Molecular Beam Epitaxy Growth Technology Of Perovskite Oxide Films And The Magnetic And Electrical Properties

With the development of solid-state electronic devices towards miniaturization,high-speed,low power consumption,and multi-function,more and more requirements are put forward for the precise control of material cell structure and interface morphology in the process of device preparation.Based on the deep understanding of the formation mechanism of low-dimensional structure and complex interface of the device and its structure-activity relationship with physical properties,the development of precise and controllable material growth process and interface control method can provide significant assistance to solve many practical problems such as improving the operation speed of the integrated circuit,prolonging the service life of electronic devices and increasing the capacity of memory.Among many functional materials,the perovskite oxide system has been the focus of researchers for a long time because of its abundant and peculiar phenomena related to interface,such as superconductivity,multiferroicity,two-dimensional electron gas?2DEG?,Although with the development of thin film preparation technology and its auxiliary means,researchers have been able to continuously develop the preparation process and method of controllable film structure and surface/interface morphology,there are still many scientific and technological problems to be solved in the study of the epitaxial mechanism of high-quality perovskite oxide films,and the relationship between semiconductor/oxide interface regulation and its related physical properties.Molecular beam epitaxy?MBE?,due to its unique methods and technical advantages in the growth of thin film materials,with the help of a variety of in-situ analysis and monitoring means?such as RHEED,reflection high-energy electron diffraction?,can achieve accurate control of material preparation at the atomic level.In the preparation of some compound films,the strategy of alternating growth of each component layer by layer can be used to precisely control the component,structure,surface,and interface.In this thesis,SrTiO₃?001?and SrMnO₃?001?homoepitaxial and heteroepitaxial thin films were prepared on SrTiO₃?001?substrate by MBE layer-by-layer alternating method.The micro mechanism of SrTiO₃?STO?and SrMnO₃?SMO?thin films layer-by-layer alternating growth was analyzed by combining the results of in-situ RHEED and surface/structure in later stages.Meanwhile,the surface/interface,lattice structures,stress-strain states,magnetic and dielectric properties of the thin films were studied.The main research contents and results are as follows:1.Based on the in-situ RHEED assisted MBE layer-by-layer alternate growth method,through the careful adjustment and optimization of molecular beam flow and alternate deposition time,we successfully realized the homoepitaxy of STO single crystal thin film with flat surface at the atomic level,and found the dependence of RHEED diffraction intensity oscillation characteristics on Sr,Ti molecular beam flow and stoichiometric fluctuation during the growth process.The influence of monolayer atomic dose on the stoichiometric ratio of film components was analyzed.2.On the basis of homoepitaxy,the heteroepitaxy of SMO/STO with MBE layer-by-layer alternate growth method is realized.The effects of different growth conditions,such as the type of oxygen source,substrate temperature,Sr:Mn flux ratio,and monolayer atomic dose,on the preparation of SMO/STO epitaxial layer were discussed.The epitaxial relationship between the SMO/STO heterogeneous film and the substrate can be summarized as:?001?_SMO||?001?_STO and[100]_SMO||[100]_STO,or the equivalent direction.The lattice expansion caused by stress-strain-induced defect states?such as oxygen vacancies,valence change of Mn ion,etc.?exists in the epitaxial layer of SMO.3.The magnetic properties of SMO?001?films grown on STO?001?substrate by MBE are analyzed and discussed.The result shows that the SMO film is ferromagnetic,which mainly comes from Mn³⁺-Mn⁴⁺double exchange coupling,stress-strain of the epitaxial layer,and oxygen vacancies in the sample.Oxygen annealing can significantly reduce the content of oxygen vacancies in the sample,leading to the conversion of more trivalent Mn ions into tetravalent Mn ions,and the weakening of macro ferromagnetism.4.In the study of dielectric properties,we found that there are four dielectric relaxation behaviors of the SMO film in different temperature and frequency ranges.Two groups of oxygen-vacancy-related dielectric relaxations are very sensitive to oxygen annealing,which are attributed to the short-range hopping motion of oxygen vacancies,and the dipole-type relaxation induced by the hopping of localized Jahn-Teller?JT?polarons,respectively.Due to the electron-phonon coupling and the formation of JT distortion of Mn³⁺ions,JT polarons are closely related to the existence of oxygen vacancies,resulting in the transition of Mn⁴⁺ions to Mn³⁺.Besides,the other two kinds of dielectric relaxations independent of oxygen vacancies can be attributed to the slowdown behavior of polarization fluctuation related to spin-glass at low temperatures and the polarization of the Maxwell-Wagner?MW?interface at the SMO/Nb:STO interface at high temperatures,respectively.