High performance La1-xAlxF3 nanocomposite coatings prepared by a co-evaporation technique

Abstract

La1-xAlxF3 nanocomposite coatings with various amounts of AlF3 in LaF3 were deposited on the fused silica substrate using double-source electron beam co-evaporation. The effects of the material composition on the chemical composition, optical properties, microstructure properties, and residual stress of these deposited coatings were investigated. The results showed that although the doping of AlF3 led to a reduction in the refractive index, the overall performance of the nanocomposites showed remarkable improvement. The increase in the AlF3 content of prepared nanocomposites caused a decrease in the crystallinity, surface roughness, residual stress, and optical loss. Compared to polycrystalline LaF3, the La1-xAlxF3 nanocomposite (x ≥ 0.3) retained an amorphous structure, showcasing smooth surface morphology and lower absorption. As a result, the total optical loss was reduced. Additionally, the tensile stress was also mitigated to a certain extent, which means that the nanocomposite material was less prone to cracking. Finally, a 193 nm-La0.70Al0.30F3/AlF3 high reflective coating was prepared, which achieved an improved reflectance of 98.7% and better film quality compared to the pure LaF3/AlF3 sample. These findings clearly indicate the significant potential of La1-xAlxF3 nanocomposites for developing deep ultraviolet band thin films, extending even into the vacuum ultraviolet band.