Stress and optical constants in thin films of BaF2-PrF3: A potential Th-free infrared low-index evaporation material

Although PrF3 can be regarded as a potential infrared low-index evaporation material used in infrared antireflection to substitute for radioactive ThF4 because its thin films have a good transparency, the lower refractive index n and extinction coefficient k in the spectral range of thermal infrared, the greater tensile stress emerging in its layers will deteriorate the reliability and durability of antireflection coatings. In our investigation, BaF2-PrF3 thin films were deposited using electron beam evaporation from the sintered pellets of PrF3 admixed with BaF2. Stress in thin films was calculated from Stoney's equation on the basis of measuring the changes of radius of curvature of thin silicon strips. The stoichiometry of thin films was determined using the energy dispersive X-ray analysis (EDX). The optical constants of thin films were determined by fitting the measured spectral transmittance curves using Lorentz oscillators as a dispersion model. It can be observed that stress in BaF2-PrF3 thin films is obviously reduced with the increasing of the concentration of BaF2 in thin films and the sintered pellets. Moreover, it was also demonstrated that incorporation of BaF2 into PrF3 can reduce the refractive index of thin films, although BaF2-PrF3 thin films have a greater dispersion than that of PrF3 thin films. In addition, the lower value of extinction coefficient is also presented in thin films, like those of rare-earth fluorides.