The infrared optical functions of Al_xGa_1-xN determined by reflectance spectroscopy

Infrared reflectivity measurements have been carried out on samples with structures of GaN/sapphire and Al_xGa_1-xN/GaN/sapphire as well as sapphire substrates. Analyses of the reflectance data of sapphire using the Kramers-Kronig technique and fitting of the reflectance spectra of GaN and Al_xGa_1-xN samples using analytical expressions have been made. The high-frequency dielectric constant ε_∞ and the transverse phonon frequency ω_TO, are found to vary from 5.15 to 4.2 and from 559.7 to 586.4 cm^-1, respectively, when the composition x is varied from 0 to 0.35 at room temperature. The E2 mode, which arises from the disordered state of the alloys, has been observed in the reflectivity spectrum of Al_xGa_1-xN, and the intensity of the peak is enhanced by increasing the Al content.