应力下SmNiO_3钙钛矿氧化物薄膜材料的电导与红外光电导

稀土镍基钙钛矿氧化物RNiO_3(R为稀土元素)可以在温度触发下发生从电子游离态到局域态的金属绝缘体转变,这一特性在传感器,数据存储,调制开关等方面具有可观的应用价值.本文通过脉冲激光沉积法,在钛酸锶(SrTiO_3)、铝酸镧(LaAlO_3)单晶衬底上准外延生长热力学亚稳态镍酸钐(SmNiO_3)薄膜材料,利用薄膜与衬底间晶格失配引入界面应力,实现对SmNiO_3电子轨道结构与金属绝缘体相变温度的调节.结合电输运性质与红外透射实验的综合表征研究,论证了双向拉伸应变引起的晶格双向拉伸畸变,可以引起SmNiO_3的禁带宽度的展宽,从而稳定绝缘体相并提高金属-绝缘相转变温度.进一步结合近边吸收同步辐射实验表征,揭示了拉伸应变稳定SmNiO_3绝缘体相的本质在于Ni—O成键轨道在双向拉伸形变作用下的弱化,使得镍氧八面体中的价电子偏离镍原子从而稳定SmNiO_3的低镍价态绝缘体相.

Electrical conductivity and infrared ray photoconductivity for lattice distorted SmNiO3 perovskite oxide film

The metal-to-insulator transitions achieved in rare-earth nickelate (RNiO₃) 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 RNiO₃ is a typical electron correlation system, in which the electronic band structure is dominant by the Coulomb energy relating to the d-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 RNiO₃ 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.