GdF3 | Optical characterization of thin films

GdF3(Gadolinium Fluoride) has a refractive index near 1.59 at 550 nm and 1.53-1.63 at 193 nm, but in most cases, is considered a candidate for below-UV or IR applications owing to its large bandgap energy.

The requirement for low-loss optical components in the deep-ultraviolet (DUV) and vacuum-ultraviolet (VUV) regions has increased drastically because of the spread of excimer lasers. The use of oxide materials is limited due to their intrinsic absorption. GdF3 is considered a candidate for producing high-index layers for wavelengths below 200 nm. GdF3 films exhibit tensile stress that is independent of the substrate but is affected by the substrate temperature and thin film thickness.

There are few material materials with little intrinsic absorption that can be employed in the DUV region, with the range of high-index materials being particularly limited by the requirement for a large bandgap energy. MgF2 and SiO2 are often used as low-index materials in such applications, whereas GdF3, LaF3, Al2O3 are often used as high-index materials.

Some researches have reported that the microstructure-either columnar or cone-shaped columnar-influences the optical and mechanical properties of this thin film. A cone-shaped columnar microstructure will make the thin film more inhomogeneous; to avoid this, coating at an elevated substrate temperature is recommended.

Furthermore, the present trend of avoiding contamination with ZnS and other poisonous materials during deposition has allowed this material to be a candidate for use in IR applications.