Atomic Scale Characterization Of Oxide Heterointerfaces And Its Electronic Properties

Due to the interactions of symmetry breaking, lattice, spin exchange charge transfer, chemical bonding and other degrees of freedom at interface, heterointerfaces often exhibit remarkable physical properties that vastly differ from those of their bulk constituents Especially for oxide heterointerfaces, they have attracted much attention of condensed matter physicists and materials scientists because of the various structures and the strongly correlated electron system. Over the past 20 years, people have made great progress in the research of heterojunction. Many novel phenomena have been found in this field, such as two-dimensional electron gas, interfacial superconductivity and interfacial magnetism. As many studies shown, the interfacial defects have significant influence on interfacial properties, yet it remains a challenging task to identify each individual interfacial defect and its electronic impact. In this work, four oxide heterointerfaces were selected as model systems. We grow the thin films and atomic-scale characterize the hetero-interfaces by using Cs-corrected scanning TEM and electron energy-loss spectroscopy. Finally, based on the experimental results, we build the interface models and implement DFT calculations. The main contents of the thesis are summarized as follows:1. Interfacial defect and its electronic impact at the SrVO3/SrTio3. The SrVO3 thin film with cubic structure shows epitaxial growth on (100) SrTiO3 substrate. The relationship between film and substrate is SrVO3(100)//SrTiO3 (100), SrVO3 [100]//SrTiO3 [100]. The stress of lattice mismatch is removed by domain structures. Atomic scale STEM images and EELS results show that the interfacial interdiffusion result in the formation of Tiv and VTi defects at the SrVO3/SrTiO3 heterointerface. And the Ti and V keep+4 valence state after diffusion. Based on the experiment results, we build the models of SrVO3/SrTiO3 interface and implement DFT calculations. The results show the electronic properties of the transition layer (SrVxTi1-xO3) vary with the ratio of Ti/V. What’s more, the calculation demonstrates that the diffusion of V weakens this interface and Ti strengthens it.2. Interfacial defect and its electronic impact at the BiFeO3/SrTiO3. The BiFeO3 thin film with pseudo cubic structure shows epitaxial growth on (100) SrTiO3 substrate. The relationship between film and substrate is BiFeO3 (XYZ)//SrTiO3 (XYZ), BiFeO3 [XYZ]// SrTiO3 [XYZ]. The growth mechanism of film show island mode (Volmer-Weberm). Atomic scale STEM images and EELS results show that the interfacial diffusion result in the formation of substitutional point defect (TiFe) at the BiFeO3/SrTiO3 heterointerface. And the Ti keeps +4 valence state after diffusion. Based on the experiment results, we build the models of BiFeO3/SrTiO3 interface and implement DFT calculations. The results show that there is two dimensional electron gas at the heterointerface of BiFeO3/SrTiO3. The TiFe interact with interfacial Bi and Fe atom resulting in the shift of Femi level. And the diffusion of Ti strengthens this interface. What’s more, we confirm the interface conductivity by using Peak Force Tapping Tunneling AFM.3. Interfacial defect and its electronic impact at the LaCrO3/SrTiO3. The LaCrO3 thin films with pseudo cubic structure show epitaxial growth on (100) SrTiO3 substrate. The relationship between film and substrate is LaCrO3(100)//SrTiO3 (100), LaCrO3[100]//SrTiO3 [100]. The growth mechanism of film show island mode (Volmer-Weberm). Atomic scale STEM images and EELS results show that the interfacial diffusion result in the formation of substitutional point defect (TiCr) at the LaCrO3/SrTiO3 heterointerface. And the Ti keeps +4 valence states after diffusion. Based on the experiment results, we bulid the models of LaCrO3/SrTiO3 interface and implement DFT calculations. The results show the existence of TiCr will reduce the electron density resulting in the lower interface conductivity. The reason should be the diffusion of Ti cause the charge redistribution within CrO2 layers.4. Interfacial defect and its electronic impact at the MgO/SrTiO3. The MgO thin films with cubic structure show epitaxial growth on (100) SrTiO3 substrate. The growth mechanism of MgO film is related to growth rate, and the relationship between film and substrate is MgO (100)//SrTiO3 (100), MgO [100]//SrTiO3 [100]. The large stress is removed by mismatch dislocation and finally results in a semi-coherent interface. Atomic scale STEM images and EELS results show that the interfacial diffusion result in the formation of TiMg-ViMg defect complex at the MgO/SrTiO3 heterointerface. And the Ti keeps +4 valence states after diffusion. Based on the experiment results, we build the models of MgO/SrTiO3 heterointerface and implement DFT calculations. The results show the existence of TiMg-VMgI defect complex cause the shift of conduction band to the higher energy level, and enlarge the band gap of MgO/SrTiO3.