High-Quality Heteroepitaxial Growth of Thin Films of the Perovskite Oxynitride CaTaON: Importance of Interfacial Symmetry Matching between Films and Substrates.

Perovskite oxynitrides have been studied with regard to their visible light-driven photocatalytic activity and novel electronic functionalities. The assessment of the intrinsic physical and/or electrochemical properties of oxynitrides requires the epitaxial growth of single-crystalline films. However, the heteroepitaxy of perovskite oxynitrides has not yet matured compared to the progress realized in work with perovskite oxides. Herein, we report the heteroepitaxial growth of CaTaON thin films with (100), (110), and (111) crystallographic surface orientations (where the subscript pc denotes a pseudocubic cell) on SrTiO substrates using reactive radio frequency magnetron sputtering, along with investigations of crystallinity and surface morphology. Irrespective of surface orientation, stoichiometric CaTaON epitaxial thin films were grown coherently on SrTiO substrates and showed clear step and terrace surfaces in the case of low values of film thickness of approximately 20 nm. A (110)-oriented film was also more highly crystalline than (100)- and (111)-oriented specimens. This relationship between crystallinity and surface orientation is ascribed to the number of inequivalent in-plane rotational domains, which stems from the symmetry mismatch between the orthorhombic CaTaON and cubic SrTiO. A CaTaON thin film grown on a lattice- and symmetry-matched orthorhombic DyScO substrate exhibited a significant crystallinity and a clear step and terrace surface even though the film was thick (∼190 nm). These results are expected to assist in developing the heteroepitaxial growth of high-quality perovskite oxynitride thin films.