Fabrication and linewidth characterization of III-nitride distributed feedback laser diodes with embedded surface gratings

III-Nitride distributed feedback (DFB) laser diodes offer an elegant chip-scale solution for single-frequency blue and green photon sources, but are challenging to fabricate due to the narrow Fabry-Pérot mode spacing, delicate p-contact layer, and lattice mismatch in the nitride material system. Here, we present MOCVD-grown InGaN-based DFBs fabricated with first- and third-order SiOx surface gratings embedded in the ITO p-cladding layer. We compare two fabrication methods: first using electron beam lithography of hydrogen silsesquioxane (HSQ), then a more cost-effective and scalable fabrication using holographic lithography. Blue DFBs exhibit single longitudinal mode behavior with 5 pm (resolution-limited) full-width at half maximum and 25 dB side mode suppression. We characterize the free-running frequency noise power spectral density of a 443 nm InGaN DFB with a first-order grating using a correlated delayed self-heterodyne frequency discriminator, yielding a free-running intrinsic linewidth of 685 kHz and a β-separation line integrated linewidth of 3.47 MHz.