Optical properties and hydrophilic behaviors of TaO_xN_y thin films with and without rapid thermal annealing

TaO_xN_y thin films were prepared using DC reactive sputtering of Ta target with the variation of (O₂ + N₂)/Ar ratio, followed by rapid thermal annealing (RTA) at 800 °C for 5 min. During RTA, the amorphous structure of the as-deposited films would transform to crystallized phases that might contain either TaON, or Ta₂O_5, or both. With the increase of (O₂ + N₂)/Ar ratio, Ta₂O₅ phase becomes more dominant, while TaON was formed in an opposite way. Elemental analysis also shows the same trend. Hydrophilicity and optical properties of these films were found to be dependent on phases formed after annealing. The optical band gap was measured and calculated to be in the range of 2.43 eV (510 nm) to 3.94 eV (315 nm). The films with low band gap values exhibited super hydrophilicity behavior under visible light irradiation, mainly due to their light absorption had been extended to visible light range. Clearly, it was due to the existence of TaON phase.