SrTiO₃ Polar Surfaces And Epitaxial Growth:the Controlat The Atomic Scale

Perovskite oxides have attracted extensive attentions due to their rich material systems,simple crystal structures and novel physical properties.The emergent phenomenon can be observed in the surface and interface of transition metal oxides,which are different from their bulks.The interfacial reconstruction in polar oxides may further extend the physical properties of these structures.In this dissertation,we focus on the typical perovskite oxide SrTiO₃ to study the unusual properties of polar transition metal oxides.The study mainly includes the strain engineering on polar surface,the growth of polar films and polar heterostructures.1.The adsorption behaviors of Sr atoms,Co atoms and Co clusters on SrTiO₃（110）-（4×1）reconstructed surface are studied by scanning tunneling microscopy and density functional theory calculations.The Ti₂O₃ vacancy and Sr adatom defect pairs would spontaneously form on the ideal surface of SrTiO₃（110）-（4×1）to release its residual strain.When Sr atoms are deposited on the surface,the quasi one dimensional Sr adatom chains would form along the[1 0]direction.Additionaly,the Sr chains tend to stay in the middle side of adjucent Ti₂O₃ vacancies.The calculations indicate the formation of such adatom chains is determined by the surface strain relief–the local lattice relaxations in response to Sr adatoms and Ti₂O₃ vacancies are incompatible,leading to the strong repulsive interaction between them.When the Sr atoms are changed to Co atoms,which have similar adsorption behaviors.When the Co atoms grow into Co clusters,they would have an inverse modulation effect to the site of initial Sr atoms on the surface.2.We show that the layer-by-layer growth of homoepitaxial SrTiO₃（111）thin films can be achieved in molecular beam epitaxy method by keeping the growing surface reconstructed.And the cation stoichiometry is optimized precisely with the reflective high energy electron diffraction as the feedback signal that changes sensitively to the variation of metal concentration during growth.With atomically well-defined surfaces,the SrTiO₃（111）films show high dielectric performance with the charge density modulated in the range of 1.9×10¹³/cm² with the back gate voltage lower than 0.2V.The outstanding charge density modulation ability of the SrTiO₃（111）dielectric layer would have very wide application in other physical experiments.3.The growth of SrTiO₃（111）/LaTiO₃（111）heterostructure with discontinuities interface is studied with scanning tunneling microscopy and X-ray photoelectron spectroscopy.Due to the difference of surface polarity,the LaTiO₃（111）-（4×4）and SrTiO₃（111）-（4×4）reconstructed surfaces have very different microstructures.Additionally,the surface polarity of SrTiO₃（111）-（4×4）can be effectively decreased,when La atoms are deposited on the surface.This polarity compensation mechanism can be the driving force for the diffusion of La to the SrTiO₃（111）surface,which is grown on the surface of LaTiO₃（111）.4.Transition metal sulfides have very rich properties,which can behave even novel features with appropriate substrate,such as SrTi O₃.Based on the growth requirement of transition metal sulfides films,we design and construct a MBE-STM combined system.Additionally,single layer and multilayer FeSe films are achieved in this system.