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

Abstract

LaTiO3−xNx films with high nitrogen contents (x≈0.4–0.8) have been grown on (001)-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 (001) and (112) orientation, whereas films on LaAlO3 are predominantly (001) oriented. N(2p) orbitals contribute to the top of the valence band, leading to a considerable decrease in the band gap energy from ∼4.0eV for the parent LaTiO3.5 to ∼2.4–2.9eV for LaTiO3−xNx (x≈0.4–0.8) films. This results in a strong visible light absorption at wavelengths below ∼500nm. The band gap energies in LaTiO3−xNx films decrease with increasing nitrogen content. The values of the band gap energies for LaTiO3−xNx as a function of the N content agree well with other titanate-based perovskite-type oxynitrides, which confirms the proposed band gap model.