Controllable Preparation Of Strontium Iridate Epitaxial Thin Films And Their Electrical Transport Regulation

In the transition metal oxide system,the 5d transition metal oxide iridates exhibits many fantastic physical properties under the strong spin-orbit coupling and electron correlation interaction,such as Mott insulator,topological crystal insulator,Dirac/Weyl semi-metal,etc.For example,the layered perovskite structure Sr₂IrO₄ has the characteristics of J_eff=1/2 Mott insulator,and its crystal structure and orbital occupation are very similar to the parent material of Cu-based superconductor La₂CuO₄,because of which it is predicted to achieve high temperature superconductivity through chemical doping.Inspired by this theoretical prediction,researchers have carried out a lot of experiments,and more and more evidence show that high-temperature superconductivity may exist in iridates.However,compared with bulk materials,there are few studies on iridates thin films.The growth of high-quality single crystal epitaxial films is an important factor that restricts the extensive development of their films.The research on chemical doping and electric field regulation of these films will enrich their physical properties and expand their new functions and potential device applications.In this thesis,we focus on the controllable preparation of 5d strontium iridate single crystal epitaxial films and the regulation of their physical properties,and explores the growth rules of high-quality epitaxial films,the novel physical properties and potential device applications.And the main results are as followings:1.With the optimization of preparation process,the growth of high-quality strontium iridate single crystal epitaxial thin films on different substrates were achieved by pulsed laser deposition.The physical properties of these films were tested and analyzed,and the relationship between the physical properties and the epitaxial strain was studied.Furthermore,the selective growth of Ruddlesden-Popper（RP）phase strontium iridate films with different phases was achieved by using a single Sr₃Ir₂O₇ or Sr₂IrO₄ target.Finally,using water-soluble Sr₃Al₂O₆ as a sacrificial layer,the preparation and transfer of freestanding Sr₂IrO₄ thin films and SrIrO₃ thin films were realized.The controllable preparation of high-quality strontium iridate single crystal epitaxial films will lay a foundation for the related physical property control research and device application.2.Chemically doped(Sr_1-xCe_x)₂IrO₄ epitaxial films were prepared by replacing Sr²⁺with a small amount of Ce⁴⁺.The microstructure,Raman spectra,surface morphology,electrical and magnetic properties of the films were characterized.The results show that with the doping of Ce⁴⁺,(Sr_1-xCe_x)₂IrO₄ films does not show metallic characteristics,but its resistance increases,and its conduction mechanism is more consistent with the 3D Mott variable-range hopping model.At the same time,Ce⁴⁺doping also significantly inhibited the canted antiferromagnetic phase transition of the thin films,and the effective magnetic moment decreased significantly.When x=0.0125 and 0.025,the phase transition temperatures decreased from 220 K to 160 K and 50 K,respectively.This may be because chemical doping in the strontium iridate thin film system is easy to cause structural defects of different RP phases,which inhibits the effect of electron doping and leads to the abnormal increase of the resistance of the system.3.By optimizing the structure of the device,the electrolyte-gated Mott transistor was fabricated with Sr₂IrO₄ single crystal epitaxial film as the channel and ionic liquid as the gate insulator.The switching current ratio of the device can reach 5orders of magnitude at room temperature.Based on the influence of channel layer thickness,test environment vacuum,and humidity on the transfer characteristics of the device,combined with the in-situ phase structure characterization,a double layer electrostatic field effect mechanism enhanced by water molecule electrolysis is proposed.Compared with the traditional electrostatic field effect,the double electric layer structure enhanced by water molecule electrolysis has stronger carrier regulation ability,which is helpful to improve the effect of the electrolyte gate on the 5d transition metal oxide material physical property electric field regulation,and develop new principle device based on 5d material.4.The transferable ion-gel film was prepared by ionic liquid and organic polymer,and an all-solid state electrolyte gated field-effect transistors were prepared with the ion-gel film as the gate insulator,and the SrIrO₃ epitaxial film as channel,which successfully realized the control of electric field on SrIrO₃ channel electrical transport characteristics.The results show that the applied electric field can significantly inhibit the metal insulator transition of SrIrO₃ thin films.Through the fitting of electrical transport characteristics before and after electric field regulation,the in-situ characterization of the film microstructure,the transport characteristics of thin films with different oxygen pressures and the systematic analysis of electrochemical measurement of SrIrO₃ thin films,the influence of oxygen vacancy formation or electrochemical hydrogen ion embedding on the electrical properties of the films during electric field regulation was eliminated,and the main mechanism of electrostatic field regulation was determined.