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Abstract
We demonstrate a wafer-level integration of a distributed feedback laser diode (DFB LD) and high-efficiency Mach-Zehnder modulator (MZM) using InGaAsP phase shifters on Si waveguide circuits. The key to integrating materials with different bandgaps is to combine direct wafer bonding of a multiple quantum well layer for the DFB LD and regrowth of a bulk layer for the phase shifter. Buried regrowth of an InP layer is also employed to define the waveguide cores for the LD and phase shifters on a Si substrate. Both the LD and phase shifters have 230-nm-thick lateral diodes, whose thickness is less than the critical thickness of the III-V compound semiconductor layers on the Si substrate. The fabricated device has a 500-µm-long DFB LD and 500-µm-long carrier-depletion InGaAsP-bulk phase shifters, which provide a total footprint of only 1.9 × 0.31 mm2. Thanks to the low losses of the silica-based fiber couplers, InP/Si narrow tapers, and the phase shifters, the fiber-coupled output power of 3.2 mW is achieved with the LD current of 80 mA. The MZM has a VπL of around 0.4 Vcm, which overcomes the VπL limit of typical carrier-depletion Si MZMs. Thanks to the high modulation efficiency, the device shows an extinction ratio of 5 dB for 50-Gbit/s NRZ signal with a low peak-to-peak voltage of 2.5 V, despite the short phase shifters and single-arm driving.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
To support future network systems, it is essential to use large-capacity optical fiber in short and middle-reach links, such as those in datacenter networks. Optical transceivers equipped with Mach-Zehnder modulators (MZMs) are key devices for increasing transmission capacity because they provide a high baud rate and multilevel modulation formats that modulate both the amplitude and phase of the optical carriers. Recently, the critical issues for optical transceivers are their cost, size, and power consumption. To reduce the cost, Si photonics technology is a promising solution because it provides large-scale photonic integrated circuits (PICs) comprising ultra-compact optical filters, low-loss spot-size converters (SSCs) for fiber coupling, polarization rotators and splitters, and Ge photodetectors [1–3]. Large-scale PICs enable us to reduce assembly cost, which is a dominant factor in the total cost. However, Si and Ge are indirect bandgap materials and therefore not suitable for fabricating lasers. Due to the lack of integrated lasers, external laser modules must be assembled with the Si-photonics chip, which increases the size and assembly cost of optical transceivers.
As a solution, III-V semiconductor LDs have been heterogeneously integrated on a Si photonics platform [4–6]. Recent progress in the direct wafer-bonding technique enables us to fabricate LDs by a wafer-level fabrication process. The III-V semiconductor LDs have been integrated with carrier-depletion Si MZMs, which provide large bandwidths of 40 GHz [7]. However, further reductions of size and power consumption are still limited by the modulation efficiency of Si MZMs. Typical Si MZMs modulate the optical phase by carrier-plasma dispersion and have a tradeoff between modulation efficiency and absorption loss. For a reasonably low absorption loss and large bandwidth, the modulation efficiency, which is characterized by the half-wave voltage length product (VπL), is typically over 1 Vcm. Due to the performance limit, further reducing both driving voltage and size simultaneously is difficult for conventional Si MZMs. This is a fundamental performance limit arising from the effective mass and mobility of carriers in Si [8].
A promising way to overcome the performance limit is to use III-V semiconductors for the phase shifter of the MZM. InP-based III-V semiconductors provide a high-efficiency optical phase shift due to the large carrier-plasma effect, band-filling effect, and Franz-Keldysh (F-K) effect; therefore, they are suitable for achieving a VπL in an MZM much lower than that in the conventional Si MZM [9–11]. This advantage is essential for reducing both size and driving voltage. However, simultaneous wafer-level integration of both an LD and InP-based phase shifter is difficult on a Si platform. The critical challenge is the integration of materials with different bandgaps for the LD and phase shifter. On a typical InP wafer platform, an epitaxial regrowth process is a promising way to reduce fabrication and assembly costs of electro-absorption modulator integrated distributed feedback (EA-DFB) lasers. Therefore, epitaxial regrowth of III-V semiconductors is desirable for heterogeneous wafer-level integration on a Si platform. However, the crystal quality of the conventional 2–3-µm-thick InP-based devices is degraded by the large thermal strain produced during epitaxial growth on Si. This is because the critical thickness for the epitaxial growth of the InP-based materials on a Si substrate is only around 430 nm [12,13]. For this reason, micro-transfer printing [6] of LDs and phase shifters is typically required. It provides an integration of different materials at low temperature and makes it easy to sort out defective devices before integration. However, each device must be precisely aligned to the Si waveguides and then bonded to them. The resulting low throughput is not suitable for integrating a large number of devices.
