimized. To employ the electron overshoot effect in the drift layer, an accurate estimation of the relationship between electron velocity and electric field is crucial. Monte Carlo simulation is employed to model electron transport in the InGaAsP (Q1.48) drift layer. Subsequently, the electric field can be adjusted to satisfy the electron overshoot condition.
In the Monte Carlo simulation, various scattering mechanisms have been examined, including polar optical phonon scattering, elastic acoustic phonon scattering, alloy scattering, and intervalley scattering. The material parameters such as the energy of polar optical phonons and the static and optic dielectric constants are calculated based on the interpolation method [27–29]. The calculated scattering rate as a function of electron energy is shown in Fig. 4.
The non-equilibrium electron transport under different electric fields in InGaAsP (Q1.48) is then simulated, as illustrated in Fig. 5(a). The distance of electron transport in the drift layer is 300 nm. As shown in Fig. 5(b), the electric field range for electron overshoot in InGaAsP (Q1.48) is found to be about 10-30 kV/cm. The maximum electron overshoot velocity can reach 4 × 107 cm/s, while the saturation velocity is about 1 × 107 cm/s.
Fig. 5. Monte Carlo simulation results of electron un-equilibrium transport in InGaAsP (Q1.48). (a) Electron transport velocity under different electric fields. (b) Average electron velocity over 300 nm as a function of electric field.
To minimize power consumption, our PD is designed to operate at a low reverse bias. Figure 6 exhibits the electric field distribution under −1 V bias. A 10 nm n-doped (1 × 1017 cm−3) cliff layer is adopted to adjust the electric field in the InGaAsP drift layer. The resulting electric field ranges from 15 to 41.2 kV/cm, ensuring electron transport at the overshoot velocity. In addition, the electric field in the depleted absorption region is maintained at a relatively high level to prevent electron accumulation. The graded doping in the un-depleted absorption layer creates a quasi-electric field to speed up electron diffusion. According to our simulations, the transit time limited 3-dB bandwidth can reach 356 GHz.
2.3 Epitaxial structure
Based on the above discussion, the detailed epitaxial structure is shown in Table 2. The 200-nm-thick InGaAs absorber consists of four 30 nm graded doped InGaAs layers and one 80 nm depleted absorption layer. The coupling waveguide consists of five InGaAsP quaternary layers, of which the top 300 nm n-InGaAsP (Q1.48) also serves as the drift layer. The 350 nm heavily n + -InGaAsP (Q1.4) and 200 nm n + -InGaAsP (Q1.29) are used as the n-contact layers. Compared with traditional PDs, a reduced doping concentration of 2 × 1018 cm−3 is adopted for the n-contact layers, to ensure acceptable free carrier loss. To maintain a low n-contact resistivity, thick contact layers are employed. The remaining two coupling layers are unintentionally doped passive waveguide layers.
Table 2. Epitaxial structure of the WG-PD
3. Device fabrication and characterization
3.1 Fabrication
WG-PDs with various active areas have been fabricated. As shown in Fig. 7(a), a triple-mesa structure is defined by a combination of inductively coupled plasma (ICP) dry-etching and subsequent wet-etching. Ti/Pt/Au and Ni/Au are adopted for p- and n-contacts, respectively. Annealing at 350°C for 1 minute is employed to ensure low contact resistance [30]. 800-nm-thick SiO2 is deposited over the device for isolation and passivation. The p- and n-contacts are exposed by dry etching to form coplanar waveguide (CPW) electrodes. Finally, the input facet of the coupling waveguide is formed by cleaving.
Fig. 7. (a) Microscopic photo of the fabricated PD and (b) SEM image of the PD with high impendence CPW.
Two types of CPWs are designed. A 110 Ω high impedance transmission line (denoted as CPW-1) is employed to compensate for the RC constant and extend the bandwidth [31]. The scanning electron microscopy (SEM) image of the fabricated PD with a high impedance transmission line is shown in Fig. 7(b). For comparison, PDs with a standard 50 Ω transmission line (denoted as CPW-2) are also fabricated.
3.2 Device characterization
The I-V curves of the fabricated PDs with different active areas are depicted in Fig. 8, indicating a dark current of less than 3 nA under 2 V reverse bias.
