Refractive index dispersion in Y2O3 thin films obtained by different methods

Abstract

We have studied refractive index dispersion in Y 2 O 3 thin films obtained by stepwise vaporization and high-frequency ion plasma spraying in different atmospheres. We have established that regardless of the method used to obtain the films, the spectral dependence of the refractive index in the visible region of the spectrum is mainly determined by transitions from the band of 2p states of the oxygen, forming the highest unoccupied level of the valance band, to the bottom of the conduction band, formed by the 4d5s states of the ytrrium. For the studied films, we have determined the parameters of the single-oscillator approximation, the dispersion energy, the ionicity of the chemical bond, and the coordination number.Key words: thin film, yttrium oxide, dispersion, refractive index.Introduction. Monolayer and multilayer films are used in fabrication of optical light filters and as antireflective coatings on optical components. Films based on rare-earth oxides, having good optical and dielectric properties and operating characteristics, have become widely used as thin-film materials. Among the rare-earth oxides, yttrium oxide Y 2 O 3 particularly stands out: films of this oxide are the least expensive to make compared with other rare-earth oxides. Furthermore, Y 2 O 3 is a good matrix for incorporation of trivalent rare-earth ions, since there is no need for local chemical charge compensation; and most compounds obtained on the basis of Y 2 O 3 activated by rare-earth metals have high luminescence and are widely used in electron beam devices and for visualization of ionizing radiation [1]. Although the optical properties of such films have also been studied previously [2, 3], the dispersion properties and their connection with the energy structure and crystal chemistry properties have not been investigated in detail. The interconnection between the conditions under which they are obtained and the optical properties of Y 2 O 3 thin films have also not been studied. This was the reason for our investigations in this work.Experimental procedure. Thin films of Y 2 O 3 of thickness 0.3-1.0 µm were obtained by stepwise vaporization under vacuum and by high-frequency ion plasma spraying on fused quartz substrates (v-SiO 2 , vitreous silica). High-frequency spraying was carried out in an atmosphere of either argon or argon with oxygen added, in a system using a magnetic field created by external solenoids for compression and additional ionization of the plasma column. As the starting material, we used ItO-I grade Y 2 O 3 . After deposition of the films, they were subjected to heat treatment in air at 950 o C. X-ray diffraction studies showed the presence of a polycrystalline structure in the films, regardless of the method by which they were obtained. Typical diffraction patterns for the thin films are shown in Fig. 1. The Y 2 O 3 films obtained by stepwise vaporization are the most disordered, as is apparent from the greater number of diffraction maxima. The Y 2 O 3 films obtained by high-frequency spraying are more order...