To reduce the total thickness to less than the critical thickness, we focus on membrane lateral p-i-n and p-n diodes. Recently, by combining the direct bonding of a thin InP template and epitaxial regrowth of multiple-quantum-well (MQW) and InP-bulk layers, directly modulated membrane buried heterostructure (BH) lasers have been fabricated on a SiO2/Si substrate [14]. This approach is the key to overcoming the hurdles for the integration of materials with different bandgaps on a Si substrate. Using the membrane structures, in this paper, we demonstrate a wafer-level integration of a DFB LD and a high-efficiency carrier-depletion InGaAsP-bulk MZM on Si waveguide circuits. The membrane DFB LD and the phase shifters of the MZM are 230-nm-thick lateral p-i-n and p-n diodes. They are integrated with a Si-waveguide Mach-Zehnder interferometer (MZI) on a silicon-on-insulator (SOI) wafer. We employed direct bonding of the MQW/InP layer and the epitaxial regrowth of the InGaAsP-bulk layer on a bonded InP template. Notably, the membrane structure also has the advantages of easy optical coupling to compact 220-nm-thick Si waveguide circuits [15] and high modulation efficiency of the phase shifters due to the high optical confinement [11,16]. The fabricated LD-MZM performed a 50-Gbit/s non-return-to-zero (NRZ) operation, which is a higher operation speed than that in our preliminary work [17]. In addition, the small VπL of 0.4 Vcm, which overcomes the performance limit of the typical Si MZM, enables us to construct 500-µm-long phase shifters and a LD-MZM with the total footprint of only 1.9 × 0.31 mm2.
2. Design
Figure 1(a) shows a schematic of the integrated membrane LD-MZM device. The DFB LD and two InGaAsP-bulk phase shifters are integrated on a Si waveguide circuit containing an MZI. The length of both the DFB laser active region and phase shifters is 500 µm. The bidirectional output of the DFB LD is coupled to an LD power monitor and the input arm of the 2 × 2 MZM. At both chip facets, 3 × 3-µm2 silica-based (SiOx) cores are formed for a low-loss fiber coupling, and integrated on the inversely tapered Si waveguide, as shown in Fig. 1(b). The refractive index of the SiOx cores is designed to match the mode field diameter of a high-numerical-aperture (NA) fiber [18]. The SiOx SSC enables easy butt coupling to an optical fiber. The 220-nm-thick Si waveguide efficiently couples to the low-refractive-index SiOx core through a 300-µm-long inverse taper with a taper tip width of 100 nm. As shown in Fig. 1(c) and (d), both the LD and phase shifters have membrane lateral p-i-n and p-n diodes with 230-nm-thick InP layers. Since the effective refractive index of the membrane InP layer is comparable to that of widely developed 220-nm-thick Si waveguides, the membrane devices can be easily coupled to mature Si photonics circuits by using tapered waveguides [17]. In addition, by controlling the Si waveguide width under the III-V membrane devices, we can control the optical confinement factor in the active III-V region.
Fig. 1. (a) Top view of integrated DFB LD and MZM on Si platform. Cross-sectional views of (b) SSC, (c) DFB LD, and (d) phase shifter of MZM.
The membrane DFB LD is a lateral current injection p-i-n diode, which has a buried MQW core, whose photoluminescence (PL) peak wavelength is around 1.52 µm for operation in the C band [15]. As shown in Fig. 2(a), the 600-nm-wide MQW core optically couples to the 840-nm-wide and 220-nm-thick Si waveguide core. By optically coupling to the low-loss Si core, the fill factor into the active region can be controlled while maintaining a small overlap with the large-loss p-type InP region. This is beneficial for reducing the internal loss of the LD. The total fill factor in the six-period quantum-well layer was designed to be around 4%. Figure 2(b) shows a side view of the DFB LD. A uniform grating is made by using a 20-nm-thick SiN layer. To achieve single-mode lasing, we designed a uniform grating on the top surface of the InP layer and a width-modulated Si core under the MQW core [15]. The moderate refractive index of the SiN layer is suitable for fabricating a relatively low-coupling coefficient (33 cm−1) cavity with a large fabrication-error tolerance. The width modulation of the Si core forms the defect mode in the stopband. Schematics of the band diagram and the design of the Si core are shown in Fig. 2(c). In the schematic, the red and blue lines show stopband edges at longer and shorter wavelengths. The band bending is formed by modulating the width of the Si core to form a defect mode [15]. With this configuration, the cavity mode can be precisely controlled by the mature Si core patterning process. Here, to achieve single-mode lasing for the defect mode, the modulation depth was set to 7 nm, which provides the largest difference in the threshold gain between the fundamental and second modes. In addition, spatial hole burning can be suppressed by the 50-µm-long modulation grating. The Si core widens from 840 to 920 nm.
