s All-Silicon Photodetector Based on Microring ... - IEEE Xplore

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 27, NO. 7, APRIL 1, 2015

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40 Gb/s All-Silicon Photodetector Based on Microring Resonators Xianyao Li, Zhiyong Li, Xi Xiao, Hao Xu, Jinzhong Yu, and Yude Yu

Abstract— We experimentally demonstrate the first all-silicon microring photodetector with 40-Gb/s operation speed based on the defect-assisted subbandgap avalanche mechanism in reverse p-n junction. A novel zigzag p-n junction providing a high responsivity of 48 mA/W upon 8 V in avalanche mode is demonstrated with a low doping concentration of 2 × 1017 cm−3 . With the optimized operation wavelength, a 3-dB optical to electrical response of ∼20 GHz and high-speed photodetections of 20–40 Gb/s are achieved experimentally, showing great potential in the application of all-silicon ultrahigh-capacity optical interconnects. Index Terms— All-silicon photodetection, microring resonator, high speed.

I. I NTRODUCTION

S

ILICON photonics has attracted increasing attention during the last decades due to its compatibility with the mature silicon fabrication technology and miniaturization of the optical functionalities and systems. All-silicon devices have long been favorite components for on-chip monolithic optical integrations. However, the 1.12 eV indirect bandgap of silicon has become one of the biggest obstacles lie ahead. Up to date, all-silicon modulators and passive waveguide devices have been demonstrated [1]–[5], but a few key components such as all-silicon lasers and photodetectors are still very challenging. In order to achieve an effective photodetection in the telecom wavelengths around 1.31/1.55 μm, several approaches have been proposed including the incorporation of photoactive materials into silicon by epitaxial growth or wafer bonding techniques [6]–[8], introducing special dopants like Si+ or B+ in the waveguide to form mid-bandgap defect states for the mid-bandgap absorption [9], [10], or employing nonlinear absorption effect with high optical power [11]. However, these schemes require either complex fabrication processes or harsh conditions for the input light. The advancement of carrier-depletion silicon modulator has provided a new solution for all-silicon photodetection, with no Manuscript received September 30, 2014; revised December 16, 2014; accepted January 6, 2015. Date of publication January 12, 2015; date of current version March 6, 2015. This work was supported in part by the National High Technology Research and Development Program of China under Grant 2012AA012202 and Grant 2013AA014402, in part by the National Basic Research Program of China under Grant 2011CB301701, and in part by the National Natural Science Foundation of China under Grant 61275065 and Grant 61107048. The authors are with the State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China (e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; yudeyu@ semi.ac.cn). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2015.2390619

Fig. 1. (a) Top-view microscope image of the photodetector after fabrication. The brown color region is aluminum ground-signal-ground electrodes. (b) Schematic view of the zigzag PN junction doping profile and waveguide structure.

need of additional fabrication steps in CMOS foundries [12]. The defect-states introduced during the formation of PN junctions can effectively serve as the photocurrent generators. Moreover the responsivity of this kind photodetector will be further improved by initiating the avalanche multiplication under a high reverse voltage or using interleaved PN junctions with better light-carrier overlap [13], [14]. High responsivities of 72.8 mA/W have been demonstrated recently, but the photodetector suffers low operation speed [13]. In this letter, we present a compact all-silicon MRR photodetector with operation speed up to 40 Gbit/s. An enhanced interleaved PN junction - zigzag PN junction is employed to achieve both efficient light-carrier interaction and high electrical bandwidth. A high responsivity of 48 mA/W is achieved for a compact racetrack ring with 10μm bending radius and 2μm-long straight waveguides upon 8V in avalanche mode. The doping concentration of the PN junction is only 2×1017 cm−3 . Our photodetector shows a 3dB optical to electrical (OE) bandwidth of ∼20 GHz at 8V and clear opening eye-diagrams at 20 Gbit/s and 40 Gbit/s. II. D EVICE D ESCRIPTION AND FABRICATIONS In our previous work, we have proposed and demonstrated an interleaved PN junction to improve the lightcarrier overlapping with high junction misalignment tolerance [15], [16]. In order to enhance the electrical bandwidth, we developed another kind of PN junction with a zigzag configuration to reduce the unnecessary junction capacitance along the waveguide edges and a high-speed modulation of 44 Gbit/s was achieved [17]. In this letter, we employ the same zigzag PN junction to explore its’ performance of photodetection. Our device was fabricated in the standard 0.18 μm-CMOS foundry at Semiconductor Manufacturing International Corporation (SMIC) in China. The waveguides were formed in a SOI substrate with 340nm-high top silicon layer and a 2μm thick oxide layer.

