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

Abstract

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 CaTaO2N thin films with (100)pc, (110)pc, and (111)pc crystallographic surface orientations (where the subscript pc denotes a pseudocubic cell) on SrTiO3 substrates using reactive radio frequency magnetron sputtering, along with investigations of crystallinity and surface morphology. Irrespective of surface orientation, stoichiometric CaTaO2N epitaxial thin films were grown coherently on SrTiO3 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 CaTaO2N and cubic SrTiO3. A CaTaO2N thin film grown on a lattice- and symmetry-matched orthorhombic DyScO3 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.

Growth and Electronic Properties of SnSe2 Films on Reconstructed, (111)-Oriented SrTiO3 Substrates

“…Substrate surfaces play an indispensable role in the growth of high-quality thin films, ,, which could also be responsible for tunning electronic properties of films. ,, Choosing a symmetry-matched supporting surface has profound benefits in heteroepitaxy. − The (111)-oriented STO substrate is desirable since its symmetry matches that of SnSe 2 . While the surface of (111)-oriented STO is terminated either by Ti 4+ or SrO 3 4– , both of which are polar, its surface free energy can be reduced and the polarity be compensated by the electronic and structural reconstructions. − …”

Semi Transparent Three-Dimensional Macroporous Quaternary Oxynitride Photoanodes for Photoelectrochemical Water Oxidation

“…Previous reports on oxynitrides have predominantly worked on powder-based photoanodes. , Similar to oxidic photoanodes, the generally used powder-based oxynitride photoanodes are prone to suffering from high resistivities and poor carrier collection yields due to the limited connectivity and severe recombination rates. , In addition to post-necking treatment and functional coating, the synthesis of thin films is a feasible solution to mitigate these restrictions and simultaneously modulate the surface features. ,,, Nevertheless, in comparison to powder-based photoanodes prepared by the electrophoretic deposition (EPD) method, the synthetic procedures for semitransparent oxynitride thin films are generally more complex. Several kinds of metal oxynitride thin films, such as LaTiO x N y , BaTaO x N y , CaNbO x N y , SrTaO 2 N, and CaTaO 2 N, have been prepared by reactive radio frequency (RF) magnetron sputtering, − pulsed laser deposition, , and spin coating of a metal oxide layer followed by post-ammonolysis . A strategy for improving the PEC performance of photoelectrodes is nanostructuring such as to improve light trapping, carrier collection, and mass transport throughout the material.…”

Fabrication of Semi-Transparent SrTaO2N Photoanodes with a GaN Underlayer Grown via Atomic Layer Deposition

“…19,21,26 Nonetheless, the fabrication process of transparent oxynitride thin films is demanding in comparison to oxides. Transparent quaternary oxynitride films have been previously obtained by pulsed laser deposition (PLD), 27,28 reactive radio frequency (RF) magnetron sputtering, [29][30][31][32] and spin-coating in conjunction with post-ammolysis. 21 Although several TCL substrates have been tested, including MgO, Al2O3 and SrTiO3, 21,32 the limited choices hamper the widespread application of transparent oxynitride-based photoanodes in tandem cell.…”

“…Transparent quaternary oxynitride films have been previously obtained by pulsed laser deposition (PLD), 27,28 reactive radio frequency (RF) magnetron sputtering, [29][30][31][32] and spin-coating in conjunction with post-ammolysis. 21 Although several TCL substrates have been tested, including MgO, Al2O3 and SrTiO3, 21,32 the limited choices hamper the widespread application of transparent oxynitride-based photoanodes in tandem cell. This is because of the high temperatures and reducing atmosphere required in these processes.…”