Study On The Epitaxial Growth And Physical Properties Of Perovskite Oxide Ultra-thin Films

The storage of massive information data is the demand of the rapid development of society in the information age.As a potentially valuable information storage carrier,semiconductor materials with a perovskite structure are considered to be ideal substitutes for traditional semiconductor storage materials.Lots of perovskite structure materials with remarkable physical properties have been studied and reported,such as colossal magnetoresistance effect,high-temperature superconductivity.The LaCoO₃ thin film material with low temperature ferromagnetism and the LaTiO₃ thin film material with2DEG are one of the materials that have been widely studied.In addition,the LaCoO₃/LaTiO₃ superlattice film composed of LaCoO₃ and LaTiO₃ has also aroused people’s research interest.The preparation of high-quality epitaxial films by PLD is the prerequisite for the research and exploration of material physical properties,and the characterization of the epitaxial film structure has become the basis for further research and analysis.This dissertation is based on the research of LaCoO₃ and LaTiO₃ two kinds of epitaxial films,LaCoO₃/LaTiO₃ superlattice film as the extension,through RHEED monitoring during the film growth process,XRD,XRR and XAS characterization of the film structure,as well as analysis of M-T,M-H and R-T.And the peculiar physical properties of LaCoO₃,LaTiO₃ and LaCoO₃/LaTiO₃ superlattice films with different structures were studied.The specific content is divided into the following three parts:（1）The first part is the study of the different structures of LaCoO₃ epitaxial films and their low-temperature ferromagnetism.The dense LaCoO₃ ceramic target was fired through an improved target manufacturing method,and the pulsed laser deposition system was used to epitaxially grow LaCoO₃ with different structures on substrates with different lattice constants and the same substrate with different orientations（<001>/<011>/<111>）,which realizes precise control of the growth of LaCoO₃ films.Then,the structure of the LaCoO₃ film was analyzed and studied.Through the magnetic test,it is found that the Curie temperature of the low-temperature ferromagnetic transition of the LaCoO₃ film with different orientations has a maximum value of 95K in the<011>orientation film,and the saturation magnetization is slightly larger than the other two orientation film in the<001>orientation.（2）The second part is the study of the conditions of LaTiO₃ epitaxial films and the two-dimensional electron gas in single crystal and amorphous LaTiO₃ films with different structures.The LaTiO₃ ceramic target used in the pulsed laser deposition system was fired,and LaTiO₃ films were grown at different deposition temperatures and oxygen partial pressures.Through RHEED,XRD,XAS and other characterization and analysis techniques,650℃and high vacuum were obtained as the optimal growth condition for LaTiO₃ thin film.Then,R-T tests were performed on single crystal LaTiO₃ films with different orientations and amorphous LaTiO₃ films with different thicknesses grown under different growth conditions and optimal growth conditions.It is found that the amorphous LaTiO₃ film also exhibits the 2DEG phenomenon similar to the single crystal LaTiO₃ film.And the sheet resistance in the amorphous LaTiO₃ film is three orders of magnitude higher than that in the single crystal film.The reason is that the irregular arrangement of the LaTiO₃ structure has hindered the flow of the 2DEG between the film and the substrate.（3）The third part is the study of Co and Ti valence changes and charge transfer in LaCoO₃/LaTiO₃ superlattice epitaxial films.The growth of LaCoO₃/LaTiO₃ superlattice epitaxial film involves two materials with different growth conditions.Therefore,the switching oxygen partial pressure method was proposed.This method is demonstrated to result in a high-quality LaCoO₃/LaTiO₃ superlattice film.In the LaCoO₃/LaTiO₃superlattice structure,due to the easy oxidation of Ti³⁺in the LaTiO₃ film layer,it is easy to lose electrons and become Ti⁴⁺,while the Co³⁺in the LaCoO₃ film layer is reduced to Co²⁺by electrons.Charges are transferred between different film layers.Through XAS,the absorption peaks of Co L-edge and Ti L-edge can be studied and analyzed,so as to understand the valence state and content of Co and Ti.By designing and studying the charge transfer in LaCoO₃/LaTiO₃ superlattices with different structures,the concept of cumulative thickness ratio is proposed.And the cumulative thickness ratio proposed was used to measure the degree of charge transfer in superlattices with different structures,and it was found that the cumulative thickness ratio has a linear relationship with the degree of charge transfer.Based on this,the charge transfer in LaCoO₃/LaTiO₃superlattices can be measured.