Anisotropic optical properties of semipolar AlGaN layers grown on m-plane sapphire

The valence band order of AlxGa 1−x N is investigated experimentally by analyzing the anisotropic dielectric functions of semipolar (112¯2) AlGaN thin films grown on m-plane Al2O3. Point-by-point fitted dielectric functions are obtained by spectroscopic ellipsometry and corresponding inter-band transition energies are extracted. The known strain situation of the sample layers is used to correct for the small strain-induced energy shifts within k · p perturbation theory. It also is used to identify transitions related to the three valence bands. Transitions with E ⊥ c from the Γ9 valence band verify an inter-band bowing parameter of b=0.9 eV. The transitions with E || c allow determining the crystal field splitting energy which can be described by a linear interpolation between the values for GaN and AlN satisfactorily.