Selective Epitaxial Growth of Ca2NH and CaNH Thin Films by Reactive Magnetron Sputtering under Hydrogen Partial Pressure Control.

Calcium compounds with N and H are promising catalysts for NH3 conversion, and their epitaxial thin films provide a platform to quantitatively understand the catalytic activities. Here we report the selective epitaxial growth of Ca2NH and CaNH thin films by controlling the hydrogen partial pressure (PH2) during reactive magnetron sputtering. We find that the hydrogen charge states can be tuned by PH2: Ca2NH containing H- is formed at PH2 < 0.04 Pa, while CaNH containing H+ is formed at PH2 > 0.04 Pa. In situ plasma emission spectroscopy reveals that the intensity of the Ca atomic emission (∼422 nm) decreases as PH2 increases, suggesting that Ca reacts with H2 and N2 to form Ca2NH at lower PH2, whereas at higher PH2, CaHx is first formed on the target surface and then sputtered to produce CaNH. This study provides a novel route to control the hydrogen charge states in Ca-N-H epitaxial thin films.

Heteroepitaxial Growth of a Ta3N5 Thin Film with Clear Anisotropic Optical Properties.

Ta3N5 is a promising semiconductor photocatalyst which can generate H2 gas from water under visible light illumination. It is expected that Ta3N5 exhibits a strong anisotropy in its physical properties stemming from its highly anisotropic crystal structure. However, such anisotropic properties have not been verified experimentally due to the difficulty in synthesizing a large single crystal. Here, we report the synthesis of (010)-oriented Ta3N5 single-crystalline thin films by solid phase epitaxy on the (110) plane of perovskite LaAlO3 substrates. The obtained epitaxial thin films of Ta3N5 exhibited clear optical anisotropy (pleochroism) as predicted by previous first-principles calculations. The optical gap for E||[100] polarization (∼2.12 eV) was smaller than that for E||[100] polarization (∼2.27 eV).

High-Quality Heteroepitaxial Growth of Thin Films of the Perovskite Oxynitride CaTaO2N: 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 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.

Strain-induced creation and switching of anion vacancy layers in perovskite oxynitrides.

Perovskite oxides can host various anion-vacancy orders, which greatly change their properties, but the order pattern is still difficult to manipulate. Separately, lattice strain between thin film oxides and a substrate induces improved functions and novel states of matter, while little attention has been paid to changes in chemical composition. Here we combine these two aspects to achieve strain-induced creation and switching of anion-vacancy patterns in perovskite films. Epitaxial SrVO3 films are topochemically converted to anion-deficient oxynitrides by ammonia treatment, where the direction or periodicity of defect planes is altered depending on the substrate employed, unlike the known change in crystal orientation. First-principles calculations verified its biaxial strain effect. Like oxide heterostructures, the oxynitride has a superlattice of insulating and metallic blocks. Given the abundance of perovskite families, this study provides new opportunities to design superlattices by chemically modifying simple perovskite oxides with tunable anion-vacancy patterns through epitaxial lattice strain.