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Fig. 2. (a) Measured transmission spectra with the applied voltage of reverse 1 V, 3 V, 5 V, 7 V, 7.6 V, 7.8 V, 8 V and 8.1 V, respectively. (b) Generated photocurrent spectra according the reverse voltages with input optical power of ∼ 0.1 mW in the straight waveguide. (c) The measured photocurrents as functions of the input optical power and applied voltages.

Fig. 1(a) shows the microscope image of a fully fabricated MRR based photodetector with single side coupled to a straight bus waveguide. We designed the MRR with 10 μm bending radius and 2 μm long straight waveguide to form a racetrack shape. The gap between the ring and bus waveguide was optimized to be 200 nm. The PN junction is embedded into the 500 nm-wide rib waveguide which has a 80 nm-thick slab. Fig. 1(b) shows the schematic view of the zigzag PN junction, which has a periodical doping profile with a period of 600 nm along the waveguide. The P- and N-type doping concentration is both 2×1017 cm−3 . Highly doping N++ and P++ region (with a concentration of 1×1020 cm−3 ) are both located 500 nm away from each waveguide edges for the ohmic contact to aluminum electrodes. III. D EVICE C HARACTERIZATIONS To investigate the DC performance of the photodetector, we packaged the input and output grating couplers in a planar manner using two identical standard cylindrical SMF-28 fibers with their end facets polished at a 40° [18]. The measured fiber-chip-fiber loss is ∼ 18 dB, suggesting a ∼ 9 dB loss at the straight waveguide of the photodetector. The waveguide propagation loss in the defect-states remained Si waveguide is 16.7 dB/cm [16], which consisted of 4.1 dB/cm waveguide loss and 12.6 dB/cm doping induced loss. We measured the transmission spectra and the generated photocurrents of the MRR with the bias voltages varying from reverse 1 V to 8.1 V, as shown in Fig. 2(a) and (b). The optical power in the straight waveguide was ∼0.1 mW. Due to the carrier depletion effect the resonance shifts when the voltage increases until 7 V, with the MRR under a under-coupling condition. From 7 V to 8 V the resonance is fixed at 1550.59 nm. When the voltage increases to 8.1 V, the resonance shifts again due to the evident thermal effect caused by the avalanche multiplication [19]. In Fig.2 (b) a dramatic increase can be seen when the wavelength is approaching the resonance of MRR. As the voltage increases, the PN junction is gradually

depleted, and the free carrier plasma loss is decreasing and the detection volume is increasing rapidly, which leads to higher extinction ratios (ER) in the transmission spectra and significant increase of the photocurrents at cavity resonances. As the bias voltage increases from 7 V to 8 V, the generated photocurrent increases due to the avalanche multiplication, but the ER of the MRR remains to be ∼ 16 dB. We guess the reason is the drift velocity of the photocarrier increases with the voltage, thus the concentrations of the carriers are maintained to be the same in the waveguide. The photocurrent spectrum upon 8V shows a peak value of 4.81μA and the photocurrent-to-dark-current ratio of ∼8, which indicates an avalanche gain of 8.7 to the photocurrent generated upon 7 V. From the Miller’s formula [20] −1 M = 1 − (V/Vb )n where V is the applied voltage, Vb is the avalanche breakdown voltage and n is a constant between 1.5∼4 for silicon [21], we evaluate Vb =8.4 V and n=2.5 for our device. Fig.2 (c) shows the generated photocurrents under different voltages when increasing the input optical power. The fitted dash lines show linear responses of our device to the input powers. From the obtained results we estimated our photodetector has a responsivity of 48 mA/W upon 8 V. To estimate the linear absorption coefficient of our device, we adopted the formula given in [13] Ilinear = η

q αl (1 − e−αtot Ld )γ Pres Eph αtot

where α1 is the linear absorption coefficient and 2 Pres = κ/(tAe−iϕ − 1) Pbus . We fitted the transmission spectra of our device with κ = 0.28, A = 0.97, t = 0.96 and assumed the optical mode spatial overlap factor γ ≈ 1, carrier collection efficiency η = 0.92. We extracted α1 to be 0.158 dB/cm at 7 V. In order to estimate the electrical characteristics of the zigzag PN junction, we measured the S11 parameters of

LI et al.: 40 Gb/s ALL-SILICON PHOTODETECTOR BASED ON MICRORING RESONATORS

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Fig. 3. Measured (dots) and fitted (lines) S11 (a) magnitude and (b) phase responses at reverse 1 V and 5 V, respectively. The inset in (b) shows the equivalent circuit model of the MRR photodetector.