The frequency response is measured by the optical heterodyne setup [32]. The RF signals generated by the PDs are detected by a power meter (Keysight E4419B) with three sets of power sensor heads covering the frequency range from DC to 50 GHz, V band (50-75 GHz), and W band (75-110 GHz). For frequencies above 110 GHz, a submillimeter power meter (VDI-Erickson power meter, PM5B) with a WR10 input sensor head is employed. Tapered and lensed fiber with 2.5 µm MFD is utilized for light coupling. The RF losses, including coaxial cables, power sensor heads, bias tee, waveguide transitions, and probes, have been calibrated.
The normalized frequency responses of PDs with an active area of 4 µm ×10 µm are plotted in Fig. 9. For the PD with a 50 Ω CPW (CPW-2), the 3-dB bandwidth is 107 GHz, while the bandwidth of the PD with a high impendence CPW (CPW-1) is improved to 127 GHz.
Figure 10 illustrates the normalized frequency responses of WG-PDs with different active areas under a low reverse bias of 1 V. For PDs with active areas of 5 × 15, 5 × 10, and 5 × 6 µm2, the 3-dB bandwidth is 86.67, 119.3, and 153 GHz, respectively. Thanks to the optimized electric field distribution within the PD, the bandwidth is not limited by the electron transit time. Instead, the RC time constant is the primary limiting factor, and downscaling the area of the PD helps enhance the 3-dB bandwidth.
The average responsivities under different reverse biases are shown in Fig. 11, which shows the responsivity is bias-independent. By adopting the proposed multi-layer coupling waveguide, a high responsivity can be secured within a short PD length. Even with a small active area of 5 × 6 µm2, the responsivity can reach 0.38 A/W at −1 V. An enhanced responsivity of 0.50 and 0.53 A/W can be obtained by extending the PD length to 10 and 15 µm, respectively, consistent with our simulations.
The RF power versus photocurrent under the fixed reverse bias of 2 V for the fabricated PDs are plotted in Fig. 12. For 5 µm × 15 µm PDs, the maximum RF power reaches −2 dBm with a saturation current of 16 mA at 100 GHz. The 5 µm × 10 µm PDs can achieve −1.2 dBm output RF power with a saturation current of 12 mA at 100 GHz. The 5 µm × 6 µm PDs exhibit a saturation current of 7 mA with a maximum RF power of −5.6 dBm at 130 GHz. With further implementation of thermal management, we anticipate a higher RF output power.
Fig. 12. Output RF power versus optical photocurrent for WG-PDs with active areas of (a) 5 × 15, (b) 5 × 10, and (c) 5 × 6 µm2.
A comparison of the performances of WG-PDs is summarized in Table 3. Our devices show competitive performance compared with previously reported WG-PDs. A high bandwidth-efficiency product (BEP) has been secured without a complicated mode size converter and additional anti-reflection coating. The experiment results indicate the effectiveness of our proposed structure for improved coupling efficiency and enhanced bandwidth.
Table 3. Performance comparison of reported WG-PDs
4. Conclusion
In this work, a novel evanescently coupled waveguide UTC-PD with ultrawide bandwidth and high responsivity has been successfully demonstrated. The coupling waveguide, epitaxy structure, device fabrication and characterization are discussed. Thanks to the gradually increasing index profile, coupling waveguide structure design, and precise electric field management, the WG-PD with an active area of 5 × 6 µm2 exhibits a 3-dB bandwidth of 153 GHz and a responsivity of 0.38 A/W, corresponds to a high BEP of 46.51 GHz. For 5 × 10 µm2 WG-PD, the 3-dB bandwidth is 119.3 GHz and the responsivity is improved to 0.50 A/W, with a high BEP of 47.72 GHz.
Funding
National Key Research and Development Program of China (2022YFB2803002); National Natural Science Foundation of China (61927811, 61991443, 62127814, 62225405, 62235005); Collaborative Innovation Centre of Solid-State Lighting and Energy-Saving Electronics.
Acknowledgements
This work was supported in part by National Key R&D Program of China (2022YFB2803002); National Natural Science Foundation of China (62235005, 62127814, 62225405, 61927811, and 61991443); 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.
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