On the structural and optical properties of cerium doped Ga₂O₃ films after high-temperature annealing

In this work, we investigated the structural and optical properties of cerium (Ce) doped β-Ga2O3 thin films, which were deposited by magnetron sputtering and subjected to high-temperature annealing. The Ce doping concentration was varied systematically from 0.04 at.% to 0.20 at.%. The as-deposited films were all amorphous, with topography that changed with Ce concentration. Crystallization of the films occurred upon annealing, and grain growth of β-Ga2O3 nanocrystals was observed via X-ray diffraction. For samples annealed at 1100 ^◦C in air ambience, X-ray photoelectron spectroscopy (XPS) indicated the presence of both Ce³⁺ and Ce⁴⁺ ions, and charge states of Ga and oxygen favoring oxygen-vacancies and Ga⁺ defects having similar concentration dependence on the Ce doping level. The optical bandgaps of the annealed films are larger than that of β-Ga2O3 (~4.9 eV), which is attributed to Al from the Al2O3 substrate diffusing into the films during the high-temperature annealing. For samples annealed at 1100 ^◦C in nitrogen, photoluminescence (PL) was significantly stronger than that of samples annealed in air; In both environments, PL intensities exhibited a similar trend with Ce doping concentrations, increasing for 0 to 0.06 at.%, and from there drastically decreasing. The variation in PL intensities correlates with the changes in oxygen-vacancies inferred from XPS analysis. Besides the emission bands from the Ga2O3 matrix, the PL spectra of Ce-doped samples show bands associated with optical transitions 5d (²D) → ²F5/2 and 5d (²D) → ²F7/2 of Ce³⁺. The photo-electric properties of the films for solar-blind detectors application were also examined. This work may contribute to the broader application of Ga2O3 based photonics and photo-electronics.