Anisotropic infrared optical properties of GaN and sapphire

Infrared polarized reflectance at three incidence angles (18°, 45°, and 75°) was measured from 300 to 1200 cm−1 for sapphire and GaN/sapphire at 295 K. The approximately 1.5 μm thick film of GaN was grown directly on c-plane sapphire by metal-organic chemical vapor deposition. The optical anisotropy of the sapphire substrate and GaN film was studied by fitting with a suitable model the five- and three-mode polarized reflectance spectra, respectively, at the three different incident angles all simultaneously. In the procedure, we adopted a fitting technique, i.e., fitting the first numerical derivative of the polarized reflectance spectra to improve the accuracy of the phonon parameters and to overcome the inconsistency between the model and measurement over the whole frequency range. Excellent agreement has been obtained between the measured and fitted first derivative reflectance spectra for both the sapphire and GaN/sapphire samples. The infrared optical parameters thus obtained for sapphire and GaN enhance those reported previously.