Fig. 2. (a) Cross-sectional mode field pattern of DFB LD. (b) Side view of DFB LD. (c) Diagram of stopband and design of Si core.
The phase shifter is a 230-nm-thick lateral p-n diode [11,16]. The PL peak wavelength of the buried InGaAsP-bulk core is around 1.3 µm. When reverse bias is applied to the phase shifter, the refractive index is changed by the large carrier-plasma effect, band-filling effect, and F-K effect. Figure 3(a) shows the calculated mode field pattern of the phase shifter. There is no Si waveguide under the phase shifter section, which enables us to increase the optical confinement in the buried core. Note that the large refractive-index contrast between the InP and SiO2-cladding films enables us to obtain high optical confinement compared with that of the typical vertical diode. Since the cross-sectional area of p-n junction is much smaller than that of vertical p-n junctions, the capacitance can be reduced while maintaining high modulation efficiency. Figure 3(b) shows the calculated relationships between the fill factors and the width of the InGaAsP-bulk core formed in the 230-nm-thick InP layer. The thickness of the core ranges from 50 to 150 nm. Here, we set the area of the InGaAsP-bulk core to 0.6 × 0.1 µm2, as we did in our previous work [16]. The fill factor of around 30% is smaller than that of the typical carrier-depletion Si MZM; however, the improvement of the refractive index change due to the carrier-plasma, band-filling, and F-K effects is large enough to overcome the VπL limit of the Si MZM. The previously reported membrane MZM with the 0.6-x-0.1-µm2 InGaAsP-bulk core showed modulation efficiency of around 0.4 Vcm in the C band [16].
Fig. 3. (a) Cross-sectional mode field pattern of phase shifter. (b) Calculated relationships between fill factor and InGaAsP width with various core thicknesses.
We designed the InP and Si tapers for mode transfer between the Si waveguide circuits and membrane InP-based devices. Figure 4(a) and (b) shows calculated side and cross-sectional views of the propagation mode fields from the LD and Si waveguide and from the waveguide to the phase shifter, respectively. As shown in Fig. 4(a), the output light from the LD, which is the optical mode coupled with the III-V and Si waveguides, first butt couples to the 1.5-µm-wide InP and 840-nm-wide Si cores. In the 50-µm-long taper, the InP and Si cores narrow to 100 and 440 nm, respectively. At the end of the taper, the light butt couples to the 440-nm-wide single-mode Si waveguide. The Si waveguide then couples to the phase shifter in the MZM, as shown in Fig. 4(b). The Si waveguide first couples to the 100-nm-wide InP and 440-nm-wide Si cores. Then, the InP core widens to 1.5 µm, and the Si core narrows to 100 nm. At the taper end, the light butt couples to the phase shifter. Thanks to the effective refractive-index matching between the InP and Si layers, both tapers are very compact, and the low aspect-ratio taper tip can be easily fabricated by using mature fabrication techniques.
Fig. 4. Calculated mode fields (a) from LD to Si waveguide and (b) from Si waveguide to phase shifter.
3. Fabrication
We fabricated the integrated LD and MZM using the following procedure. First, a Si waveguide layer was patterned on a 220-nm-thick Si layer on an SOI wafer. Then, an SiO2 cladding film was deposited, followed by polishing of the SiO2 surface by a chemical mechanical polishing process. Next, an InP wafer containing the MQW layer was bonded by using an oxygen plasma-assisted bonding method, followed by removal of the InP substrate [Fig. 5(a)]. After that, the MQW layer except for the LD area was etched, with a 50-nm-thick InP template layer remaining on the wafer [Fig. 5(b)]. Then, a 100-nm-thick n-type InGaAsP bulk layer and InP capping layer were regrown on the template [Fig. 5(c)]. Here, the InGaAsP bulk layer has a donor concentration of 3 × 1017 cm−3, and the InP layer above and below the core is intrinsic. The InGaAsP-bulk layer was regrown by metalorganic vapor phase epitaxy (MOVPE) at around 600°C. Both the LD and phase shifter cores were patterned [Fig. 5(d)] and then buried in a 230-nm-thick intrinsic InP layer by MOVPE [Fig. 5(e)]. After that, a 50-nm-thick InGaAs was regrown for a contact region, and then donor and acceptor regions were formed by Si ion implantation and Zn thermal diffusion, respectively. During this process, Zn atoms slightly diffuse into the n-type InGaAsP core and form the p-n junction in the core for the phase shifter [11]. Next, a SiN layer was deposited and then patterned to form a uniform grating on the InP surface. Then, an InP mesa region and InP tapers were patterned, followed by electrode formation. Finally, in the backend process SiOx core was deposited and patterned to form SSCs.
