Anisotropic dielectric function, direction dependent bandgap energy, band order, and indirect to direct gap crossover in <i>α</i>-(Al_<i>x</i>Ga_1−<i>x</i>)₂O₃ ( 0≤x≤1)

Mueller matrix spectroscopic ellipsometry is applied to determine anisotropic optical properties for a set of single-crystal rhombohedral structure α-(AlxGa1−x)2O3 thin films (0 ≤ x 1). Samples are grown by plasma-assisted molecular beam epitaxy on m-plane sapphire. A critical-point model used render dielectric function tensor and direct electronic band-to-band transition parameters, including the direction dependent two lowest-photon energy transitions associated with bandgap. We obtain composition dependence lowest separate bandgap bowing parameters perpendicular (bEg,⊥ = 1.31 eV) parallel (bEg,|| 1.61 electric field polarization lattice c direction. Our density functional theory calculations indicate from indirect characteristics between α-Ga2O3 α-Al2O3, respectively, we identify switch in band order where occurs whereas α-Al2O3 c. estimate that change at approximately 40% Al content. Additionally, characteristic critical point changes M1 type M0 van Hove singularity α-Al2O3.