Surface chemical composition and optical properties of nitrogen-doped Ba_0.6Sr_0.4TiO₃ thin films

The effect of nitrogen (N) doping on surface chemical composition and optical properties of Ba_0.6Sr_0.4TiO₃ (BST) thin films have been investigated using x-ray photoemission spectroscopy (XPS) and transmittance measurement. It was found that the nitrogen doping induced Ti^3+/4+ states and formed Ti–N bonds in BST films. With lower implantation energy, the oxygen vacancies can be effectively eliminated because of N dopants substitution. Unfortunately, the oxygen vacancies possibly increased when enhance implantation energy. In addition, the N-doped BST films indicated larger amount of perovskite phase and lower content additional phase at surface. The chemical shifts in XPS confirmed the forming of Ba–N and Sr–N bonds in N-doped BST films. The optical properties for as-grown and N-doped BST films were considered at wavelength range from 190 to 1100 nm. The optical constants, including refractive index n and extinction coefficient k, were calculated by fitting transmittance spectra with single Tauc-Lorenz and Lorenz models. The refractive index decreased largely with nitrogen implantation. Oppositely, the extinction coefficient increased with N dopants. The dispersion of refractive index in transparent region agreed well with a single electronic oscillator for all samples. The band gaps of N-doped BST films were much smaller than that of as-grown BST films because of localized N 2p states above the valence band edge.