The Study Of Lattice And Electronic Structure Of Complex Oxides Single Crystal Thin Films

The perovskite single crystal oxide film not only has extremely important applications in many fields,but also it has novel properties under the control of the lattice structure and charge.It is an important quantum material and has received extensive attention.However,the mechanism involved in controlling the properties of complex oxides,especially at the interface,is very unclear.As the most important research object under control,the lattice change and electronic structure change,the characterization of its precise state is very difficult,especially the lattice structure and electronic structure of thin layer or superlattice interface layers.At present,the challenge is how to control,characterize and reveal the specific variation in electronic structure and lattice structure,including the concentration of charge doping,the effect of lattice distortion on bond length and bond angle,etc.This paper includes six chapters:Chapter 1 mainly describe the details of research background;Chapter 2 mainly introduces the fabrication of single crystal thin?film samples and related characterization methods.Chapter 3 mainly introduces the strain state and mechanism of lattice reconstruction at the interface in superlattices.Chapter 4 introduces the electron antidoping by modulating the breathing distortion in BaBiO3.Chapter 5 is the study of ionic liquids gating LaNiO3 single crystal epitaxial oxide films.Chapter 6 is the summary of this work and innovation points.In the first chapter,we firstly introduce the properties and strain of the oxide thin film interface;introduce the specific situation and calculation method of oxygen octahedral rotation in perovskite-type thin film,and then introduce the basic concept of anti-doping effect and the anti-doping effect in different material systems;finally,the latest developments of ionic liquids in gating the properties of oxide thin films are introduced.The second chapter mainly introduces the working principle and the equipment of the experimental techniques used in this paper.Firstly,introducing the working principle and the application of molecular beam epitaxy system and pulsed laser deposition system which are used to fabricate single crystal thin film.Then introduced the working principle of the reflection high-energy electron diffraction to monitor the growth of the film in real-time,the working principle of the quartz crystal microbalance used to calibrate the growth rate of the film,then introduced the specific working principle of the common characterization methods of single crystal film,and briefly introduced the process of micro-nano processing used to fabricate small devices.The third chapter mainly studies the interfacial strain state and the lattice reconstruction mechanism of ultrathin LaMnO3+? in the short-period(LaMnO3+?)N/(SrTiO3)N(2)superlattice.At the interface,the stoichiometry of the interface layer is generally different from the bulk due to strain,so we first introduce a virtual bulk.The stoichiometry of the virtual bulk is the same as the LaMnO3+? layer at the interface.The lattice structure of virtual bulk is derived by Poisson ratio.Due to introducing the virtual bulk,the biaxial strain and hydrostatic strain at the interface can be accurately calculated,which can provide a strain calculation method for the design of the interface.In addition,two series of superlattice were systematically measured for half-integer diffraction peaks.By fitting the half-integer diffraction peaks,the angle of oxygen octahedral rotation at the interface can be obtained.The fitting results indicate that the oxygen octahedral rotation along the in-plane axes and out-of-plane axis is conservation.The average value of the rotation angle is always close to the rotation angle of the oxygen octahedral in the virtual bulk,which can provide theoretical guidance for the rotation of the oxygen octahedral in the interface design.The fourth chapter mainly introduces the electronic anti-doping effect by controlling the concentration of oxygen vacancies in BaBiO3 thin films and reveals the new anti-doping mechanism.In this chapter,we first calculated the DOS of BaBiO3-? thin films and the lattice breathing distortion after electron doping by the first-principles calculation principle.The calculation results show that as the concentration of doped electrons in BaBiO3-? increases,the band gap also increases.As a result,the lattice breathing distortion is reduced.This reveals that the electron anti-doping effect in BaBiO3 is caused by lattice breathing distortion.In the experiment,we used a pulsed laser deposition system assisted by a high oxygen pressure reflection high energy electron diffraction and fabricated a series of high-quality single crystal thin film samples with different oxygen vacancy concentrations by controlling the annealing time and annealing oxygen pressure,and then performed a series of X-ray absorption spectra and emission spectra,ellipsometry,Raman shift spectroscopy,and X-ray diffraction to prove that the breathing distortion in BaBiO3 decreases with the increase of oxygen vacancy concentration.The XAS and XES of O K-edge show that the band gap of the thin films increases as the concentration of oxygen vacancies increases.This can indicate that the oxygen vacancy in BaBiO3 can realize the electronic anti-doping effect by modulating the lattice breathing distortion.Finally,the fitting process of X-ray half-integer diffraction peaks and the optimization fitting process of the rotation angle value obtained by fitting and the lattice breathing distortion are optimized to obtain the optimal rotation angle of oxygen octahedral and the breathing distortion percent.In Chapter 5,we mainly studied the properties of ionic liquids to gating the oxide thin films in this chapter.In this work,we used high-quality single crystal epitaxial LaNiO3 thin films grown by molecular beam epitaxy.By measuring the transport properties of the film under different gating voltages,the measurement results reveal that the resistance of the film increases as the increase of the gating voltage,and the carrier concentration decreases.The Hall measurements reveal that LaNiO3 is p-type conduction,it can explain that under the gating of ionic liquid,electron injection will combine with conductive carriers and annihilate,which reduce the concentration of carriers and increase the resistance.The in-situ X-ray diffraction measure can prove that at gating voltage(VG<0.7 V),the lattice expands in the out-of-plane direction,when the gating voltage is reduced to 0 V,the lattice structure of the thin film can be recovered to the pristine structure,which can prove that the electron can be injected into the film under the ionic liquid gating,and the electron can combine with ligand hole which can cause the lattice structure to expand.The in-situ Ni K-edge XANES can prove that the valence of Ni in LaNiO3 has not changed,but as the gating voltage increase,the valence of Ni will shift to the low valence state.The experimental results can explain that no defects are formed in the thin film under VG<0.7 V,and the variation in film properties is caused by electron injection.