Effects of terbium doping on the structural and optical properties of Ga₂O₃ films after high temperature annealing

In this study, we investigated the effects of rare earth terbium (Tb) doping on the structure and optical properties of Ga₂O₃ thin films. The films were deposited on sapphire substrates with Tb concentration ranging from 0.53 at.% to 15.35 at.% and subsequently subjected to high temperature annealing. For films with lower Tb concentrations (i.e., less than 1.25 at.%), X-ray diffraction indicates the formation of the monoclinic phase β-Ga₂O₃; film crystallinity, characterized by the sizes of crystalline Ga₂O₃ particles, improves as the annealing temperature increases from 800 °C to 1100 °C. In films with higher Tb concentration, transmission electron microscopy reveals the formation and distribution of terbium gallium oxide (i.e., Tb₃Ga₅O₁₂) and Tb oxides (i.e., TbO_1.75) in different regions of the films. Strong Tb³⁺ photoluminescence (PL) was observed in films with lower Tb concentrations, whereas both Tb³⁺ PL and emissions from oxygen vacancies decrease significantly with increasing Tb concentration. The ultraviolet–visible spectra of the films show that the optical transmittance decreases with increasing Tb concentration, while the optical bandgap has a weak dependence on Tb concentration. X-ray photoemission spectra indicate a decrease in oxygen vacancies but an increase of Tb³⁺ content as Tb concentration increases, consistent with the changes observed in the PL spectra. This study may contribute to expanding the applications of Ga₂O₃-based photonics.