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 1200 A film thickness was surface-treated by N2-H2 mixed gases plasma and additionally anneal-treated in N2 gases at 400 deg C for 2 h. The absorption edges of plasma-treated sample and plasma plus anneal-treated one shifted from 363 nm (3.4 eV) to 428 nm (2.9 eV), and 354 nm (3.5 eV) to 428 nm (2.9 eV), 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 (2.65 eV) due to the metallic behavior of extinction coefficient k in lower energy range (2.5 eV). X-ray photoemission spectroscopy reveals the formation of TiO2-xNx and TiN deep into about 120 A 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 TiO2-xNx, 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.