## 3 Membrane Photonic Devices on Si Substrate
### Multi-Bandgap Photonic Devices Using Advanced Epitaxial Growth Technique
#### 3.2.2 DFB Laser Integrated with MZM Using Epitaxial Regrowth of InGaAsP  
To support advanced high-baud-rate modulation formats for data centers, a WDM transmitter PIC that includes an MZM with a distributed feedback (DFB) laser array is desirable. For this purpose, heterogeneously integrated PICs using low-loss Si photonics devices for optical multiplexer and Mach-Zehnder interferometers and III-V-based devices for laser and phase modulators are promising. Therefore, it is essential to use BJ regrowth on a III-V-on-insulator substrate, which enables the integration of different-bandgap active regions on the same plane. Figure 15a shows the process flow of integrating BHs that include different bandgap active regions [71]. A six-period InGaAsP MQW-DH optimized for DFB lasers was originally grown on an InP substrate and transferred to a Si-photonics platform using direct bonding. The 220-nm-thick Si waveguide layer was fabricated in advance underneath the MQW layer. After removing the excess MQW region, a 20-nm InP buffer layer, 100-nm-thick Si-doped InGaAsP bulk layer optimized for phase shifters, and 30-nm InP cap layer were regrown on an InP membrane. We adopted a thin InP buffer layer to obtain a high confinement factor in the membrane phase shifter (>40%) for high modulation efficiency. These two active regions were then simultaneously patterned to mesa-stripes and buried with undoped InP. The total III-V layer thickness was 230 nm, which was thinner than the critical thickness determined by thermal strain. Figure 15b shows the normalized PL spectra for two BHs measured with a PL microscope. Figure 15c shows the PL-intensity map and PL-wavelength map for the two BHs placed in close to evaluate PL at one mapping. The different bandgap materials for the C-band DFB laser (\(\lambda_{\text{PL}}\): 1.52 \(\upmu\)m), MZM (\(\lambda_{\text{PL}}\): 1.3 \(\upmu\)m) and passive InP waveguide were integrated on the Si-photonics platform. The PL signals observed at the vicinity of InGaAsP MQW corners originate from the remaining selectively grown InGaAsP bulk layer because the growth rate accelerates at these regions. The micro-PL map revealed that both the MQW-BH and InGaAsP-bulk BH were uniform, although the InGaAsP bulk layer was selectively grown on an InP-on-insulator substrate. Since the InP buffer layer was only 20 nm, this growth technique should also be applicable for BJ regrowth, which enables the fabrication of EADFB lasers. Figure 15: (**a**) Process flow of integrating different-bandgap BHs using epitaxial regrowth; (**b**) normalized PL spectra for directly bonded indium gallium arsenide phosphide (InGaAsP) MQW and epitaxially grown InGaAsP bulk buried with InP; (**c**) PL-intensity and PL-wavelength maps for both BHs. Figure 16a shows a schematic of a DFB laser integrated with a MZM on a Si-photonic platform using our epitaxial regrowth technique. Both the DFB laser and phase shifters were 500-\(\upmu\)m long and 230-nm thick and optically connected to the Si waveguides and Si multimode interferometers (MMIs) underneath the III-V membrane layer. The Si interseater SSCs were also integrated for the low-loss fiber coupling. The lasing emission was obtained from both ends of the DFB laser, and two output waveguides were integrated so that the lasing characteristics with and without an MZM can be measured. Figure 16b,c shows cross-sectional SEM images of the fabricated chip for the DFB-laser section and phase-shifter section, respectively. The _p-i-n_ and _p-n_ diodes were fabricated using Zn thermal diffusion and Si ion implantation to the InP layers. Figure 16d shows the lasing spectrum of the output light measured by butt-coupling the HNAF. The laser current was 50 mA and MZM DC voltage was 4.5 V. These results indicate single-mode operation with a side-mode suppression ratio of around 55 dB. We also input radio-frequency signals to one arm of the MZM to evaluate its dynamic characteristics. Figure 16e shows the measured eye diagram for a 28-Gbit/s signal without 50-\(\upOmega\) termination.