The Topotactic Phase Competition In The Growth Of SrCoO_x/LaAlO₃ Thin Films

The development and progress of science and technology have benefited from people's constant pursuit of high performance and new materials.The development of modern integrated circuits puts ever-increasing demands on the miniaturization,integration,and high performance of devices.The growth and mechanism of materials have become increasingly important.Perovskite oxides have attracted a wide range of research interests due to their rich physical properties,including ferromagnetic/antiferromagnetic,metal insulator transition,superconductivity,and multiferroic properties.In perovskite oxides,the properties of SrCoO_x?SCO?can be regulated over a wide range due to its unique oxygen ion mobility.It is an ideal material for studying charge-lattice-spin-orbit coupling in strongly correlated electron systems.In the fully oxidized state,SrCoO₃?P-SCO?is a ferromagnetic conductive perovskite.When the oxygen content of P-SCO is reduced to a certain extent,the crystal structure of the P-SCO will be changed from the perovskite structure to the antiferromagnetic insulating brownmillerite structure SrCoO2.5?BM-SCO?.These two structures can undergo reversible topological phase transitions without changing the crystallinity,thus making SrCoO_x have a broad application prospects,for example,as an electrode material,a catalyst in an oxidation-reduction reaction,or an epitaxially strained magnetic switch,etc.However,the maintenance of Co4+ in P-SCO requires high energy and it requires high temperature and high oxygen pressure to synthesize P-SCO,and the synthesized P-SCO tends to convert to more stable Co3+-containing BM-SCO at normal temperatures and pressures.In addition,although Co3+ in BM-SCO is relatively stable,there are a large number of oxygen vacancies in the structure,and these oxygen vacancies also lead to poor structure and physical stability of BM-SCO.Therefore,the study of the topological phase transition between P-SCO and BM-SCO will help to understand the mechanism of the unique physical properties such as high oxygen ion mobility of such materials,which will facilitate the regulation of oxygen vacancies in these materials.This helps to improve the electrode performance,catalytic efficiency,etc.This article systematically introduces the structure and properties of perovskite oxides,and focuses on the research progress of SrCoO_x thin films.Using pulsed laser deposition to study the growth of SrCoO_x epitaxial films.we found temperature-dependent competition in the growth of SCO/LaAlO₃?LAO?films and calculated the Gibbs free energy of P-SCO and BM-SCO using first-principles calculations.The trend of free energy at high temperatures gives a theoretical explanation of competition and compares the electronic structures of P-SCO and BM-SCO films.The paper is divided into five chapters,the specific content is as follows:The first chapter summarizes the structure and properties of perovskite oxide materials,and introduces the characteristics and potential applications of the SCO system,analyzes the current research status and existing problems of the system,and clarifies the main research contents and significance of this paper.In chapter two,the basic principles of pulsed laser deposition for preparing epitaxial films and various means for characterizing the structure and physical properties of SCO films are introduced.These include X-ray diffraction?XRD?characterization of thin-film crystal structures,X-ray reflectance?XRR?versus thin films,roughness and film thickness tests,as well as spectroscopic ellipsometry and X-ray photoelectron spectroscopy analysis of the electronic structure of the film.In Chapter 3,the process of preparing SCO thin films by pulsed laser deposition is introduced.We have found that when SCO thin films are grown on LAO substrates,the brownmillerite crystal structure BM-SCO and the perovskite crystal structure P-SCO can be obtained respectively by adjusting the growth temperature under suitable growth oxygen pressure,laser energy,and the like.However,due to the large number of oxygen vacancies in P-SCO,it is difficult to observe its intrinsic ferromagnetic metallicity.On the?LaAlO₃?0.3-?SrAl0.5Ta0,5O₃?0.7 substrate,it is difficult to obtain good P-SCO thin films because the oxygen vacancy concentration is too high.Using the method of oxidizing BM-SCO with NaCIO solution,we obtained P-SCO with low ferromagnetism through post-treatment.Chapter 4 discusses the topotactic competition between BM-SCO and P-SCO at different growth temperatures.We analyzed the lattice parameters of SCO/LAO thin films grown at different temperatures and found that there is a topotactic competition between BM-SCO and P-SCO at a certain growth temperature.Combining the first-principles calculations,we analyzed the trends of Gibbs free energy of P-SCO and BM-SCO at high temperatures.We believe that Gibbs free energy of two phases is almost consistent at a certain temperature.The important reason.Through spectroscopic ellipsometry and X-ray photoelectron spectroscopy,we found that the electronic structures of these two phases are very similar.This is mainly because the concentration of oxygen vacancies in P-SCO is too high.Although the perovskite structure still remains,it contains a large amount of Co-O tetrahedrons,resulting in little difference in the physical properties of the two.Chapter 5:Summarized the full text.