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  • Anisotropic dielectric function, direction dependent bandgap energy, band order, and indirect to direct gap crossover in α -(Al x Ga 1−x ) 2 O 3 (0≤x≤1)
Title: Anisotropic dielectric function, direction dependent bandgap energy, band order, and indirect to direct gap crossover in α -(Al x Ga 1−x ) 2 O 3 (0≤x≤1)
Mueller matrix spectroscopic ellipsometry is applied to determine anisotropic optical properties for a set of single-crystal rhombohedral structure α-(Al x Ga 1− x ) 2 O 3 thin films (0 [Formula: see text] x [Formula: see text] 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 ([Formula: see text] = 1.31 eV) and parallel ([Formula: see text] = 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 α-Ga 2 O 3 and α-Al 2 O 3 , respectively, and we identify a switch in band order where the lowest band-to-band transition occurs with polarization perpendicular to c in α-Ga 2 O 3 whereas for α-Al 2 O 3 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 α-Ga 2 O 3 to M 0 type van Hove singularity in α-Al 2 O 3 .  more » « less
Award ID(s):
2044049 1808715
NSF-PAR ID:
10393091
Author(s) / Creator(s):
; ; ; ; ; ; ; ;
Date Published:
Journal Name:
Applied Physics Letters
Volume:
121
Issue:
5
ISSN:
0003-6951
Page Range / eLocation ID:
052101
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
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