Epitaxial recrystallization and luminescence of CaAl₂O₄:Eu²⁺ thin films prepared on sapphire substrates

Eu²⁺ doped CaAl₂O₄ thin films were prepared on c-, a-, and r-plane sapphire substrates by a sputtering method, and then post-annealed for the recrystallization. Post-annealed films have out-of-plane epitaxial orientations of (010) and (001) CaAl₂O₄ on c- and a-plane sapphire substrates, respectively, whereas the films on r-plane were composed of two epitaxial phases of (210) CaAl₂O₄ and (210) CaAl₄O₇. In-plane relationships between films and substrates were also determined by an X-ray pole figure method, demonstrating that the films were epitaxially recrystallized, but not single crystals. The emission wavelengths were independent of the orientations of epitaxial films, showing a blue emission at 448 nm under 325 nm excitation, but the emission intensity depended on the film structure.