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

Abstract

The anisotropic dielectric functions (DF) of corundum structured m-plane α-(Al x Ga1-x )2O3 thin films (up to x=0.76) grown on m-plane sapphire substrate by metalorganic chemical vapor deposition (MOCVD) have been investigated. Infrared (IR) and visible-ultraviolet (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 cm-1 to 6000 cm−1 by a complex Lorentz oscillator model yields the anisotropic IR active phonons Eu & A2u 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 Ga1-x )2O3 of b ⊥=2.1 eV and b ∥=1.7 eV.

Si-doped (AlGa)2O3 growth on a-, m- and r-plane α-Al2O3 substrates by molecular beam epitaxy

“…For the (101 ¯0) UID Ga 2 O 3 layers, the E go fitted under the assumption of the indirect and direct transitions were 4.78 and 5.31 eV, respectively. The E ind,go of 4.78 eV is much smaller than the theoretical values of 5.34 eV 2) and the values determined by spectroscopic ellipsometry (5.6 eV, 22) 5.62 eV 39,40) ), transmission spectra (5.3 eV), 9,41) and X-ray photoelectron spectroscopy (XPS) (5.3 eV, 42) 5.41 eV 5) ). The measured absorption spectrum might be dominated by directlike energy band absorption.…”