Preparation Of Heteroepitaxial SrMnO₃ Monocrystalline Thin Films And Study Of Interface And Defects

The development of basic devices such as electronic transistors,semiconductor lasers and semiconductor solar cells in the modern microelectronics and optoelectronics industry is mostly due to the establishment of semiconductor heterogeneous interface science.The tremendous progress of microelectronics and optoelectronics industry has brought mankind into the information technology era,thus completely changing the way of human production and life.Unlike conventional semiconductor systems,transition metal ions in transition metal oxides have d or f orbital electrons that are not filled with the shell layer,and these electrons have very strong correlation effects,which will lead to a variety of exotic properties,such as high-temperature superconductivity,magnetic ordering,giant magnetoresistance and multiferroic properties,and will be expected to be applied to the next generation of new multifunctional electronic devices.At present,there are still many scientific problems to be solved in the study of epitaxial growth,defect mechanism and nature of physical properties of strongly correlated surfaces/interfaces in perovskite oxides,especially the study of novel physical properties in strongly correlated systems is inextricably linked to the fine control of material structure,surface and interface preparation.Molecular beam epitaxy(MBE)has unique technical advantages in the growth of thin film materials due to its high purity elemental atomic beams,in-situ monitoring/feedback control of the growth process,clean ultra-high vacuum environment,and flexible means for fine control of surface and interface,which enables precise control of cation stoichiometry,surface/interface atomic configuration and associated defects.In this thesis,the preparation of Sr Mn O₃(SMO)heterogeneous epitaxial single crystal films on Sr Ti O₃(STO)substrates and La Al O₃(LAO)substrates has been realized using MBE technique based on in situ reflection high-energy electron diffraction(RHEED)real-time monitoring means,respectively.The growth pattern,surface/interface,defect analysis and electrical properties of the films were investigated,and the main research contents and results are as follows:1.The precise tuning of the stoichiometric ratio of SMO single-crystal films on STO substrates was successfully achieved by MBE technology,and the influence patterns of different cation stoichiometric ratios on the surface,structure,defects and electrical response characteristics of the films were obtained.Firstly,the growth conditions and processes of SMO/STO heterogeneous epitaxial films were initially explored by MBE co-deposition growth method.On this basis,the laws of fine regulation of SMO cation stoichiometry ratio by layer-by-layer MBE were systematically investigated and obtained by observing and analyzing the characteristics of RHEED intensity oscillations during the growth process with in situ RHEED.We analyzed and studied the effects of different cation stoichiometric ratios on the surface and structural composition of thin films in detail,especially the interfacial lattice swelling and film defect formation due to deviated stoichiometric ratios.Finally,in order to investigate the effects of defects caused by deviated chemical components and the effects of their migration on the electrical properties of the films,the changes of dielectric response of SMO films with different stoichiometric ratios were studied and discussed.2.LAO substrates with single cut off surface and atomic-level flatness were obtained by combination of chemical etching and heat treatment.Based on this,high-quality SMO films were heterogeneously epitaxialized by layer-by-layer MBE under different growth conditions of substrate starting surfaces(mixed interface and single interface)and growth starting layers(Sr-O and Mn-O).Subsequently,the mechanism of the lattice expansion phenomenon at the SMO/LAO interface was explained by means of TEM,EDS and EELS analyses.Different types of interfacial defects formed by different strain were compared,and it was confirmed that higher concentrations of oxygen vacancies are generated in films under tensile strain,while cation vacancies are more likely to be generated at the interfaces of films under compressive strain.3.A combination of experimental and theoretical calculations was used to investigate the mechanism of residual strain relaxation in SMO/STO films(tensile strain).Through systematic SEM,TEM and EELS characterization and analysis,we have performed a detailed microstructural characterization and compositional analysis of the cracked regions at the film/interface.Combined with the calculation and analysis of the theoretical calculations for the formation energy of oxygen vacancies under different strain states and different crystal orientation distribution,we summarized the residual strain relaxation mechanism in SMO films related to the orientation distribution of oxygen vacancies under different strain states.