The improvement of optical reactivity for TiO₂ thin films by N₂-H₂ plasma surface-treatment

To improve the optical reactivity of TiO₂ thin film in visible-light region, sputter-deposited anatase film on slide glass substrate with 1200Å film thickness was surface-treated by N₂-H ₂ mixed gases plasma and additionally anneal-treated in N ₂ gases at 400°C for 2h. The absorption edges of plasma-treated sample and plasma plus anneal-treated one shifted from 363nm (3.4eV) to 428nm (2.9eV), and 354nm (3.5eV) to 428nm (2.9eV), while the absorptance for the two corresponding samples increased by 16% and 26%, respectively, in comparison with the as-deposited sample. Spectral absorption is well explained by Tauc-plot extrapolated band-gap using extinction coefficient k obtained from spectroscopic ellipsometry for the surface layers of the three samples, even though in the case of plasma plus anneal-treated sample showed only nominal band-gap value (2.65eV) due to the metallic behavior of extinction coefficient k in lower energy range (2.5eV). X-ray photoemission spectroscopy reveals the formation of TiO_2-xN_x and TiN deep into about 120Å thickness from the film surface for both plasma-treated and plasma plus anneal-treated samples. Secondary ion mass spectrography also detected high concentration of N element at the surface of the two corresponding samples. From the fact of formation of TiN and TiO_2-xN_x, N-doping in this work was proved as N substituted O in TiO₂ lattice. We confirmed this substitutional N-doping causes the narrowing of band-gap that resulted in the observed significant red shift of the absorption edge to the visible-light region.