Nitrogen Doping in Epitaxial Self-Oxidized BaTiO₃ Ferroelectric Thin Films

Epitaxial oxynitride films have promising genuine electronic properties but are very challenging to engineer due to a detrimental imbalance between nitriding and oxidation. The crystalline growth of BaTiO₃ thin films doped by atomic and ion-nitrogen plasma-assisted molecular beam epitaxy has been studied on SrTiO₃(001) substrates. Several conditions for nitrogen insertion in the perovskite lattice of BaTiO₃ were considered. The N-doped BaTiO₃ layers are compared to undoped BaTiO₃ films produced with an atomic oxygen plasma source only. Oxynitride layers were elaborated on two different perovskite surfaces: SrTiO₃(001) single crystal substrates and a La_2/3Sr_1/3MnO₃ back electrode on SrTiO₃(001). This approach permits an in-depth study of the films’ specific properties including crystalline structure, chemical composition, ferroelectric behavior, and optical response. The chemistry and crystalline structure of the films are found to depend modestly on the substrate nature, while the growth is strongly dominated by self-oxidation and the presence of the ca. 1% substitutional nitrogen (N) in the oxynitride films. Structural and ferroelectric properties are similar for N-doped and undoped BaTiO₃ films, while significant changes in optical absorption are observed upon N-doping, confirming recent theoretical predictions. This new class of compounds is expected to be very well suited for novel applications based on band engineering in multifunctional materials.