Surface chemical composition and optical properties of nitrogen-doped Ba0.6 Sr0.4 Ti O3 thin films

The effect of nitrogen (N) doping on surface chemical composition and optical properties of Ba0.6 Sr0.4 Ti O3 (BST) thin films have been investigated using x-ray photoemission spectroscopy (XPS) and transmittance measurement. It was found that the nitrogen doping induced Ti3+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.