## 2 Epitaxial Growth on InP-on-Insulator Substrate
### Growth of Multi-Quantum-Well Double-Hetero Structure  
To integrate III-V photonic devices with different bandgap active regions on a Si substrate, the growth of a ternary or quaternary compound semiconductor is also necessary. In particular, a strained MQW is widely used as an active region for state-of-the-art DMLs and electro-absorption modulators [63; 64; 65; 66]. We demonstrated the growth of MQWs on InP-on-insulator followed by epitaxial regrowth of InP to fabricate a BH, by keeping the total thickness to less than 350 nm. We prepared an InP-on-insulator substrate that includes 50-nm-thick InP membrane layer by direct bonding. Then, a 120-nm-thick InP buffer layer, 6-period MQW, and 30-nm-thick InP cap layer were grown on it. Both InGaAsP/InGaAsP and InGaAsInGaAs have been demonstrated as MQW material [47; 67; 68]. The growth temperature was estimated to be around 630 \({}^{\circ}\)C, and the pressure was about 6.7 kPa under hydrogen carrier gas. The precursors for In, Ga, Al, As, and P were TMI, TEG, trimethyl-aluminum, AsH\({}_{3}\), and PH\({}_{3}\), respectively. The epitaxial layer quality was characterized by PL measurements. Figure 6a compares the PL spectra of an InGaAlAs MQW grown on an InP-on-insulator substrate and the same MQW grown on a standard InP substrate as a reference. To achieve high crystal quality, we controlled the residual strain in the MQW on InP-on-insulator substrate to be -680 ppm (tensile), which almost cancels the compressive thermal strain induced at the growth temperature [47; 69]. The peak PL intensity of the MQW grown on the InP-on-insulator substrate was 7.2 times greater than that of the reference grown on the InP substrate because both the excitation and collection efficiency of PL were enhanced due to the underlying InP/SiO\({}_{2}\)/Si interfaces [46; 51]. The slightly broadened full width at half maximum of the PL (30.0 to 33.4 meV) seems to be because of the higher excitation energy density, and there were no significant differences for spectral shapes or peak PL wavelength. Figure 6b shows the PL-intensity map measured with a PL microscope. The PL intensity was uniform and there were neither dark spots nor dark lines over the 500 \(\times\) 500 \(\upmu\)m\({}^{2}\) area, corresponding to a TDD of less than 4.0 \(\times\) 10\({}^{2}\)/cm\({}^{2}\). After the growth of the MQW on the InP-on-insulator substrate, the BH was formed using epitaxial regrowth of undoped InP. Figure 6c shows the PL-intensity map of an 80-\(\upmu\)m-long and \(\sim\)0.8-\(\upmu\)m-wide MQW-BH after regrowth of the InP layer. The PL intensity was very  
Figure 5: (**a**–**c**) Process flow of fabricating buried-heterostructure (BH) using MQW-DH on SiO\({}_{2}\)/Si substrate; (**d**) cross-sectional SEM image of BH on SiO\({}_{2}\)/Si substrate; (**e**) atomic force microscope (AFM) image of surface of InP membrane on SiO\({}_{2}\)/Si substrate after epitaxial growth. uniform in the active region. These PL characteristics clearly represent the superior quality of the MQW, although the growth was carried out on an InP-on-insulator substrate. To structurally evaluate the MQW-BH on the InP-on-insulator substrate, we carried out transmission electron microscope (TEM) observation. Figure 7a shows a cross-sectional bright-field scanning transmission electron microscope (BF-STEM) image of the BH fabricated using selective etching of the MQW followed by regrowth of InP. The BF-STEM image clearly shows that the BH was successfully fabricated on the InP-on-insulator substrate without dislocations. The relatively dark region along the original surface of the InP-on-insulator is attributed to the existence of As and Ga that originate from the etch-stop layer. Figure 7b,c show high-angle annular dark-field STEM (HAADF-STEM) images of the BH fabricated on the InP-on-insulator substrate and of MQWs grown on an InP substrate, respectively. Note that the former is the fabricated laser (i.e., after the epitaxial regrowth of InP, thermal diffusion of Zn, ion implantation of Si, etc.