Electronic band structure, band-to-band transitions, and anisotropic dielectric functions of orthorhombic NdGaO3

A set of ultrawide bandgap single-crystal NdGaO3 samples cut with crystallographic surface orientations (001), (101), and (110) are investigated by using generalized spectroscopic ellipsometry. We report the complex anisotropic dielectric functions in the spectral range from 0.73 to 9.0 eV, for polarization along directions a, b, and c of the orthorhombic unit cell. A Kramers–Kronig consistent parameterized model was utilized to identify critical point structures and their association with band-to-band transitions, and the results from density functional theory (DFT) calculations for the electronic band structure of NdGaO3 are compared. A Cauchy parameter analysis for the three refractive indices is performed in the below bandgap spectral range. Dielectric functions along lattice directions a, b, and c and their respective bandgap energy values are very similar. Below the bandgap energy, the refractive index differences for directions a and b are very small (0.34% on average) while index differences with direction c (approximately 1.25%) lead to effective positive uniaxial optical properties (nc>na≈nb) toward the phonon modes spectral range. Our ellipsometry analysis determines the lowest band-to-band transitions of NdGaO3 to occur at Ea=6.46(6)eV, Eb=6.29(5)eV, and Ec=6.77(7)eV, i.e., Ec>Ea>Eb, while our DFT calculations predict the first transition to be lowest along the a axis. We discuss our findings in light of previous reports and present all parameters of our theoretical and experimental analyses.