Comprehensive microstructural and optical characterization of the thermal stability of aluminum-titanium oxynitride thin films after high temperature annealing in air

The thermal stability of two Al_yTi_1-y(O_xN_1-x) layers prepared by cathodic vacuum arc deposition with different oxygen content was studied after high temperature annealing of the samples in air. These layers were designed to be part of solar-selective coating (SSC) stacks. Compositional and microstructural characterization of the thin films was performed before and after the thermal treatment by elastic recoil detection (ERD), transmission electron microscopy and Raman spectroscopy.
Al_yTi_1-yN sample was stable after 2 hours of annealing at 450ºC. Initial stages of the formation of a surface oxide layer after annealing at 650 ºC were observed both by ERD and Raman analysis. Contrarily, no changes were found after 2 hours annealing treatment either at 450 and 650ºC in the composition and microstructure of Al_yTi_1-y(O_xN_1-x) sample. In both samples the formation of a surface anatase TiO₂ film was reported after 2 hours annealing at 800°C. These compositional and microstructural changes were correlated with the optical properties determined by spectroscopic ellipsometry. A transition from metallic to dielectric behaviour with increasing annealing temperature was observed. These results complete the durability studies on the designed SSCs based on Al_yTi_1-y(O_xN_1-x) materials, confirming that these stacks withstand breakdown at 600ºC in air.