Magnetron sputtered TiAlON composite thin films. II. Optical and electrical properties

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Abstract
The optical and electrical properties of well characterized composite TiAlON thin films are presented. The samples display a low reflectance in the visible–near-UV spectral range (below 16%) and high absorption coefficients (1–4×105 cm−1). Standard deposition conditions, defined with respect to the desired functional properties as decorative coatings, yield the darkest shades with flat reflectance spectra (between 10% and 12%). Spectroscopic ellipsometry studies show that these features are intrinsically related to the two-phase structure. Differences among the samples result from the porosity level inherent to columnar microstructures. The generalized Maxwell Garnett theory is used to estimate the level of porosity in standard TiAlON. A void fraction of 0.15 is calculated, well correlated with the available microstructural observations. The electrical resistivities of 2 μm thick TiAlON films are determined by the van der Pauw method. At room temperature, values between 10−3 and 10−2 Ω m are measured for the different samples. The negative temperature coefficients of resistivity support the description as a separate grain structure. The conduction mechanism can be postulated as thermally activated tunnelling from one conductive (Ti,Al)Nx grain to the next, across the dielectric barrier formed by the oxide matrix. However, the temperature dependence of log(ρ/ρ0) does not follow a simple T−1/2 power law. The observed exponent increases continuously from this limiting value as the temperature is raised.

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