Metal-Insulator Transition In Sm_0.6Nd_0.4NiO₃/LaAlO₃Epitaxial Thin Film

The metal-insulator (MI) transition in transition metal oxides has been attracting intensive attention in condensed matter physics. Experimental and theoretical studies to unravel the mechanism of MI transition have been ongoing for nearly half a century. One such correlated system is the rare earth nickelate series which have the general formula ReNiO3(where Re is non-La trivalent rare earth ion). These nickelates belong to the perovskites structural family and display a first-order metal-insulator phase transition. The low temperature state of ReHiO3is generally classified as a charge-transfer semiconductor. As temperature increasing, the material transitts to metallic phase above metal-insulator transition temperature (TMI). TMI decreases monotonically with increasing the radius (r) of the Re3+ion, from600K (LuNiO3) to135K (PrNiO3). The change in resistivity at MI transition temperature can be quite large,2-3orders of magnitude across a narrow temperature window, and the possible applications are wide ranging, from sensors, optoelectronic switches to memory. However, there is no common understanding on mechanism for the MI transition of ReNiO3, and the sample cannot be reproduced between different researchers because the ReNiO3is difficult to prepare at normal pressure and temperature. So, more studies are needed to ravel the correlations between the MI transition and structure change. In this thesis, due to the potential application, we focused on the structural and electrical properties of Sm0.6Nd0.4NiO3(SNNO) thin film, which shows an MI transition near room temperature.This thesis includes four chapters:the first chapter introduces the related background knowledge. The second chapter describes the deposition of Sm0.6Nd0.4NiO3/LaAlO3thin films and the effect of growth condition on MI transition properties. The current induced MI transition in Sm0.6Nd0.4NiO3/LaAlO3thin film is contained in the third chapter. And the forth chapter describes the microstructure change near the MI transition.In chapter one, the related background knowledge was introduced. First, the MI transition mechanisms, such as band theory, Mott transition and Anderson transition, were introduced. Then, the recent progress on the provskite nickelate was reviewed. Finally, the research outline and its significance of this thesis were explained.In chapter two, the deposition and characterization of the epitaxial Sm0.6Nd0.4NiO3/LaAlO3thin films were described. First, the fundamental mechanism of pulse laser deposition (PLD) was simple introduced. PLD is a physical vapor deposition technique. It is considered as one of the best tools for epitaxial thin film deposition. Then, the traditional solid state sintering method for PLD target preparation and the selection of the LaA103substrate were described. After that, we introduced the growth process of thin films and techniques to characterize the properties of thin films, such as AFM for surface topography, XRD for structure, and four probes method for electrical measurement. Thereafter, the influence of growth oxygen pressure and substrate temperature on MI transition was discussed. It was found that the MI transition temperature (TMI) of the films decreases remarkably as the oxygen pressure decreases, while the out-of-plane lattice constants decrease slightly. We believe that these are the results of competition between the changes of Ni valence, elongation of Ni-O band length, bending of Ni-O-Ni angle and the formation of Ni3-δ-O2--Ni3+δ charge ordering. On the other hand, we found that although the structure of thin films almostly does not change as the substrate temperature increases to900℃, the MI transition changes significantly. Among the films deposited at different temperature, we chose the one deposited at850℃, which shows steepest MI transition at TMI at280K for further studies.In chapter three, the modification of MI transition by applied DC current on Sm0.6Nd0.4NiO3/LaAlO3thin film was discussed. First, the effect of temperature on the basic properties of Sm0.6Nd0.4NiO3/LaAlO3thin film, such as structure, surface topography and resistivity, were introduced. The fitting of p-T curve indicated that the conduction mechanism in the insulator state is controlled by the Dual-conduction mechanism of thermal activated conduction and Mott’s variable range hopping. After that, the decrease of TMI from280K to200K as the increase of applied DC current was discussed, and the metal-insulator transition phase diagram was constructed. Finally, base on the nonlinear Ⅰ-Ⅴ curve, we suggest that the MI transition induced by DC current at low temperature (<Tmi) is due to the destruction of the localized Ni3+δ-O2--Ni3-δ charge ordering by the applied DC current.In chapter four, the rotation of NiO6octahedron near the MI transition was characterized by the in situ synchrotron radiation high resolution X-ray diffraction. Many physical phenomenon such as MI transition and magnetic transition is closely related to the rotation of06octahedron. It can obtain the relationship between the microstructure and MI properties by measure the half-order Bragg diffraction peaks. Below and above TMI, we found that the intensity of (1/2,1/2,3/2) half-order Bragg reflection of thin film which originates from the rotation of the NiO6octahedra along [010] changes, and the intensity of (1/202) half-order Bragg reflection which originates from the rotation of the NiO6octahedra along [001] keeps constant. These results suggeste that the thermal induced MI transition accompanies with the change of structure. However, both of the two half-order Bragg reflections of thin film remain unchanged with applied DC current at low temperature, indicating that there is no rotation of NiO6octahedron accompanying the current induced MI transition at low temperature. We believe that the electrically driven MI transition is pure behavior of electron without structure change.