Silicon photonics provides a promising platform for energy-efficient interconnects within supercomputers and data centers. However, developing a complementary metal–oxide–semiconductor compatible high-speed photodetector with low dark current has long presented a challenge in the field. In this paper, we report the first O-band InAs quantum dot (QD) waveguide photodiode (PD) heterogeneously integrated on silicon. Record low dark currents as low as 0.01 nA, responsivities of 0.34 A/W at 1310 nm and 0.9 A/W at 1280 nm, and a record high 3 dB bandwidth of 15 GHz was measured. Avalanche gain was observed and a maximum gain of up to 45 and a gain bandwidth product (GBP) of 240 GHz were achieved, which are also record high results for any QD avalanche photodetector (APD) on silicon. Additionally, we demonstrate a device sensitivity of at 10 Gb/s and open-eye diagrams up to 12.5 Gb/s. These QD-based PDs are able to operate as p-i-n PDs or APDs under different bias conditions and offer a promising alternative to heterogeneous InGaAs-on-silicon and SiGe counterparts in low-power optical communication links. They also leverage the same epitaxial layers and processing steps as heterogeneously integrated QD lasers, significantly simplifying the processing and reducing the cost of a fully integrated QD transceiver on silicon.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
With data consumption in our society increasing exponentially, future supercomputers and data centers will need to send and process data at rapidly increasing speeds and lower energy. Photonics has become the dominant connectivity solution for reach from meter to kilometer and bandwidth from 10 to 400 Gb/s per lane and will soon reach expected aggregated bandwidths of many petabytes/s . Silicon (Si) photonics can naturally leverage the existing advanced technology infrastructure behind the Si complementary metal–oxide–semiconductor (CMOS) industry and has emerged as a new and promising integrated photonic platform in the past decade. While pure Si modulators are capable of reaching data rates over 100 Gb/s, light generation and absorption still rely on non-Si materials.
Arguably the most challenging task in developing a fully integrated Si photonic interconnect is the development of an on-chip laser on Si. Due to the indirect bandgap of germanium, Si-Ge lasers are extremely limited in efficiency and have yet to be proven as viable materials to develop lasers on Si. Additionally, most Si photonic integrated circuits (PICs) contain Ge-on-Si photodiodes (PDs) for photodetection in 1310 and 1550 nm windows. However, these PDs often suffer from relatively high dark current and high dislocation densities in comparison to alternatives made from III-V semiconductor materials.
Monolithic integration offers a platform to incorporate direct bandgap material onto Si. However, this requires the growth of thick buffer layers in order to mitigate threading dislocations in the active device layers, which limits the performance and design flexibility in the integration of active devices. The thick buffer layers prevent efficient, evanescent coupling between Si waveguides and the active devices. Also, the defects within the buffer layers degrade the reliability of devices such as lasers .
Therefore, heterogeneous integration of a direct bandgap material is required to provide a viable gain medium for an efficient laser and a high-quality, low-defect density material for PDs . The integration of indium arsenide (InAs) QDs as a gain material on Si has proven to be a promising platform for CMOS-compatible, uncooled on-chip lasers . For example, due to the three-dimensional confinement of carriers, QD lasers promise higher temperature stability and lower threshold current densities when compared to quantum well lasers [5,6]. In addition, due to the size distribution of the dots, QD lasers have a wide gain bandwidth, which allows for a larger channel count in a wavelength division multiplexing (WDM) link and is particularly attractive for comb lasers .
Furthermore, quantum dot (QD) PDs heterogenously integrated on Si were made using the same epitaxial layers and fabrication process for a recent 1310 nm hybrid QD Si comb laser with error-free operation for 14 channels . The three-dimensional carrier confinement in the QDs leads to dark currents in these QD PDs as low as 10 pA (), which is the lowest dark current of any PD on Si, to our best knowledge. These hybrid Si QD PDs also demonstrate a maximum 3 dB bandwidth of 15 GHz at 1300 nm, and external responsivity of 0.34 A/W at 1310 nm and 0.9 A/W at 1280 nm. An avalanche gain of up to 45 and a gain-bandwidth product (GBP) of 240 GHz were observed, which are the highest for any QD APDs on Si. Open-eye diagrams up to 12.5 Gb/s were taken and temperature studies have been done on these APDs, which exhibit high performance up to 60°C, showing that these APDs can be practically used in an uncooled, WDM system on a Si photonic platform.
