Polarized reflectance spectroscopy and spectroscopic ellipsometry determination of the optical anisotropy of gallium nitride on sapphire

The refractive indices n_⊥ (E⊥c) and n_∥ (E ∥ c) of the hexagonal GaN on sapphire substrates have been determined in the transparent region using the polarized reflection measurements. It is found that the difference in the refractive indices for E⊥c and E ∥ c is below 3% over the entire wavelength range measured, and ∈_∞ the high-frequency dielectric constant, is 5.14 for E⊥c and 5.31 for E ∥ c. Ellipsometry angles, Δ and ψ, have been calculated using the results of n_⊥, n_∥ and the thickness of the film, and an excellent agreement has been obtained between the calculated results and ellipsometric measured data.