Journal
APPLIED PHYSICS LETTERS
Volume 121, Issue 5, Pages -Publisher
AIP Publishing
DOI: 10.1063/5.0087602
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Funding
- National Science Foundation (NSF) [NSF DMR 1808715, NSF/EPSCoR]
- RII Track-1: Emergent Quantum Materials and Technologies (EQUATE) [OIA-2044049]
- Air Force Office of Scientific Research [FA9550-18-1-0360, FA9550-19-S-0003, FA9550-21-1-0259]
- ACCESS, an AFOSR Center of Excellence [FA9550-18-1-0529]
- Knut and Alice Wallenbergs Foundation
- University of Nebraska Foundation
- J. A. Woollam Foundation
- Japan Society for the Promotion of Science Overseas Challenge Program for Young Researchers [1080033]
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Mueller matrix spectroscopic ellipsometry is used to study the anisotropic optical properties of rhombohedral alpha-(AlxGa1-x)(2)O-3 thin films. The results show that the direction dependence of the lowest bandgap and band-to-band transitions changes with composition, and there is a band order switch at approximately 40% Al content.
Mueller matrix spectroscopic ellipsometry is applied to determine anisotropic optical properties for a set of single-crystal rhombohedral structure alpha-(AlxGa1-x)(2)O-3 thin films (0 <= x <= 1). Samples are grown by plasma-assisted molecular beam epitaxy on m-plane sapphire. A critical-point model is used to render a spectroscopic model dielectric function tensor and to determine direct electronic band-to-band transition parameters, including the direction dependent two lowest-photon energy band-to-band transitions associated with the anisotropic bandgap. We obtain the composition dependence of the direction dependent two lowest band-to-band transitions with separate bandgap bowing parameters associated with the perpendicular (b(Eg,&updatedExpOTTOM;) = 1.31 eV) and parallel (b(Eg,||) = 1.61 eV) electric field polarization to the lattice c direction. Our density functional theory calculations indicate a transition from indirect to direct characteristics between alpha-Ga2O3 and alpha-Al2O3, respectively, and we identify a switch in band order where the lowest band-to-band transition occurs with polarization perpendicular to c in alpha-Ga2O3 whereas for alpha-Al2O3 the lowest transition occurs with polarization parallel to c. We estimate that the change in band order occurs at approximately 40% Al content. Additionally, the characteristic of the lowest energy critical point transition for polarization parallel to c changes from M-1 type in alpha-Ga2O3 to M-0 type van Hove singularity in alpha-Al2O3. Published under an exclusive license by AIP Publishing.
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