Air-based sputtering deposition of TiNxOy films for solar selective absorber coatings applications

Highlights: •TiN_xO_y thin films with tunable properties were produced by air-based sputtering. •The structures and properties of TiN_xO_y films changed gradually by varying air/Ar. •The trace components in air barely influenced TiN_xO_y films during sputtering. •Gradient TiN_xO_y-based coatings show promising performance for SSA applications. -- Abstract: Titanium oxynitride (TiN_xO_y) thin films with tunable electrical and optical properties were produced by sputtering using air as a reactive gas at a high base pressure of 1.3 × 10⁻² Pa. Pure N₂ and O₂ with a fixed ratio of 79/21 was also used as a reference reactive gas (denoted as S-air) at a low base pressure of 6.7 × 10⁻⁴ Pa to investigate the influence of trace components in air on the characteristics of the films. By merely changing either the air/Ar or the S-air/Ar flow ratio within suitable processing windows, crystal phases, microstructures, electrical resistivities, and optical properties could be tailored. In practice, the TiN_xO_y thin films produced by such air-based sputtering could turn from conductive, semiconductive, to dielectric and would correspondingly change from opaque, translucent, to transparent. For solar selective absorber (SSA) coatings evaluations, bilayer and gradient TiN_xO_y-based SSA structures were tested, respectively. The gradient TiN_xO_y SSA coatings exhibited the best performance with solar absorptance of 86.3% and thermal emittance of 11.3%. This facile air-based sputtering technique with such a gradient coating design has great potential for solar selective absorber coatings applications.