High performance LaAlF nanocomposite coatings prepared by a co-evaporation technique.

LaAlF nanocomposite coatings with various amounts of AlF in LaF 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 AlF led to a reduction in the refractive index, the overall performance of the nanocomposites showed remarkable improvement. The increase in the AlF content of prepared nanocomposites caused a decrease in the crystallinity, surface roughness, residual stress, and optical loss. Compared to polycrystalline LaF, the LaAlF 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-LaAlF/AlF high reflective coating was prepared, which achieved an improved reflectance of 98.7% and better film quality compared to the pure LaF/AlF sample. These findings clearly indicate the significant potential of LaAlF nanocomposites for developing deep ultraviolet band thin films, extending even into the vacuum ultraviolet band.