High Efficiency and High Bandwidth Double-Cladding Waveguide Photodetector Array for 400 Gbit/s Communication

Abstract

A parallel array of 10 side-illuminated waveguide photodetectors (WGPDs) with double-cladding structure was designed and fabricated. In order to achieve high coupling efficiency to the fiber, the thicknesses of InGa_0.24As_0.53P cladding layers and In_0.53Ga_0.47As core layer were optimized. The array exhibited a uniform responsivity of 0.54 A/W at 1310 nm without anti-reflection (AR) coating and dark currents lower than 1.3 nA at −5 V. Each photodetector (PD) showed a bandwidth of over 30 GHz, amounting to 400 Gbit/s transmission capacity for the whole chip. In addition, numerical analysis showed that the fiber alignment tolerance to the chip edge along vertical and horizontal directions, when using a lensed fiber, were 1.8 μm and 4.6 μm, respectively. The simple fabrication, easy alignment capability and high performance make the photodetector array a competitive solution for future 400 Gbit/s parallel communication.

1. Introduction

Driven by the rapid development of big data, cloud computing, and 5G communication networks, the demand for data transmission capacity has grown exponentially [1,2]. In order to meet the heavy traffic in data centers, fiber-based optical interconnections have become a promising scheme, due to their large capacity, high speed, and high reliability [3,4,5]. At present, the optical communication rate has gradually developed from 25 Gbps/40 Gbps to 100 Gbps/400 Gbps [6]. As the core component of optical receivers, photodetectors play a key role in converting optical signals into electrical signals. High-efficiency, high-sensitivity, and broadband photodetector (PD) arrays are crucial for optical transmission systems [7,8,9].

Surface-illuminated photodetectors cannot achieve high efficiency in the high frequency region due to the theoretical bandwidth-efficiency limitation [10,11,12]. To overcome this limitation, side-illuminated waveguide photodetectors (WGPDs) are an attractive alternative [13,14,15]. Since light and photogenerated carriers are transported in the perpendicular direction, the internal quantum efficiency and bandwidth can be designed almost independently. However, the drawback of conventional InP-In_0.53Ga_0.47As-InP waveguide photodetectors is the poor coupling efficiency with the fiber due to the thin In_0.53Ga_0.47As layer [16]. Since the In_0.53Ga_0.47As layer serves as both the carrier transport layer of the PD and the core layer of the waveguide, there is a compromise between coupling efficiency and bandwidth. HO KO et al. [17], in 2018, reported a sport size converter (SSC) integrated waveguide photodetector (WGPD) with a bandwidth of 45 GHz and a responsivity of 0.73 A/W. Sun et al. [18], in 2018, showed an evanescently coupled (EC) WGPD with a bandwidth of 30 GHz and a responsivity of 0.13 A/W and Lin et al. [14], in 2019, investigated an EC WGPD with a bandwidth of 65 GHz and responsivity of 0.19 A/W. These WGPDs are not suitable for PD arrays, due to their complex structures and fabrication processes.

In this paper, we report a parallel array with 10 high-efficiency side-illuminated WGPDs. Each PD contains additional InGa_0.24As_0.53P cladding layers sandwiching the In_0.53Ga_0.47As absorber layer. The coupling efficiency of this structure to the fiber is higher than that of the conventional waveguide photodetector, since the vertical optical field distribution is enlarged by the additional InGa_0.24As_0.53P layer. Meanwhile, the bandwidth is not sacrificed as it is still determined by the thickness of the In_0.53Ga_0.47As layer. The measurement results showed that the 10 WGPDs exhibited uniform responsivities over 0.54 A/W at 1310 nm without anti-reflection (AR) coating on the edge facet and low dark currents of about 1.3 nA at 5 V reverse bias. The PD array has the total transmission speed exceeds 400 Gbit/s, and it provides a low-cost solution for both monolithic integration and heterogeneous integration platforms (PLC, SiP, etc) [19,20].

