Identifying orientation-dependent optical properties of single-crystalline β-Ga₂O₃ films

We explore the effect of crystallographic anisotropy on the optical properties of high-quality β-Ga₂O₃ thin films by conducting experimental measurements and theoretical simulations. High resolution x-ray diffraction measurements confirm the high-quality and the single crystalline quality, ruling out the effect of defects for all films. Raman spectra reveal the presence of anisotropy, as evident from a stronger B g ( 2 ) mode related to Ga_IO₄ chain libration in (100) orientation. Conversely, a stronger A g ( 10 ) mode corresponding to tetrahedral bonds is evident in the (010) orientation, while it is suppressed in the (100) orientation. Low-temperature photoluminescence spectra indicate the presence of intrinsic impurity-related emissions in the ultraviolet and blue regions for all films. An anisotropic bandgap is observed, wherein the lowest bandgap value is related to the (010) oriented sample. Spectroscopic ellipsometry measurements confirm different refractive indices and extinction coefficient values depending on crystallographic orientation, which are in good agreement with the theoretical values obtained by density functional theory, confirming the anisotropic characteristics. The theoretical calculations of charge density show that the strength of covalent bonding depends on the β-Ga₂O₃ orientation, while experimental findings demonstrate that as the covalent bonding character increases, the film bandgap and the refractive index decrease. The anisotropy of β-Ga₂O₃ with respect to the crystal orientation leads to variations in the extinction coefficient, refractive index, and bandgap energy.