The key process is the on-Si regrowth of the InGaAsP-bulk layer for the high-efficiency MZM. This process enables us to integrate both the LD and MZM in a wafer-level integration process. We have checked the quality of the InGaAsP-bulk film regrown on a test wafer. Figure 6(a) shows the measured PL spectrum of the InGaAsP-bulk layers regrown on an InP-template bonded on a SiO2/Si substrate and a conventional InP wafer as a reference. In the experiment, the thicknesses of the InP template and InGaAsP-bulk layer were 50 and 150 nm, respectively. Here, a donor was not doped into the InGaAsP layer. There is no clear difference in the spectrum between the two InGaAsP-bulk layers. Since the InGaAsP-bulk layer thickness is less than the critical thickness, high-quality crystal was obtained by using the epitaxial regrowth on the Si substrate. Figure 6(b) shows the measured PL intensity map of the InGaAsP-bulk layer regrown on the bonded InP template. We confirmed a uniform intensity distribution over a large area. Notice that, in our previous work, we already confirmed that the bonded MQW layer was not damaged during the epitaxial regrowth process [12,13].
Fig. 6. (a) PL intensity spectrum of regrown InGaAsP-bulk layer on bonded InP template and InP wafer. (b) PL intensity map of regrown InGaAsP-bulk layer on bonded InP template.
Figure 7(a) shows a microscope image of the fabricated LD-MZM device. The small footprint, excluding the fiber-coupling region, is only 1.9 × 0.31 mm2 thanks to the compact Si waveguide circuits and short tapers and phase shifters. Figure 7(b) and (c) show cross-sectional scanning electron microscope (SEM) images, and (d) and (e) show transmission electron microscope (TEM) images of the DFB LD and phase shifter, respectively. The small active cores are buried in the membrane InP layers on the Si-waveguide circuits.
Fig. 7. (a) Microscope image of fabricated device. Cross-sectional SEM images of (b) LD and (c) phase shifter. Cross sectional TEM images of (d) LD and (e) phase shifter.
4. Measured characteristics
First, we measured the optical output power from the LD monitor port and the MZM output ports using a large-diameter photodetector (PD) directly facing the chip facet. The diameter of the PD is large enough to detect the total output power from the two MZM output ports. Figure 8(a) shows the measured output power at 25°C. From the LD monitor port, the output power was 6.2 mW at the LD injection current of 80 mA. The threshold current was only 4.5 mA thanks to the small active area of the membrane DFB LD. Although the maximum output power is dominated by thermal rollover, the output power can be increased by increasing the active-region length [15]. The total output power from the MZM output ports was 4.4 mW at 80 mA. The low losses of the InGaAsP phase shifters, InP/Si tapers, and Si waveguides contribute to obtaining the high output power. We also measured the fiber-coupled output power from the two MZM output ports by butt coupling them to the high NA fiber. Since we didn’t apply DC voltages to the phase shifters, the light was emitted from both MZM output ports. To evaluate the total output power, we summed the measured optical power from the two MZM output ports. Figure 8(a) also shows the measured total output power from the two MZM output ports. The total output power was around 3.2 mW at the injection current of 80 mA. From the measured results, fiber coupling loss was estimated to be only around 1.5 dB, thanks to the low-loss SSC with the inverse Si taper and SiOx core.
Fig. 8. (a) Measured L-I curves. (b) Measured Vπ of stand-alone MZM device. (c) Measured EO response at DC voltage of 3 V. (d) Measured relationship between 3-dB bandwidth and DC voltage.
Next, we characterized the Vπ of the fabricated MZM. In the experiment, light with various wavelengths was input from an external tunable laser diode to a stand-alone MZM fabricated on the same wafer. The length of the phase shifter of the stand-alone MZM was also 500 µm. Figure 8(b) shows the relationship between the Vπ and wavelength of the stand-alone MZM. In the C band, the measured Vπ ranged from 7.6 to 8.2 V, which corresponds to the VπL of around 0.4 Vcm. The measured value is around three times smaller than that of the typical carrier-depletion Si MZM (> 1 Vcm). The wavelength dependence of the Vπ was relatively low and almost comparable to that of our previously reported InGaAsP/Si carrier-accumulation phase shifter [19]. We also checked the Vπ of the integrated LD-MZM device by sweeping the DC voltage of one of the phase shifters and applying LD current of 5 mA. We confirmed the measured Vπ was around 7.5 V, which was close to the measured value of the stand-alone MZM in the C band.
