Optical and structural properties of sol-gel SiO2 layers containing cobalt

SiO2 coatings containing cobalt were prepared using the sol-gel method and the Si to Co nominal atomic ratio in the coatings was varied from 1.3 to 7. 1. The structure and optical properties of the coatings, heat-treated in ai r at 300 and 500 degreesC, were characterized using optical, x-ray diffract ion, and Auger depth profile measurements. The optical transmission data in the UV-visible range, in samples with low cobalt concentrations, show only the absorption bands associated with the tetragonal Co2+ regardless of the heat treatment temperature. Coatings with a larger amount of Co treated at 300 degreesC show absorption bands associated with both tetragonal and oct ahedral Co2+. When these coatings are heat treated at 500 degreesC, most of the cobalt migrates to the free surface of the coatings, where it is oxidi zed by the atmospheric oxygen and forms a top layer Of Co3O4. The thickness of the cobalt oxide layer depends on the heat treatment temperature and on the Co concentration. To describe the absorption bands of both tetragonal and octahedral Co, the Lorentz oscillator model was used to represent the c omplex effective dielectric function of the SiO2 cobalt doped layers. The f requency dependence of the optical constants, in the UV-visible range for t he top cobalt oxide layer, was obtained with a Jellison-Modine model. (C) 2 001 American Vacuum Society.