Authors Hilfiker, M. ; Stokey, M. ; Korlacki, R. ; Kilic, U. ; Galazka, Z. ; Irmscher, K. ; Zollner, S. ; Schubert, M.
Title Zinc gallate spinel dielectric function, band-to-band transitions, and Gamma-point effective mass parameters
Date 30.03.2021
Number 59982
Abstract We determine the dielectric function of the emerging ultrawide bandgap semiconductor ZnGa2O4 from the near-infrared (0.75·eV) into the vacuum ultraviolet (8.5·eV) spectral regions using spectroscopic ellipsometry on high quality single crystal substrates. We perform density functional theory calculations and discuss the band structure and the Brillouin zone G-point band-to-band transition energies, their transition matrix elements, and effective band mass parameters. We find an isotropic effective mass parameter (0.24·me) at the bottom of the G-point conduction band, which equals the lowest valence band effective mass parameter at the top of the highly anisotropic and degenerate valence band (0.24·me). Our calculated band structure indicates the spinel ZnGa2O4 is indirect, with the lowest direct transition at the G-point. We analyze the measured dielectric function using critical-point line shape functions for a three-dimensional, M0-type van Hove singularity, and we determine the direct bandgap with an energy of 5.27(3) eV. In our model, we also consider contributions from Wannier–Mott type excitons with an effective Rydberg energy of 14.8·meV. We determine the near-infrared index of refraction from extrapolation (1.91) in very good agreement with results from recent infrared ellipsometry measurements (·8····v=1.94<br />) [M. Stokey, Appl. Phys. Lett. 117, 052104 (2020)].<br />This work was supported in part by the National Science Foundation under award DMR 1808715, in the framework of GraFOx, a Leibniz-Science Campus partially funded by the Leibniz Association-Germany, by Air Force Office of Scientific Research under Award Nos. FA9550-18-1-0360 and FA9550-19-S-0003, by the Nebraska Materials Research Science and Engineering Center under Award No. DMR 1420645, by the Swedish Knut and Alice Wallenbergs Foundation supported grant Wide-bandgap semi-conductors for next generation quantum components, and by the American Chemical Society/Petrol Research Fund. Mathias Schubert acknowledges the University of Nebraska Foundation and the J. A. Woollam Foundation for financial support. DFT calculations were in part performed at the Holland Computing Center of the University of Nebraska, which receives support from the Nebraska Research Initiative.
Publisher Applied Physics Letters
Citation Applied Physics Letters 118 (2021) 132102

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