the photodetector by using a 40 GHz vector network analyzer (VNA). We extract the RC parameters by curvefitting the S11 parameters with an equivalent circuit model. The measured and fitted S11 magnitude and phase responses under 1 V and 5 V are shown in Fig. 3(a) and (b) respectively. In the circuit model given in the inset of Fig.3 (b), Cp , Cj and Co are the capacitances induced by the pads, PN junction and the oxide layer, Rj and Ro are the resistance of the junction and the Si substrate. We have obtained Cp = 14 fF, Co =130 fF, Rj = 32 and Ro = 700 by the curve-fitting. With reverse voltages change from 1 V to 5 V, the extracted Cj reduces from 40.5 fF to 30 fF due to the expansion of the depletion region and the electrical 3-dB bandwidth of ∼37 GHz and ∼49 GHz are estimated at 1 V and 5 V [16]. We also characterized the OE frequency responses of our device by the 40 GHz VNA. The schematic diagram of the measurement system is shown in Fig.4 (a). Microwave signals coming from one port of the VNA were biased to a commercial 40 Gbit/s modulator to form optical small-signals and added to the photodetector. The optical signals were then coupled into the MRR by the planar packaged fibers and transferred into the electrical domain by the photodetector under reverse biases as shown in Fig.4 (b). The generated electrical signals were collected by a ground-signal-ground (GSG) probe to another port of the VNA. We used a commercial 50 GHz PD as a reference for bandwidth calibration. Because the responsivity of the photodetector is very weak under 7 V, we only got the OE responses near 8 V as shown in Fig.4 (c). We see that the frequency responses decrease rapidly when the voltage increase from 7.8 V to 8.1 V due to the avalanche build up effect. The 3-dB OE bandwidths are 22.8 GHz, 20.5 GHz and 10 GHz at 7.9 V, 8 V and 8.1 V respectively. As the electrical bandwidth of our device is normally several tens of GHz, the speed is mainly limited by the photon-lifetime or the carrier transit time according to the following equation −2 = (2πτc )2 + (τs /0.45)2 + (2πRC)2 f3dB

Fig. 4. (a) Picture of the packaged silicon chip for high-speed photodetection. (b) The schematic diagram of the OE frequency response measurement system. (c) Measured OE frequency response of our MRR under reverse voltages from 7.8 V to 8.1 V.

where τc and τs are the photon-lifetime and carrier transit time. Since the MRR is under a over-coupling condition near 8 V with the Q factors remain unchanged thus we estimate the rapid decrease of the OE bandwidth is caused by the increasing carrier transit time when the voltage is near 8 V. IV. H IGH S PEED P HOTODETECTION To demonstrate the performance of high-speed photodetection, we carried out the eye-diagram measurements. The schematic diagram of the measurement system is shown in Fig. 5(a). The continuous-wave (CW) light was modulated at first by implementing a non-return-zero pseudorandom binary sequence (PRBS) 231–1 signals to a commercial 40 Gbit/s modulator with a Vpp of 3 V and a DC bias of 1.5 V. The modulated optical signals were introduced to the silicon chip by grating couplers. The generated photocurrents were collected by a 40 GHz GSG probe upon 8 V. A 38 GHz SHF driver was adopted to convert the photocurrents into voltage signals and then amplify them. A 50 resistor was also loaded to the pads to reduce the electrical signal reflection as shown in Fig 4(b). A 65 GHz electrical module of Tektronix was used to monitor the eye-diagrams. We changed the input wavelength from 1550.4 to 1550.8 nm with a 0.002 nm step to find the optimum wavelengths for different operation speed with an estimated input power of ∼ 9 mW in the straight waveguide. Fig.5(b) to (e) present the measured photodetection eye-diagrams. As can be seen clear and opening eyes at

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Fig. 5. (a) Schematic diagram of eye measurement system. (b) 20 Gbit/s, (c) 25 Gbit/s, (d) 30 Gbit/s and (e) 40 Gbit/s photodetection eye-diagrams upon 8 V.

20 Gbit/s, 25 Gbit/s, 30 Gbit/s and 40 Gbit/s were achieved, showing a promising future in all-silicon photodetection. V. C ONCLUSION In conclusion, we firstly demonstrate a compact all-silicon MRR photodetector operating at a bit-rate up to 40 Gbit/s. By employing a novel zigzag PN junction high responsivity of 48 mA/W is achieved in a ∼ 65 μm-long MRR upon 8 V with a low doping concentration of only 2×1017 cm−3 . A 3-dB OE bandwidth of ∼20 GHz and opening eye-diagrams of 20 Gbit/s to 40 Gbit/s have been achieved. Further improvement of the responsivity and bandwidth can be realized by optimize the period and doping concentration of the PN junction, as well as the Q factor of the MRR. ACKNOWLEDGMENT The authors thank SMIC for the fabrication support of the current Si photonics research. Thank Tektronix in China for the discussion on high-speed measurements. R EFERENCES [1] H. Xu et al., “High-speed silicon modulator with band equalization,” Opt. Lett., vol. 39, no. 16, pp. 4839–4842, 2014.

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