We also evaluated the dynamic characteristics of the fabricated LD-MZM. First, we characterized the EO bandwidth. The RF signal was applied to one of the phase shifters with DC bias through a bias-T. Here, the signal and ground electrodes of the phase shifter were connected with a 50-Ω load, which is beneficial for demonstrating a higher operation speed than the case without it in the preliminary work [17]. Figure 8(c) shows the measured and averaged frequency response at DC voltage of 3 V. Here, the LD current was 60.6 mA. The measured 3-dB bandwidth was around 31 GHz at the DC bias of 3 V. The bandwidth is comparable to that of a recent Si MZM (∼35 GHz) [20]. Figure 8(d) shows the relationship between the 3-dB bandwidth and applied DC voltage. With increasing DC voltage, the 3-dB bandwidth increases due to the reduction of the junction capacitance. The 3-dB bandwidth saturates at DC voltage over 3 V.
Lastly, we measured eye diagrams for NRZ signals. Figure 9(a) shows the experimental setup. We set the LD current to 60.6 mA. The RF signals were input from a pulse-pattern generator (PPG) to one of the phase shifters. The electrode of the phase shifter had a 50-Ω load. The monitor and MZM output ports were coupled by a high-NA fiber with refractive-index-matching oil. The fiber at the monitor port was terminated through an optical isolator. The fiber at the MZM output port was connected to an optical switch through another optical isolator. One of the outputs from the optical switch was connected to the optical spectrum analyzer to check the output spectrum, and the other output was connected to an erbium-doped fiber amplifier (EDFA). The amplified signals were detected by a high-speed p-i-n PD, whose photocurrent was fed into a sampling oscilloscope to measure the eye diagrams.
Fig. 9. (a) Measurement setup. (b) Measured spectrum of output light. Measured eye diagrams for NRZ signal of (c) 28, (d) 40, and (e) 50 Gbit/s, respectively.
Here, the LD-MZM did not have phase tuning heaters. Considering the phase- and power-balances between the two arms, the DC bias of the phase shifter was 2 V to maximize the extinction ratio at the lasing wavelength, although the 3-dB bandwidth at that voltage was smaller than the maximum value. We checked the output spectrum with the DC bias and without RF input. Figure 9(b) shows the measured spectrum of the output light, which indicates single-mode lasing with the side-mode-suppression ratio (SMSR) of 56 dB and stopband width of around 2.5 nm. The lasing wavelength of around 1556 nm was well controlled by the width-modulated Si core cavity of the LD. Then, we input NRZ signals into one of the phase shifters. The pattern length was pseudo-random binary sequence 231−1. The peak-to-peak voltage at the PPG was set to 2.5 V. Figures 9(c), (d), and (e) show the measured eye diagrams for 28-, 40-, and 50-Gbit/s NRZ signals, respectively. The input RF signal at the data rate of 50 Gbit/s is shown in Fig. 9(a). The eyes opened to the 50-Gbit/s NRZ signal. The measured extinction ratio was 5 dB at 50 Gbit/s. Thanks to the high modulation efficiency of the phase shifter, the eyes opened with the reasonably low peak-to-peak voltage, even though the phase shifter length is only 500 µm. Here, we demonstrated single-arm driving because of the constraints of the experimental setup. However, if we apply push-pull driving, the driving voltage can be further reduced. In addition, by integrating phase-tuning heaters, we can set a higher DC voltage with a high extinction ratio and thus obtain operation at much higher speed with a large 3-dB bandwidth.
5. Conclusion
We demonstrated a wafer-level integration of a DFB LD and high-efficiency InGaAsP MZM using membrane lateral diodes on a Si platform. Thanks to the membrane structure, the LD and phase shifters, whose bandgaps were different, were integrated by the direct bonding of the MQW layer and the regrowth of the InGaAsP-bulk layer. The integrated LD-MZM showed fiber-coupled output power of around 3.2 mW and Vπ of 7.5 V with the 500-µm-long phase shifters. The high-efficiency phase shifters enable us to construct an LD-MZM device with a footprint of 1.9 × 0.31 mm2. Despite the short phase shifters, the fabricated device showed clear eye opening with 50-Gbit/s NRZ signal with the peak-to-peak voltage of only 2.5 V. The device overcomes the performance limit of conventional carrier-depletion Si MZMs. These results indicate the heterogeneously integrated membrane III-V semiconductor devices can reduce the cost, size, and power consumption of the future optical transceivers.
Disclosures
The authors declare no conflicts of interest.
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