Effect of heat treatment on the optical, structural, and morphological properties of Eu³⁺-doped CaTa₄O₁₁ thin films

Radio-frequency-sputtered CaTa₄O₁₁:Eu³⁺ thin films were obtained on sapphire substrates at 300 °C, with subsequent annealing at various temperatures. The CaTa₄O₁₁:Eu³⁺ thin films heat treated at 900 and 1000 °C exhibited a hexagonal phase with a dominant (111) peak, which is in contrast with the samples as-grown and annealed at 800 °C that exhibited an amorphous structure. The 280 nm-thick CaTa₄O₁₁:Eu³⁺ thin film heat treated at 900 °C exhibited numerous pebblelike particles, and the average grain size increased significantly at 1000 °C. Under 273 nm excitation, the CaTa₄O₁₁:Eu³⁺ thin films produced a dominant emission band at 614 nm, suggesting that the Eu³⁺ ions were located at sites without a center of inversion. The strongest emission intensity was observed for the thin film heat treated at 1000 °C, with a 4.48 eV bandgap. These results indicate that 1000 °C is the optimal temperature for annealing red light-emitting CaTa₄O₁₁:Eu³⁺ thin films.