2. Design and Simulation

The PD array includes 10 waveguide PIN photodetectors with a spacing of 750 µm, which is compatible with the commercially available trans-impedance amplifier electrode spacing, and the three-dimensional (3D) schematic structure of a single PD is shown in Figure 1. To achieve great coupling to a fiber in the y direction, each PD has a 5 μm wide high-mesa structure. The photodetector epitaxial structure is symmetrical, which includes a semi-insulating InP substrate, a 0.5 μm thick n⁺-doped (Si: 5 $\times$ 10¹⁸ cm⁻³) InP cladding layer, an n-doped (Si: 1 $\times$ 10¹⁸ cm⁻³) InGaAsP cladding layer, an intrinsic In_0.53Ga_0.47As absorption layer, a p-doped (Zn: 1 $\times$ 10¹⁸ cm⁻³) InGaAsP cladding layer, a 0.5 μm thick p⁺-doped (Zn: 3 $\times$ 10¹⁸ cm⁻³) InP cladding layer and a 0.08 μm thick p⁺-doped (Zn: 1 $\times$ 10¹⁹ cm⁻³) In_0.53Ga_0.47As ohmic contact layer. The In_0.53Ga_0.47As absorption layer is sandwiched between n-InP/InGaAsP and p-InGaAsP/InP cladding layers. To obtain wide vertical optical field distribution in the WGPD, and make the InGaAsP cladding layers transparent to the input light at 1310 nm, we defined the InGaAsP as InGa_0.24As_0.53P (λ_g = 1.24 μm).

The quantum efficiency η in the WGPD can be described by the following equation [21]:

$$\eta =\left(1-R\right)\times \left(1-e^{-\alpha \Gamma L}\right)\times {\eta }_C$$

(1)

where R is power reflection loss on the facet of the WGPD, α is the absorption coefficient of In_0.53Ga_0.47As at 1310 nm, Γ is the optical confinement factor of the In_0.53Ga_0.47As layer, L is the WGPD length and ${\eta }_C$ is the coupling efficiency to fiber. In this structure, with a constant InP cladding thickness of 0.5 µm, the Γ and ${\eta }_C$ in the WGPD is simultaneously determined by the In_0.53Ga_0.47As layer and the InGa_0.24As_0.53P layer. In order to improve responsivity with a large 3 dB bandwidth, the In_0.53Ga_0.47As layer thickness D1, the InGa_0.24As_0.53P layer thickness D2 and the WGPD length L were investigated. Large photo-absorption characteristics and a large mode field in the waveguide structure should be obtained in order to achieve high responsivity.

The optical transmission of the structure was simulated by three-dimensional finite-difference time-domain (3D FDTD) methods. The simulated cross-section optical power distributions of the WGPD with different InGa_0.24As_0.53P layer thicknesses D2 are depicted in Figure 2. The mode field in the WGPD clearly expanded in the z-axis direction by introducing the InGa_0.24As_0.53P layer, which could effectively reduce the coupling loss caused by the mode field mismatch between the WGPD and the fiber. Figure 3a exhibits the simulated quantum efficiency versus InGa_0.24As_0.53P thickness D2 under different In_0.53Ga_0.47As thicknesses D1. For a fixed value of D1, the quantum efficiency did not vary monotonically with D2, which was attributed to the inverse relationship between the coupling efficiency ${\eta }_C$ and the optical confinement factor Γ as the InGa_0.24As_0.53P thickness D2 changed. Nevertheless, it could also be observed from Figure 3a that the quantum efficiency with nonzero InGa_0.24As_0.53P layer thickness D2 was significantly higher than that of D2 = 0; particularly, when D1 = 0.2 μm and D2 = 0.9 μm, when the quantum efficiency doubled. Therefore, for the same D1, the quantum efficiency could be specified relatively independent of the bandwidth.

In addition, the simulated optical power transmission of the absorption region is shown in Figure 3b. Since the light is injected directly into the edge of the absorber layer, the input light is mostly absorbed at the front end of the PD. At a fixed In_0.53Ga_0.47As thickness D1 = 0.2 μm. The WGPD length L dependent quantum efficiency for the values of InGa_0.24As_0.53P thickness D2 = 0.85 and 1 μm is also presented in Figure 3c, from which it can be seen that the quantum efficiency was almost stable when the detector length exceeded 20 μm. In this case, in order to reduce the photodetector series resistance and diode capacitance, the optimized parameters of the WGPD were chosen to be D1 = 0.2 μm, D2 = 0.85 μm and L = 20 μm for further simulations and experiments.

Fiber misalignment tolerance to WGPD with optimized parameters is also plotted in Figure 3d. At −1 dB power reduction, the alignment tolerances of the chip along the vertical and horizontal directions were 1.8 μm and 4.6 μm, respectively. Therefore, the double-clad WGPD could be easily coupled with the fiber.

