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

The anisotropic dielectric functions (DF) of corundum structured m-plane a-(Al x Ga1-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 (2u ) 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 e (infinity) and the shift of the Gamma-point transition energies with increasing Al content. This results in anisotropic bowing parameters for a-(Al x Ga1-x )(2)O-3 of b (perpendicular to) = 2.1 eV and b ( divide divide ) = 1.7 eV.