# A Widely Tunable Three-Section DBR Lasers for Multi-Species Gas Detection
## 2 Experimental Details  
Figure 2a shows the DBR laser's photograph. It is composed of three segments. The epitaxial layers of the device were grown on an n-InP substrate by the low pressure metal-organic chemical vapor deposition (MOCVD) with a three-step procedure. The active region composed of four 1.2% compressive strained 7-nm InGaAs quantum wells and five  
Figure 1: Absorption spectrums of different gas species in the 1720-1840 nm wavelength range which are extracted from the HITRAN (high-resolution transmission molecular absorption) 2016 database.  
0.25% tensile strained 12-nm InGaAsP barriers embedded in the separate confinement heterostructure (SCH) was grown in the first step, exhibiting a 1.78 um photoluminescence (PL) peak at room temperature (RT). After the first epitaxy, a 150-nm SiO\({}_{2}\) layer was deposited as the mask, the active layers in the phase and DBR sections were removed selectively by wet chemical etchings. We usually use the selective 311 etchant (H\({}_{2}\)SO\({}_{4}\):H\({}_{2}\)O\({}_{2}\):H\({}_{2}\)O = 3:1:1) to etch InGaAsP material, but the etching rate of 311 solutions is too fast for InGaAs materials. There is almost no report on the wet etching scheme of butt-joint integrated of InGaAs quantum-wells. According to the article [22], the less phosphorus composition in InGaAsP, the faster the etch rate. After experimental exploration, we adopted optimized solutions (H\({}_{2}\)SO\({}_{4}\):H\({}_{2}\)O\({}_{2}\):H\({}_{2}\)O = 2:1:5) which can avoid severe undercut at the interface of the passive and active regions. In the second growth step, the 340-nm InGaAsP (PL peak at 1.45 um) waveguide layer was butt-jointed with the MQWs (multiple-quantum wells) layers in the gain section. Figure 2b shows the SEM image of the butt-joint interface with no voids. The good butt-joint morphology is a key factor to device performance. Figure 2c shows the comparison of PL spectrum from the gain and passive waveguide section. The effective index change in the grating material strongly affects the tuning range [21]. A longer wavelength (smaller bandgap) of butt-joint grating material results in a larger index change in a fixed carrier injection, thus yielding a larger tuning range. Here, we used a 1.45-um InGaAsP (1.45Q) as the butt-joint layer which has a 340 nm wavelength detuning compared to the wavelength at the gain section (\(\lambda_{\text{g}}\) = 1.78 um), ensuring both a sufficient low absorption loss and a large tuning range.  
After waveguide layer and the MQWs were butt-jointed by the MOCVD epitaxial growth, grating corrugations were fabricated in the 1.45Q of the DBR section using conventional holographic lithography and reactive ion etching (RIE), respectively. Afterwards, a p-InP cladding layer and a p+-InGaAs contact layer were implemented in the third step regrowth. When the whole layer structure was finished, a ridge waveguide with the width of 3 um was formed by photolithography and wet chemical etching. Then, a 50-um isolation gap between the three sections (DBR section, phase section, and gain section) was accomplished by moving the p+-InGaAs contact layer away. The following He\({}^{+}\) implantation was implemented to these gaps to increase the electrical insulation. To provide the lateral current confinement, a SiO\({}_{2}\) insulating layer was deposited with a 3-um-width current injection window opened above the ridge waveguide. A Ti-Au metal layer was used for a P-contact metallization. AuGeNi/Au was evaporated on the bottom side of the wafer after thinning the substrate. The chips composed of a 350-um gain section, a 100-um phase section and a 200-um DBR section were cleaved with both sides uncoated (Figure 2a).  
Figure 2: (**a**) Optical microscope image of the DBR (distributed Bragg reflector) laser. (**b**) Scanning electron microscope (SEM) image of the butt-joint interface. (**c**) Photoluminescence (PL) spectra of the butt-joint material and the active material [23].