Nitrogen diffusion in doped TiO₂ (110) single crystals: a combined XPS and SIMS study

Rutile TiO₂ (110) single crystals were doped with nitrogen by heating at 675 °C in flowing NH₃ gas. This caused a red shift in the band edge and an increase in visible region absorption. Nitrogen depth profiles obtained using dynamic secondary ion mass spectrometry (SIMS) could best be fitted by assigning three distinct diffusion coefficients. X-Ray photoelectron spectroscopy (XPS) indicated the presence of two surface nitrogen states with binding energies of 395.6 eV (substitutional N) and 399.8 eV (interstitial N). Angle resolved XPS measurements allowed us to link the XPS environments to the SIMS diffusion profiles. Subsequent air annealing at 500 °C led to asymmetric diffusion of the interstitial nitrogen into the bulk, removal of substitutional nitrogen into the gas phase, a decrease in the concentration of Ti³⁺ and a blue shift in the band edge. These changes could be related to variation in the optical band gap, and it was found that the principle cause of band gap narrowing was substitutional rather than interstitial nitrogen.