## 3 Membrane Photonic Devices on Si Substrate
### Multi-Bandgap Photonic Devices Using Advanced Epitaxial Growth Technique  
#### 3.2.1 Multi-Wavelength Laser Array Using Selective-Area Epitaxial Growth  
To support wide-wavelength range WDM systems, such as local area network (LAN) WDM or coarse WDM (CWDM), each laser in the array should be made of an MQW that has an optimized PL peak wavelength. This is because the typical full-width at half maximum of the MQW gain spectrum is \(\sim\)50 nm, whereas LAN-WDM requires 35.6 nm and CWDM requires a 60- or 140-nm lasing wavelength range [73]. To integrate a WDM laser array with single epitaxy, we demonstrated the selective-area epitaxial growth of an MQW using an InP-on-insulator substrate. Figure 13a shows a schematic of the selective-area growth of the InGaAs MQW active layer. Due to the very high selectivity of the growth rate between the semiconductor surface and SiO\({}_{2}\) mask, the epitaxial layer thickness and crystal composition can be controlled by changing the mask width (\(w_{\text{m}}\)) and distance between the masks (\(w_{\text{g}}\)), which enables us to control the bandgap of the MQW layer. Therefore, the growth of the WDM laser array with a single growth becomes possible [33; 74; 75]. To control the PL wavelength for CWDM application, we selectively grew InGaAs 6-period MQWs on an InP-on-insulator substrate with the mask width ranging from 0 to 55 \(\upmu\)m and the distance between the masks fixed at 40 \(\upmu\)m. The epitaxial layer consists of an InP buffer layer (\(\sim\)70 nm), MQW layer, and InP cap layer (\(\sim\)25 nm) for blanket epitaxy (without selective growth mask). Figure 13b shows a cross-sectional SEM image of an MQW selectively grown on the InP-on-insulator substrate at the vicinity of the SiO\({}_{2}\) mask. The total III-V layer thickness with the largest mask was estimated as \(\sim\)310 nm, which is less than the critical thickness determined by thermal strain. Figure 13c shows the PL spectra for eight MQWs measured with a PL microscope. Although the peak PL wavelength for the widest selective-area mask is unclear due to the H\({}_{2}\)O atmospheric absorption peaks at around 1380 nm, we obtained multiple active regions with PL peak wavelengths over \(\sim\)150 nm by carrying out only one selective epitaxial growth on the InP-on-insulator substrate. We also observed uniform PL intensity with and without selective masks for a 40 \(\times\) 200 \(\upmu\)m\({}^{2}\) area, as shown in Figure 13d, which is wide enough to fabricate active regions of lasers. The eight-channel membrane DML array on a Si substrate was fabricated using the selectively grown MQWs [69]. We controlled the PL wavelength of the MQWs for LAN-WDM application. Figure 14a,b shows cross-sectional BF-STEM images of the selectively grown lasers on a Si substrate for the shortest wavelength channel and longest wavelength channel, respectively. Due to the different growth rate caused by selective epitaxy, each BH had a different total thickness but all were thinner than 350 nm. The lasers have a lateral-current injection structure and surface grating similar to those discussed in Section 3.1. The active-region length was 140 \(\upmu\)m. Figure 14c shows the lasing spectra of eight channel DMLs with bias currents from 15 to 23 mA. The deviations between the designed wavelength and measured lasing wavelengths were less than 2 nm, and the mean channel spacing was 860 GHz. In addition, the wavelength detuning between the PL-peak and lasing spectra was almost constant (\(\sim\)35 nm) for all channels. As a result, the lasing wavelength of each channel being almost at the amplified spontaneous emission peak. This indicates precise controllability of the lasing wavelength of multi-wavelength lasers with optimal gain mediums using selective-area epitaxial growth. Finally, dynamic characteristics of the DML array were evaluated at 25 \({}^{\circ}\)C. We demonstrated 25.8-Gbit/s direct modulation with a 2\({}^{31}-\)1 PRBS NRZ signal, and any pre-emphasis or equalization were not used.