2. DEVICE DESIGN AND FABRICATION
Devices were fabricated on a 100 mm silicon-on-insulator (SOI) wafer with a top Si thickness of 400 nm and a buried oxide thickness of 1000 nm. First, grating couplers and 320 nm thick passive Si waveguides were etched onto an SOI substrate. Then, a GaAs-based p-i-n epitaxial structure with an active region of eight layers of InAs QDs, with QD density of , totaling 320 nm in thickness, was bonded directly to the SOI substrate using an plasma-assisted direct bonding process . One monolayer of InAs was grown for the QDs, which were 7 nm in height and 20 nm in diameter.
Next, the Ni/Pd/Au was deposited and then lifted off to define the p-contact. GaAs mesas were made by a mixture of dry etching using and a wet chemistry etching using citric acid , which has a high selectivity to the etch stop layer. Then, the layer was etched in a solution of , which has a high selectivity to the -GaAs contact layer in the lower cladding. Pd/Ge/Au was deposited followed by lift-off to define the -contact. Finally, the devices were passivated with 600 nm thick , before vias were formed and probe pad metal was evaporated. Figure 1 shows a cross-section schematic and scanning electron microscope (SEM) photo of the device after fabrication.
3. RESULTS AND DISCUSSION
A. Direct Current Characterization
Figure 2 plots the I-V curves of an PD at different temperatures. A low dark current of 10 pA () at at 300 K was demonstrated from a PD, which is the lowest dark current reported for any PD on Si, to our best knowledge . This value is 3 orders of magnitude lower than the lowest ever reported dark current ( at ) in p-i-n Ge-on-Si detectors . The dark current density of a PD at was , and the dark current density of a PD at was , which showed linear scaling with area, signifying that the main contribution to the dark current is from surface leakage current and not from the bulk of the device. This can be attributed to the high crystal quality and low dislocation density of the III-V material, as well as sufficient surface passivation of the PD mesa.
The dark current was measured to be 50 μA around , which is near the breakdown voltage. The temperature dependence on the breakdown voltage reveals that impact ionization of free carriers is the primary physical mechanism responsible for the breakdown of the device.
The responsivity of an PD, an PD, and an PD at 1310 nm was measured and is shown in Fig. 3. Light from a 1310 nm laser is coupled from a cleaved fiber into a grating coupler, which then directs the light along an Si waveguide and couples light evanescently into the PD.
After extracting an estimated total loss of 10 dB from fiber connections and coupling with the grating coupler, responsivity of 0.34 A/W was measured at for an PD. Figure 4 displays responsivity with a change in the input optical wavelength. A decrease in responsivity is seen with an increase in wavelength at biases below . At 1280 nm, responsivity of 0.9 A/W is achieved at , and at 1310 nm, the responsivity decreases to about 0.15 A/W at .
At shorter wavelengths, carriers are generated in the excited state within the QDs and require less energy to escape the QDs. For instance, the carriers generated in the QDs at 1280 nm have sufficient energy to escape from the QDs and be collected as photocurrent at a reverse voltage bias greater than 4 V. At longer wavelengths, carriers are generated at lower energy states within the QDs and require more energy in order to escape the QDs . We verified the ground energy and excited energy states after observing lasing around 1210–1220 nm and 1300–1310 nm in lasers fabricated from the same structure. Furthermore, the bias dependence on responsivity is also due to the quantum-confined Stark effect (QCSE). As electron and hole wave functions and energy levels in the QDs shift with an applied electric field, the absorption coefficient of the QD layers also shifts .
The spectral response for an PD is plotted in Fig. 5. Both a wavelength dependence and a bias dependence are seen in the responsivity. At higher voltage biases, the bias dependence on the responsivity is primarily due to an increase in avalanche gain with an applied electric field. Furthermore, the gain is also wavelength-dependent, suggesting that the carrier injection and multiplication processes may differ with wavelength . This could be due to differing carrier populations within each energy level of the QDs with a change in the input optical wavelength. We believe that avalanche multiplication occurs in the GaAs spacer layers between the QDs [15,16]. It is also possible that multiplication occurs within the InAs QD material, as suggested in .
