Anisotropic Dielectric Functions, Band-to-Band Transitions, and Critical Points in α-Ga₂O₃

Authors

Matthew J. Hilfiker, University of Nebraska-LincolnFollow
Rafal Korlacki, University of Nebraska-LincolnFollow
Riena Jinno, Cornell University
Yongjin Cho, Cornell University
Huili Grace Xing, Cornell University
Debdeep Jena, Cornell University
Ufuk Kilic, University of Nebraska-LincolnFollow
Megan Stokey, University of Nebraska-LincolnFollow
Mathias Schubert, University of Nebraska-LincolnFollow

Document Type

Article

Date of this Version

2-2021

Citation

Applied Physics Letters 118, 062103 (2021)

DOI: 10.1063/5.0031424

Comments

Copyright 2021 by AIP Publishing.

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Applied Physics Letters 118, 062103 (2021) and may be found at https://doi.org/10.1063/5.0031424

Abstract

We use a combined generalized spectroscopic ellipsometry and density functional theory approach to determine and analyze the anisotropic dielectric functions of an α-Ga₂O₃ thin film. The sample is grown epitaxially by plasma-assisted molecular beam epitaxy on m-plane sapphire. Generalized spectroscopic ellipsometry data from multiple sample azimuths in the spectral range from 0.73 eV to 8.75 eV are simultaneously analyzed. Density functional theory is used to calculate the valence and conduction band structure. We identify, for the indirect-bandgap material, two direct band-to-band transitions with M₀-type van Hove singularities for polarization perpendicular to the c axis, E_0,⊥=5.46(6) eV and E_0,⊥=6.04(1) eV, and one direct band-to-band transition with M₁-type van Hove singularity for polarization parallel to E_0,∥=5.44(2) eV. We further identify excitonic contributions with a small binding energy of 7 meV associated with the lowest ordinary transition and a hyperbolic exciton at the M₁-type critical point with a large binding energy of 178 meV.