Investigations of Dependence of Optical Behavior of β-Ga₂O₃ Substrate on Their Crystallographic Orientations

b-Ga₂O₃ has shown enormous potential in various deep UV applications. Due to lack of symmetry in monoclinic structure, the anisotropy in the optical properties of b-Ga₂O₃ is still not sufficiently understood. To address this research gap, we carried out detailed experimental and theoretical investigations of three crystallographic orientations, (100), (010), and (001) of high quality single crystalline b-Ga₂O₃substrates grown via melt growth method. We correlated the obtained spectroscopic ellipsometry and advanced optical spectroscopy findings with the density functional theory (DFT) calculations. XRD rocking curve analyses exhibits narrow FHWMs in the range of 200-300 arcseconds for all the films demonstrating the superior crystalline quality, confirming very low defect density and hence, ruling out the effect of structural defect influence on the optical properties. In addition, measurements based on the room temperature Raman spectroscopy reveal that different strong vibration modes depend on the film orientation, indicating that it also governs the intrinsic point defect structural configuration (i.e. tetrahedron or octahedron ones). Further, absorption measurements show an anisotropic behavior of bandgap edge, and result in distinct bandgaps, as well as an energy shift from 4.4 eV to 4.5 eV, depending on film orientation. Spectroscopic ellipsometry measurements at 632.8 nm show a similar anisotropic orientation dependence for the refractive index of all films. Moreover, temperature-dependent photoluminescence (PL) spectra and PL excitation (PLE) spectra of all films confirm that the emission produced by all studied b-Ga₂O₃ samples comprises different emission bands, which can originate from self-trapped excitons in shallow defect bands below the conduction band, while the PLE results confirm a similar bandgap shift for different orientations. Advanced temperature-dependent time-resolved photoluminescence (TRPL) findings further reveal that the carrier transition mechanism results in a shorter luminescence lifetime than trap states associated with conventional defects. As our experimental findings are in good agreement with the DFT calculations of the optical and electronic properties, they offer valuable new insights into the origin of the anisotropic behavior in the b-Ga₂O₃ films with different orientations of [100] crystallographic group, thus advancing the understanding of the optical properties of such emerging materials.