Infrared dielectric functions, phonon modes, and free-charge carrier properties of high-Al-content Al_xGa_{1- x}N alloys determined by mid infrared spectroscopic ellipsometry and optical Hall effect

We report on the analysis of a combined mid-infrared spectroscopic ellipsometry and mid-infrared optical Hall effect investigation of wurtzite structure c-plane oriented, crack-free, single crystalline, and high-Al-content Al_xGa_1-xN layers on 4H-SiC. For high-Al-content Al_xGa_1-xN, a two mode behavior is observed for both transverse and longitudinal branches of the infrared-active modes with E₁ symmetry, while a single mode behavior is found for the longitudinal modes with A₁(LO) symmetry. We report their mode dependencies on the Al content. We determine and discuss static and high frequency dielectric constants depending on x. From the analysis of the optical Hall effect data, we determine the effective mass parameter in high-Al-content Al_xGa_1-xN alloys and its composition dependence. Within the experimental uncertainty limits, the effective mass parameters are found isotropic, which depend linearly on the Al content. The combination of all data permits the quantification of the free electron density N and mobility parameters μ.