The gain of the device at 1310 nm was measured as a function of bias voltage and is shown in Fig. 6. Unity gain was confirmed to be at by measuring and observing a linear increase in excess noise with voltage bias at biases higher than . The external responsivity at unity gain and with 8 dBm optical input power at room temperature is 0.06 A/W, and the maximum external responsivity at room temperature is 2.7 A/W. A maximum gain of about 45 was seen at room temperature, with avalanche gain seen up to a stage temperature of 60°C, displaying the temperature robustness of the devices. A temperature dependence on the gain, as well as a decrease in gain at high biases, have been observed and are due to the increase of dark current with temperature and bias. As temperature increases, more carriers gather sufficient energy to escape the QDs through thermionic emission and contribute to dark current.
B. High-Frequency Measurements and Analysis
The output frequency response of the PDs was measured at and at room temperature using an HP light wave component analyzer (LCA) at an input wavelength of 1300 nm and plotted in Fig. 7. Measurements revealed a maximum 3 dB bandwidth of 8 GHz for an PD, 11 GHz for an PD, and 15 GHz for an PD.
The frequency responses are transit-time-limited at low biases, and RC-limited at high biases, before avalanche gain dominates in the PDs. At low biases, photogenerated carriers take time in order to escape from the QDs before being collected by the contacts. With an increase in the applied electric field, photogenerated carriers escape from the QDs within a shorter period of time . With a high enough applied electric field, the frequency response is maximized as it approaches the RC limit of the device.
The frequency response was also measured under bias voltages in which high multiplication gain occurs, as shown in Fig. 8. An inductive peaking effect was observed, which has been previously explained in other APDs to be caused by impact ionization, which introduces a phase delay between the AC photocurrent and the applied electric field .
A maximum GBP of 240 GHz was measured at a bias voltage of , as shown in Fig. 9. This is higher than most traditional InP-based receivers based on APDs, which is around 100–200 GHz due to their larger impact ionization coefficient value . This number also compares to SiGe APD counterparts showing higher GBP due to the low value of Si. But they often suffer from higher dark currents due to dislocations at lattice-mismatched Ge/Si interfaces.
The GBP decreases at voltage biases higher than because after avalanche breakdown, the dark current increases at a significantly faster rate than the photocurrent does, reducing the total gain. Furthermore, as voltage bias increases, avalanche buildup time increases and limits the total carrier transit time, slowing down the frequency response.
These PDs provide enough multiplication gain to produce a sufficiently high signal-to-noise ratio and clear eye diagrams without the need of a transimpedance amplifier (TIA). A high-speed pseudorandom binary sequence (PRBS) signal was amplified by a 20 dB high-speed power amplifier. Then, a 1310 nm optical signal couples to a PD that is biased through an RF probe and a bias tee. The output electrical signal is monitored by a DCA86100C sampling scope in the form of an eye diagram.
Figures 10(a)–10(c) show the electrical eye diagrams of an APD at 5, 10, and 12.5 Gbps, respectively. At a gain of 40, a signal-to-noise ratio greater than 7 and 5 dB at 5 and 10 Gbps were obtained, respectively. We have also obtained open-eye diagrams at 12.5 Gbps, as shown in Fig. 10(c), where the data rate was limited by the pattern generator.
A bit error rate (BER) test was conducted using an Anritsu Bit Error Rate Tester at 10 Gb/s. At a gain of 28, the sensitivity was measured to be about at a BER of and at a BER of , as shown in Fig. 11. This sensitivity is a few decibels higher than that of a typical Ge-on-Si p-i-n PD . The sensitivity can be increased by wire bonding the QD PD to a TIA, as done in a case with a Ge-on-Si APD .
In this paper, we presented QD PDs heterogeneously integrated on Si using the same epitaxial layers and fabrication process for a QD laser on Si. These QD PDs exhibited dark current as low as 10 pA (), which is the lowest dark current of any PD on Si, to our best knowledge. These PDs also show a maximum 3 dB bandwidth of 15 GHz at 1300 nm, and external responsivity of 0.34 A/W at 1310 nm and 0.9 A/W at 1280 nm. A GBP of 240 GHz was observed, which is the highest for any QD APDs on Si. Open-eye diagrams were measured up to 12.5 Gb/s, and temperature studies have been done, demonstrating high performance up to 60°C and showing that these APDs can be used uncooled in an Si photonic interconnect within a WDM system.
The authors thank Min Ren, Yuan Yuan, Andrew Jones, and Yingtao Hu for their helpful discussions.
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