High-Quality Heteroepitaxial Growth of Thin Films of the Perovskite Oxynitride CaTaO2N: Importance of Interfacial Symmetry Matching between Films and Substrates

摘要

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 CaTaO_(2)N thin films with (100)_(pc), (110)_(pc), and (111)_(pc) crystallographic surface orientations (where the subscript pc denotes a pseudocubic cell) on SrTiO_(3) substrates using reactive radio frequency magnetron sputtering, along with investigations of crystallinity and surface morphology. Irrespective of surface orientation, stoichiometric CaTaO_(2)N epitaxial thin films were grown coherently on SrTiO_(3) substrates and showed clear step and terrace surfaces in the case of low values of film thickness of approximately 20 nm. A (110)_(pc)-oriented film was also more highly crystalline than (100)_(pc)- and (111)_(pc)-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 CaTaO_(2)N and cubic SrTiO_(3). A CaTaO_(2)N thin film grown on a lattice- and symmetry-matched orthorhombic DyScO_(3) 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.