Determining the optical properties of a mixed-metal oxide film, Co3−x−yCrxFeyO4, with spectroscopic ellipsometry and atomic force microscopy

Abstract

The optical properties of a mixed-metal oxide thin film from the Co3−x−yCrxFeyO4 family has been determined from combined analysis of ellipsometry, atomic force microscopy, and transmittance measurements. These types of films are useful as solar absorbing films on a variety of float glass substrates to lower the solar heat gain admitted through a glass pane in a window, skylight, or door. A commercial product with a film composition in this family is sold under its registered trademark SOLARCOOL® glass. Each constituent metal oxide film was analyzed with variable angle of incidence spectroscopic ellipsometry and transmittance (T) measurements to determine their optical constants and film thickness. Surface roughness was measured with atomic force microscopy and included in the optical model as a known variable. The oxide films were optically modeled with a bulk layer and a known surface roughness layer. The mixed-metal oxide film was optically modeled with an effective medium approximation layer consisting of each constituent metal oxide Fe2O3, Cr2O3, and Co3O4. Best fits between measured and calculated data were obtained by allowing both the film thickness and the optical constants of each constituent oxide phase to vary in the model. Each metal oxide phase (Fe2O3,Cr2O3, Co3O4, and Co3−x−yCrxFeyO4) deposited onto heated soda-lime-silica float glass was identified with thin film x-ray diffraction analysis. The ratio of the metals in the mixed-metal oxide film was determined with x-ray fluorescence analysis. The film thickness was independently measured from in lens cross-sectional field emission scanning electron microscopy images to constrain the modeled film thickness. Spectra for the optical constants of each metal oxide and the mixed-metal oxide are presented and discussed.