Investigation of atomic and electronic structures of LaAlO3/CaTiO3 and MoS2/SiO2 interfaces by Cs-corrected STEM and EELS

Abstract

Hetero interfaces have a definition of interface of two different solids or liquids. Hetero interfaces are usually defined in few nanometer scale. Though their dimension is lower than nanostructures and films, hetero interfaces play a crucial role in determining electrical, magnetic, optical properties. Since interfaces are usually defined in the depth of few-nanometer scale, some analysis techniques such as x-ray diffraction (XRD), scanning electron microscopy (SEM), x-ray photoelectron spectroscopy (XPS) have limit to explain phenomenon at the interfaces in detail. <br/> Transmission electron microscopy (TEM) is one of most powerful analysis techniques that facilitate the structural and chemical study of hetero interfaces. Using high voltage-accelerated electron beam, TEM makes it possible to observe the interested local area in nanoscale. Especially, spherical aberration corrected (Cs-corrected) scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy (EELS) are the most commonly used technique to observe interfaces intuitively. These analysis methods facilitate investigating atomic arrangement of hetero interfaces, secondary interphases in case of inter diffusion, and bonding characteristics at the interfaces.<br/> In this thesis, we applied TEM based techniques to directly observe atomic and electronic structures of hetero interfaces and their impact on electrical properties. Furthermore, we investigated the origin of the formation of interfacial structures. <br/> First, we investigated the relationship between degrees of octahedral tilt and interfacial conductivity of perovskite heterostructures depend on the thickness of the pseudo-substrate. We compared electrical properties of LaAlO3/CaTiO3 (LAO/CTO) interfaces on SrTiO3 substrate with various thickness of CTO to find critical thickness in which metal-insulator transition occurs. Using integrated differential phase contrast (iDPC) scanning transmission electron microscopy (STEM) imaging and, we measured octahedral tilt angle of CTO film from CTO/STO interface to LAO/CTO interface to further explain thickness dependent metal-insulator transition. This work demonstrates that it is octahedral tilt angle and symmetry of perovskite structure that determines electrical property of interfacial conductivity of perovskite based hetero interfaces. To explain the relationship between electrical property and atomic structure of the LAO/CTO/STO heterostructure, first principles calculation is necessary.<br/> Next, we directly observed atomic and electronic structures of MoS2/SiO2 interfaces. We measured inter layer distance of the MoS2 films and investigated bonding characteristics. Through this work, we verified the impact of amorphous SiO2 layer on structures of MoS2 thin films. First, atomic structure of AS-MoS2 film is affected by the SiO2 layer. Second, we confirmed the existence of S-O bonding at the interface layer using EELS. We demonstrated that even amorphous SiO2 layer gave affect the formation of MoS2 thin film in atomic and electronic structural manner. We believe that this work can explain enhanced mobility or conductivity of MoS2 when it is deposited on SiO2/Si.<br/>