High performance La_1-xAl_xF₃ nanocomposite coatings prepared by a co-evaporation technique

La_1-xAl_xF₃ 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 La_1-xAl_xF₃ 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-La_0.70Al_0.30F₃/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 La_1-xAl_xF₃ nanocomposites for developing deep ultraviolet band thin films, extending even into the vacuum ultraviolet band.