Determination of anisotropic optical properties of MOCVD grown m-plane α-(Al _x Ga_1−x )₂O₃ alloys

The anisotropic dielectric functions (DF) of corundum structured m -plane α -(Al x Ga 1− x ) 2 O 3 thin films (up to x = 0.76) grown on m -plane sapphire substrate by metalorganic CVD have been investigated. IR and visible–UV spectroscopic ellipsometry yields the DFs, while X-ray diffraction revealed the lattice parameters ( a , m , c ), showing the samples are almost fully relaxed. Analysis of the IR DFs from 250 to 6000 cm −1 by a complex Lorentz oscillator model yields the anisotropic IR active phonons E u and A 2 u and the shift towards higher wavenumbers with increasing Al content. Analyzing the UV DFs from 0.5 to 6.6 eV we find the change in the dielectric limits ε ∞ and the shift of the Γ-point transition energies with increasing Al content. This results in anisotropic bowing parameters for α -(Al x Ga 1− x ) 2 O 3 of b ⊥ = 2.1 eV and b ∣∣ = 1.7 eV.