We report the deposition of high-quality SiO2 and SiNxOy layers onto GaN/sapphire templates by photochemical vapor deposition (photo-CVD). It was found that the 0.845 nm root-mean-square roughness observed from the photo-CVD SiNxOy layer was much smaller than that observed from photo-CVD SiO2 layer with the same thickness. GaN metal-insulator-semiconductor (MIS) capacitors with these insulating layers were also fabricated. With an applied electric field of 4 MV/cm, it was found that the leakage current densities were 1 x 10(-8) and 6 x 10(-7) A/cm(2) for the capacitors with photo-CVD SiNxOy and photo-CVD SiO2 insulating layers, respectively. It was also found that the breakdown field of the capacitors with photo-CVD SiNxOy could reach 13 MV/cm. The interface state density at the SiNxOy/GaN interface was also found to be reasonably low. © 2005 The Electrochemical Society. All rights reserved.