Determination of the anisotropic dielectric function for wurtzite AlN and GaN by spectroscopic ellipsometry

We demonstrate that variable angle spectroscopic ellipsometry (SE) is capable of determining both ordinary and extraordinary dielectric functions (DFs) in the transparent region of group III nitride wurtzite films even if the optical axis is oriented normal to the surface plane. In contrast to the so far used prism coupling technique, the SE method is neither restricted to layers deposited on lower refractive index substrates nor to available laser wavelengths. Application to AlN and GaN films grown on 6H-SiC substrates yields experimental data which can be represented in simple analytical form. The difference in energy dispersion for ordinary and extraordinary DFs is related to the effective optical band gap depending on the light polarization. Extrapolation of data to lower photon energies allows estimation of the ordinary and extraordinary high-frequency dielectric constants.