Electrical conductivity and infrared ray photoconductivity for lattice distorted SmNiO<sub>3</sub> perovskite oxide film

The metal-to-insulator transitions achieved in rare-earth nickelate (<i>R</i>NiO₃) receive considerable attentions owning to their potential applications in areas such as temperature sensors, non-volatile memory devices, electronic switches, etc. In contrast to conventional semiconductors, the <i>R</i>NiO₃ is a typical electron correlation system, in which the electronic band structure is dominant by the Coulomb energy relating to the <i>d</i>-band and its hybridized orbitals. It was previously pointed out that lattice distortion can largely influence the electronic band structures and further significantly affect the electronic transportation properties, such as the resistivity and metal-to-insulator transition properties. Apart from directly measuring the transportation performance, the variations in the origin of carrier conduction and orbital transitions relating to the strain distortion of <i>R</i>NiO₃ can also be reflected via their optical properties. In this work, we investigate the optical properties of samarium nickel (SmNiO₃) thin films when lattice distortions are induced by interfacial strains. To introduce the interfacial strain, the SmNiO₃ thin films are epitaxially grown on the strontium titanate (SrTiO₃) and lanthanum aluminate (LaAlO₃) single crystal substrates by using the pulsed laser deposition. A bi-axial tensile distortion happens when the SmNiO₃ thin films are grown on SrTiO₃ due to the smaller lattice constant of SmNiO₃ than that of SrTiO₃, while the one grown on LaAlO₃ is strain-relaxed. We measure the infrared radiation (IR) transmission spectra of the SmNiO₃ thin films grown on various substrates. The obtained IR transmission spectra are fitted by a Drude-Lorentz model and further converted into the curves of photoconductivity versus IR frequency. Comparing the difference in photoconductance between low frequency and high frequency reflects the two different origins of the conduction, which are related to intraband transition and band-to-band transition, respectively. The smaller photoconductance is observed for SmNiO₃/SrTiO₃ than for SmNiO₃/LaAlO₃ at low frequency, and this is expected to be caused by the suppression of free carriers as reported previously for tensile distorted SmNiO₃. The consistence is obtained when further measuring the electronic transportation such as temperature-dependent electrical resistivity, as a higher resistivity is observed for SmNiO₃/SrTiO₃ than for SmNiO₃/LaAlO₃. The combination of the investigation of electrical transport with that of infrared transmission indicates that the tensile distortion in structure stabilizes the insulating phase to eliminate a pronounced metal-to-insulator transition and elevates the transition temperature. This is related to the respective twisting of the NiO₆ octahedron when tensile distortion regulates the valance state of the transition metal and further opens the band gap, which is further confirmed by results of the X-ray absorption spectrum.