Sol-gel processing of LaF3 thin films

Abstract

Lanthanum fluoride thin films were prepared on silica glass substrates by a sol-gel method using trifluoroacetic acid (TFA) as a fluorine source. A coating solution was produced from lanthanum acetate and trifluoroacetic acid dissolved in 2-propanol. Trifluoroacetate gel films were formed by spin coating the sol, which were converted to LaF3 thin films by heat-treatment at 300-500°C for 10 min. The formation of LaF3 is considered to result from the reaction between lanthanum ions and the fluorine species generated by the decomposition of trifluoroacetate ions at elevated temperatures. The film fired at 300°C had a smooth surface with a grain size of 30 nm. Cracks and voids were only observed in the films fired at 400 and 500°C. The films fired at 300 and 400°C exhibited transmittance of greater than 90% in the visible region.