**4. Conclusions**

In conclusion, the GaN-based DFB LDs that integrated with the surface and sidewall gratings were investigated by the FDTD method. The surface and sidewall gratings alongside the p-contact metal stripe on the ridge waveguide were fabricated by EBL and ICP etching on the same epitaxial

wafer. The DFB LD with the 20th order, 80% duty-cycle surface gratings showed a single wavelength emission at 398.86 nm with an FWHM of 0.32 nm under the electrical pulsed driving, and the DFB LD with the 20th order, 80% duty-cycle sidewall gratings obtained a peak emission at 399.95 nm with an FWHM of 0.23 nm. Additionally, both of the DFB LDs showed a narrower linewidth compared to that of the F–P LDs. Moreover, the FWHM of the DFB LDs with the ridge width of 2.5 μm was obviously lower than that of the DFB LDs using the same type of gratings with ridge widths of 5 or 10 μm, which indicates that the narrow ridge width was favorable for the narrowing of the linewidth. Furthermore, the sidewall grating DFB LDs possessed a slightly higher slope e fficiency than that of the surface grating DFB LDs with the same ridge width and period of gratings. Given that, the DFB LD with sidewall grating required a lower fabrication cost and achieved better device performance compared to the surface grating DFB LDs, which makes it a better choice for these applications. In addition, in order to further improve the performance of the DFB LDs, the optimization of the structure of the gratings and the conduction of the cavity surface coating process are required to reduce threshold current and improve slope e fficiency. Additionally, the side-mode suppression ratio and the high resolution spectral measurement are also required in future work.

**Author Contributions:** Conceptualization—Z.D. and J.L.; data curation—Z.D. and M.L.; methodology—Z.D. and M.L.; project administration—J.L.; supervision—J.L.; validation—W.X. and S.L.; writing original draft—Z.D.

**Funding:** This work was funded by Science Challenge Project (No. TZ2016003-2), National Natural Science Foundation of China (No. 61804140), and National Key R&D Program of China (No. 2017YFB0403103).

**Acknowledgments:** The authors would like to thank the Nanofabrication facility in Suzhou Institute of Nano-tech and Nano-bionics (CAS) for equipment access.

**Conflicts of Interest:** The authors declare no conflict of interest.
