KAUST RepositoryAbstract: Gallium nitride (GaN), a wide-bandgap III-V semiconductor material with a bandgap wavelength λ g = 366 nm (for Wurtzite GaN) and transparency window covering the visible spectrum, has a large number of applications for photonics and optoelectronics. However, the optical quality of this material suffers from growth imperfections due to the lack of a suitable substrate. Recent studies have shown that GaN grown on (-201) β − Ga 2 O 3 (gallium oxide) has better lattice matching and hence superior optical quality as compared to GaN grown traditionally on Al 2 O 3 (sapphire). In this work, we report on the fabrication of GaN waveguides on Ga 2 O 3 substrate, followed by a wet-etch process aimed at the reduction of waveguide surface roughness and improvement of side-wall verticality in these waveguides. The propagation loss in the resulting waveguides has been experimentally determined to be 7.5 dB/cm.