3. Chip Fabrication

The epitaxial structure was grown by the metal-organic chemical vapor deposition (MOCVD) technique on a semi-insulating InP substrate. PD array mesas were fabricated by contact ultraviolet lithography technology and inductively coupled plasma (ICP)etching. The SiO₂ passivation film was deposited by plasma enhanced vapor deposition (PECVD). Then polyimide was spun for planarization and patterned for metal contacts. After that, N electrodes and coplanar microwave lines were deposited by magnetron sputtering of AuGeNi/Au and Ti/Au, respectively. Finally, the fabricated PD array was precisely cleaved to a 20 μm length by an automated cleavage machine.

4. Measurements and Result Analysis

The dark current measurements of the PD array were performed by the KEITHLEY 4200 Semiconductor Tester at room temperature. Figure 4 shows the I–V characteristic curves of these photodiodes at a bias voltage from −5 to 1 V. The dark currents of these photodiodes with an active region of 5 × 20 μm² were in the range of 0.6–1.3 nA at 5 V reverse bias, which benefited from effective mesa passivation and small etching damages. These low dark current values were critical for the high sensitivity of the PD array.

Photo-response measurements were carried out using a 1310 nm laser source, a tapered fiber with a 2.5 μm spot size and a KEITHLEY 4200 Semiconductor Tester. The fiber output power and the bias voltage were fixed at 1.02 mW and −5 V, respectively. The results of measured photocurrent and calculated responsivity are shown in Figure 5a and Figure 5b, respectively. All 10 WGPDs exhibited a uniform responsivity over 0.54 A/W without an AR coating. The quantum efficiency of the single PD was deduced to be 51%, which was consistent with the simulated value. By depositing an AR coating on the input facet, the quantum efficiency was expected to exceed 72% as the reflection loss would be negligible. Therefore, the double-cladding scheme could achieve high quantum efficiency without introducing a spot size converter in WGPD, which facilitated compact and large-scale on-chip integration.

The frequency response measurement setup was based on a Vector network analyzer in the test range from 10 MHz to 40 GHz. Figure 6 shows the measurement result at −5 V bias. All 10 units of the PD array exhibited a uniform −3 dB bandwidth over 30 GHz, and the whole chip was suitable for 400 Gbit/s data transmission system. The typical bandwidth calculation formula [22] is as follows:

$$\frac{1}{f_{3dB}^2}\cong \frac{1}{f_t^2}+\frac{1}{f_{RC}^2}$$

(2)

$$f_t\cong \frac{3.5\overline{v}}{2\mathsf{\pi }D}$$

(3)

$$f_{RC}\cong \frac{1}{2\mathsf{\pi }\left(C_P+C_j\right)\left(R_L+R_S\right)}$$

(4)

where f_t and f_RC are the carrier transit-time limited bandwidth and the resistance–capacitance (RC) time constant limited bandwidth. $\overline{v}$ is the average velocity of electron and hole. D is the intrinsic layer thickness. C_P and C_j are the parasitic capacitance and diode capacitance. R_L and R_s are the load resistance and series resistance, respectively. The calculated f_t was about 147.6 GHz and f_RC was 34.3 GHz, thus, the calculated f_3dB was about 33.5 GHz. Obviously, the main factor that limited the device’s bandwidth came from the series resistance and the diode capacitance, which greatly reduced the RC bandwidth. Bandwidth of photodetectors could be further improved by optimizing alloy conditions, to decrease the series resistance, and shrinking device size, to reduce diode capacitance.

5. Conclusions

We designed and fabricated a PD array containing a 10 double-cladding waveguide structure. The PD array overcame the limitation of coupling efficiency and bandwidth in conventional WGPDs, and was easy to fabricate. The 0.2 µm In_0.53Ga_0.47As absorber thickness, 0.85 µm InGa_0.24As_0.53P cladding thickness, and 20 µm photodetector length were optimized to achieve a high quantum efficiency of 52%, twice that of the conventional InP-In_0.53Ga_0.47As-InP WGPD. The 10 PDs all exhibited good responsivity of 0.54 A/W at 1310 nm, uniform low dark current of 1.3 nA at 5 V reverse bias, and 3 dB bandwidth beyond 30 GHz, in good agreement with the theoretical expectations. The high-frequency characteristics of PD arrays would be greatly improved by optimizing alloy conditions and reducing device size. Moreover, the simulated alignment tolerance to the chip along vertical and horizontal direction was 1.8 μm and 4.6 μm, respectively. These results demonstrate that the PD array has application advantages as a low-cost, high-efficiency, high-speed, and high-sensitivity parallel communication system.