Metal-Insulator Transition In Nd_1-xY_xNiO₃/LaAlO₃ Epitaxial Thin Film

The metal-insulator transition phenomenon is always an important topic in condensed matter physics. Among these materials, perovskite-type rare earth nickelates (RNiO3, R:Rare Earth, R≠La) attract many interests because of their sharp insulate-metal transition, which make them potential application in devices such as sensors, thermochromic film manufacturing. Howerver, the phase transition temperature of RNiO3 materials is not near room temperature. Therefore, finding some way to tune the phase transition temperature of RNiO3 to near room temperature is very important for the application of device. Consequently, it is significant to study how to tune the phase transition temperature and the effect of doping, strain or electric field to the structure and phase transition property. In this thesis, based on growing NdNiO3/LaAlO3 epitaxial thin films successfully, the influence of the film thickness on the phase transition property was firstly studied. Secondly, the phase transition temperature of thin film was tuned to near room temperature by doping different Y element to the NdNiO3. Meanwhile, the effect of doping on the structure and phase transition property was also investigated. Finally, the electric-field drived metal-insulator transition of Nd0.7Y0.3NiO3/LaAlO3 epitaxial thin film were revealed and memristor effect was explored.The first chapter is a literature review of background and advance of latest research. Firstly several mechanisms about metal-insulator transition such as energy band theory, Anderson transition, Peierls transition, Wilson transition and Mott transition were introduced. Secondly, the metal-insulator transition and the explanation based on band theory of RNiO3 were presented. At last, the ways to trigger phase transition and the methods to tune phase transition temperature of RNiO3 films were given.The second chapter is an introduction of preparation method of RNiO3 thin film, and the characterization tools used in this thesis. Pulsed laser deposition (PLD) method used in this work was described in detail. The characterization methods such as X-ray diffraction (XRD), Raman spectroscopy and Fourier transform infrared spectroscopy, X-ray absorption fine structure spectroscopy (EXAFS), and the four-probe method were introduced. The home made setup for temperature-dependent resistance measurement, and the setup for combining temperature-dependent Infrared spectroscopy and electric field measurement were presented.In the third chapter, NdNiOs/LaAlO3 epitaxial thin film was prepared by using PLD method. By optimizing the preparation condition, includes the synthesis of PLD target, the selection of LaAlO3 substratethe substrate temperature and oxygen pressure, NdNiO3 thin film with sharp metal-insulator transition was obtained. The change of structure and effective carrier across the phase process, and the effect of thin film thickness on the phase transition temperature were investigated. The result indicates that all the thin films with different thickness have clear insulator to metal transition. The phase transition temperature can be tuned from 90 K to 121 K by changing the thin film thickness. Temperature dependent Raman and infrared conductivity study show the structure and carriers variation in the process of phase transition which further confirms the resistivity result. This thickness-induced compressive strain enlarges NiO6 distortion, and results in the increase of transition temperature.In the fourth chapter, the effect of Y atoms doping on the metal-insulator transition property of Nd1-xYxNiO3(x=0.3,0.4) thin films was studied. Both samples show clear metal-insulator transition. By adjusting the content of introduced Y atoms, the transition temperature can be tuned to near room temperature (340-360K). The infrared transmission intensity decreases with temperature, suggesting the thermochromism property of the film. The EXAFS result indicates the structural change across the phase transition. Furthermore, the introduced Y atoms decrease the intensity of pre-edge peak A in the XANES spectrum and extend average Ni-O bond length, suggesting weaker hybridization of Ni-3d and O-2p states. Meanwhile, the charge-disproportionated effect becomes more pronounce in high Y atom concentration sample. This result reveals the physical origin of transition temperature increasing with Y atoms content.In the fifth chapter, the effect of electric field on the metal-insulator transition of Nd0.7Y0.3NiO3 epitaxial thin films was studied. Electric field drived metal-insulator transition was observed in both voltage-drived and current-drived modes. IV curve shown aswitch effect in voltage-drived mode while negative differential resistance (NDR) phenomenon in current-drived mode. The insulator to metal property in Nd0.7Y0.3NiO3 film is accompanied by charge ordering as the increase threshold voltage follows an exponential relation with temperature. The result of infrared transmission spectra and Raman spectrum further confirm the electric field drived insulator to metaltransition in Nd0.7Y0.3NiO3 film. By utilizing FET structure, the phase transition temperature can be tuned by changing gate voltage. Finally, memristive effect of the thin films induced by applying voltage pulse was observed.