Zinc gallate spinel dielectric function, band-to-band transitions, and Γ-point effective mass parameters

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 Γ-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 Γ-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 Γ-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 (ε∞=1.94) [M. Stokey, Appl. Phys. Lett. 117, 052104 (2020)].

Modeling of Optical Band-Gap Values of Mixed Oxides Having Spinel Structure AB2O4 (A = Mg, Zn and B = Al, Ga) by a Semiempirical Approach

“…More recently, for n -ZGO films grown by the MOCVD technique, a band-gap value of 5.25 eV has been reported on the basis of a detailed cathodoluminescence study. Moreover, this band-gap value agrees nicely with the direct band-gap value (5.27 eV) of the ZGO single crystal obtained by Hilfiker et al by spectroscopic ellipsometry analysis. From this detailed analysis of different techniques employed, it seems reasonable to assume a value of 5.2 ± 0.05 eV for the fundamental energy gap of n -ZnGa 2 O 4 from which an average optical band-gap value of 4.75 eV can be derived by means of eqs 6 and .…”

Manganese and Iron Doped Cadmium Gallate Spinel as a Multifunctional Inorganic Material

“…[8][9][10] In various spinels, ZnGa 2 O 4 has given rise to interesting magnetic, optical, diagnostic, etc. [11][12][13][14] The n-type cadmium gallate spinel could observed the similar trends. [15][16][17] Cadmium gallate and zinc gallate, both, are shown normal spinel having cubic structure.…”

Growth optimization, optical, and dielectric properties of heteroepitaxially grown ultrawide-bandgap ZnGa2O4 (111) thin film

“…The bandgap energy of a single crystal ZGO sample was observed to be 4.6 eV by Galazka et al, 18 using transmittance and reflectance spectroscopy and a calculated refractive index of 1.90 and 1.97 for wavelengths of 2 and 1 μm, respectively. 19 Recently, Hilfiker et. al.…”

“…have shown the variation in the dielectric function in the near-infrared to the deep-ultraviolet region with the critical point (CP) structures within the dielectric function, which are known to be caused by band-to-band transitions and associated exciton contributions. 19 However, the effect of growth temperature and oxygen pressure during the fabrication of the ZGO thin film has a significant effect on the optical properties, bandgap, and dielectric function.…”