In the present study, both the GaN buffer layer approach and the sapphire SiN treatment method were used to develop GaN films by metalorganic chemical vapor deposition (MOCVD) on a c-plane (0001) sapphire substrate. The growth was controlled in situ by 632.8 nm laser reflectometry. The GaN-grown layer structure was investigated via high-resolution X-ray diffraction (HRXRD). The GaN structural properties improved upon SiN treatment. In addition, spectroscopic transmittance was used to determine the change in the bandgap energy of GaN upon SiN treatment. Spectroscopic ellipsometric (SE) data (ψ and Δ) acquired in the wavelength range 400–1700 nm, were analyzed using a multilayer approach. The extracted refractive indices were found to follow a Cauchy-type dispersion. Upon SiN treatment, there is a blueshift and a decrease in the refractive index. At 600 nm, the GaN refractive index decreases from 2.395 to 2.374. The SE refractive indices measurements agree with the spectroscopic reflectometry (SR) results.
