Pulsed laser deposition and characterisation of perovskite-type LaTiO3−xNx thin films

LaTiO_3−xN_x films with high nitrogen contents (x ≈ 0.4–0.8) have been grown on (0 0 1)-oriented MgO and LaAlO₃ substrates by pulsed reactive crossed-beam laser ablation from a La₂Ti₂O₇ target in a one-step process. These films have a perovskite-type structure, similar to bulk polycrystalline LaTiO₂N. The nitrogen content in the films can be changed by selecting the nitriding source, i.e. NH₃ or N₂ gas pulse, and by varying the substrate temperature. Films deposited on MgO show a preferential (0 0 1) and (1 1 2) orientation, whereas films on LaAlO₃ are predominantly (0 0 1) oriented. N(2p) orbitals contribute to the top of the valence band, leading to a considerable decrease in the band gap energy from ∼4.0 eV for the parent LaTiO_3.5 to ∼2.4–2.9 eV for LaTiO_3−xN_x (x ≈ 0.4–0.8) films. This results in a strong visible light absorption at wavelengths below ∼500 nm. The band gap energies in LaTiO_3−xN_x films decrease with increasing nitrogen content. The values of the band gap energies for LaTiO_3−xN_x as a function of the N content agree well with other titanate-based perovskite-type oxynitrides, which confirms the proposed band gap model.