Comparison of the Optical Properties of some High-Index Oxide Films prepared by Ion Beam Sputter Deposition with those of Electron Beam Evaporated Films

A typical thin film of a high-index oxide coating material deposited by electron-beam (E-beam) evaporation has a refractive index lower than the bulk value for the material. This indicates that the film is porous, having a density less than the bulk density. In addition, such films are often inhomogeneous, having a higher refractive index near the substrate surface. Thin films of the same materials deposited by ion beam sputter deposition (IBD) techniques tend to have higher and more uniform refractive indices, indicating that IBD films are denser and more homogeneous. We have measured the index of refraction n, the extinction coefficient k and the degree of inhomogeneity of several high-index oxide materials deposited by IBD and by standard E-beam evaporation. The materials investigated were the oxides of zirconium, tantalum, hafnium, niobium and aluminum. In all cases, the IBD films were found to have higher refractive indices than their evaporated counterparts. The IBD films, in general, exhibited an inhomogeneity which was the reverse of the inhomogeneity found in the E-beam films. This indicates a difference in the nucleation and growth of the films. Tantala films show an absorption band in the visible spectral region resulting from electron trapping sites. Thus, this material is probably not suitable for coatings for visible and UV laser systems.