Optical constants and film density of TiNxOy solar selective absorbers

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Abstract
Thin TiNxOy films were deposited on glass substrates in an activated reactive evaporation process. The influence of the reactive gas composition and pressure during deposition on the film density ρMass was investigated by grazing incidence x‐ray‐diffraction measurements. The density increases from 3.8 to 4.8 g/cm3 when lowering the total gas pressure ptotal from 10−3 h Pa down to 2.5×10−4 h Pa, whereas the ratio of the partial gas pressures of nitrogen and oxygen has no measurable effect on the film density. Data on the refractive index are presented, obtained from reflectance and transmittance measurements in the optical wavelength range (λ=0.3–2.5 μm). Bruggeman effective medium theory was applied to explain the optical behavior ñ(λ) as a result of the low density of the coatings. The high porosity of the films suppresses the metallic character of the phases TiN and TiO contained in the TiNxOy film. Its dielectriclike behavior in the infrared region is essential for low thermal emittances when deposited on copper to form a solar selective absorber.