Influence of preparation atmospheres on the structural and luminescent properties of Tb-doped CaF2 thin films

The effect of preparation conditions, Ar/H₂ and air atmospheres, on structural and luminescent properties of CaF₂:Tb thin films was investigated. The structure of the CaF₂:Tb thin films was confirmed by X-ray powder diffraction. Cracks and big grain sizes on the thin film's surface, prepared in air atmosphere, were observed in the scanning electron microscopy images. Atomic force microscopy images for the film prepared under air atmosphere showed big particle sizes with some cracks on the surface, while a dense and smaller particle size was formed for the film prepared under Ar/H₂ atmosphere. The measured transmittance showed that the thin film prepared under Ar/H₂ exhibited slightly higher transmittance in the visible to near infrared regions. Weak emission intensity from the Tb³⁺ ion with a decrease in lifetime was observed from the film prepared under air. Whereas the film prepared under an Ar/H₂ atmosphere showed a homogenous surface morphology with intense luminescence intensity from the Tb³⁺ ion. The variation in the Tb³⁺:⁵D₄ lifetime was analysed using rate equation analysis. The presence of Tb⁴⁺ ions in the thin films was confirmed by X-ray photoelectron spectroscopy. This work highlighted the importance of the preparation atmosphere on the photoluminescence properties of CaF₂:Tb³⁺ thin films.