Atomic-Scale Control and Characterization of Oxide Heterostructures: Correlating Interfacial Structure and Novel Functionalities

Complex oxide materials providing numerous physical properties can be fabricated in different forms, for instance in thin films or heterostructures, due to their highly adaptable crystal structures. In the last decade, the heterostructures of complex oxides have been in high-demand due to their large number of impressive functionalities, which do not exist in their bulk forms but emerge at the interfaces. Recent technical improvements of epitaxial growth techniques enable fabricating high-quality oxide heterostructures, where the phenomena occurring at their interfaces can be tailored depending on the choice of the constituents. However, the key factor dominating the interface functionalities is the control of the interface sharpness. Prominent combination of state-of-the-art atomic-layer-by-layer molecular beam epitaxy (ALL oxide-MBE) and aberration-corrected scanning transmission electron microscopy (STEM) is utilized in the thesis, for the atomic-layer-precise synthesis and atomic-resolution characterization of the heterostructures, respectively. Atomically-resolved STEM imaging [i.e. high-angle annular dark-field (HAADF), annular bright field (ABF)] and spectroscopy [i.e. electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDXS)] techniques are combined with dedicated conductivity measurements as well as atomic force microscopy and X-ray diffraction. Using these results, the local structure, octahedral distortions, and chemical properties are correlated with the functionalities of the systems. For the STEM data analysis, “Oxygen-octahedra picker tool” and “STEM-SI Warp” software tools are used in order to quantify and to post-correct the (spectrum) images, respectively. Complex functional oxide heterostructures consisting of iso-structural or hetero-structural contacts – mainly based on La2CuO4 and its derivatives – are concerned with the aim of not only tailoring the novel interface properties, which are directly linked to the local structural and chemical properties but also identifying the interface sharpness. First, La1.6A0.4CuO4 / La2CuO4 bilayers composed of a metallic (M) and an insulating phase (I), where A represents a divalent dopant (namely, Ba2+, Sr2+, and Ca2+), are studied. After the growth optimization, detailed characterization of the structures – especially extensive STEM investigations – substantiated the importance of the elemental distribution at the interfaces: Despite the perfect epitaxial growth, the dopants were found to be inhomogeneously distributed depending on the dopant size. This distribution defines whether the final superconducting properties emerge due to the striking “interface effect” or due to “classical” homogeneous doping. Moreover, a clear correlation between dopant size, dopant distribution and local lattice deformations is underlined suggesting a relationship between the nature of superconductivity (interface vs bulk) and Jahn–Teller distortions of the anionic sublattice. The second example of homoepitaxial (i.e. iso-structural) systems considered in this work is two-dimensionally-doped (i.e. δ-doped) La2CuO4 superlattices, where specified La–O atomic layers in the La2CuO4 crystal structure are substituted with A–O layers, in which A is an acceptor dopant (A = Ba2+, Sr2+, Ca2+). STEM-EELS analyses substantiated that despite the differences on the cation redistribution lengths, δ-doping results in asymmetric dopant distribution profiles at the interfaces. Such distribution is correlated with a qualitative model based on thermodynamic considerations and growth kinetics: As far as the dopant redistribution mechanism is concerned, the main factor leading to intermixing in the substrate direction is thermal diffusion. On the other hand, the wider distribution in growth direction is a consequence of the high lateral mobility of the atoms, which triggers the tendency to cationic intermixing at the surface. Moreover, the substrate temperature variation also affects the cationic distribution length, while the tensile strain induced by the substrate may influence the asymmetric profile. Furthermore, the contacts of different materials, namely, lanthanum cuprate and lanthanum nickelate systems, are studied. High-temperature superconductivity at the interface of lanthanum cuprate (La2CuO4, 214-phase) and strontium (Sr)-doped lanthanum nickelate (La2-xSrxNiO4, 214-phase) heterostructures, and high-temperature thermoelectricity of lanthanum cuprate (La2CuO4, 214-phase) and lanthanum nickelate (LaNiO3, 113-phase) heterostructures are reported. For the former, i.e. La2CuO4/La2–xSrxNiO4 contacts, the ability to tune the superconducting properties simply by changing the structural parameters is presented. More importantly, STEM techniques combined with dedicated conductivity measurements evidenced the decoupling between the electronic charge carrier and the cation (Sr) concentration profiles at the interface, which induces the formation of a hole accumulation layer dictating the final superconducting properties. This phenomenon is rationalized in the light of a generalized space-charge theory. As far as the La2CuO4/LaNiO3 heterostructures are concerned, the variation of the individual layer thicknesses (with constant total film thickness) influences the physical properties: As the thickness of the individual layers is reduced, the electrical conductivity decreases and the sign of Seebeck coefficient changes. Independent from the functionalities, the differences in chemical sharpness of lanthanum cuprate–lanthanum nickelate interfaces are further realized, although all the interfaces are structurally sharp: In the case of La2CuO4/La2– La2CuO4, 214-phase xSrxNiO4/La2CuO4 contacts, the La2CuO4–La2–xSrxNiO4 interface is sharper concerning the elemental intermixing, while the La2–xSrxNiO4–La2CuO4 interfaces exhibit a wider Sr distribution. On the other hand, the decrease of individual layer thickness in La2CuO4/LaNiO3 multilayers results in strong intermixing while thicker cuprate–nickelate layers maintain sharper interfaces. In the case of hetero-structural epitaxy of different materials (phases), e.g. superconducting–ferromagnetic La2CuO4/LaMnO3 multilayers, substantial differences in cationic redistribution and the local octahedral network is observed. STEM investigations demonstrate that Sr redistribution in 113-LaMnO3 and 214-La2CuO4 phases is different and this directly affects the sharpness of the interfaces. In particular, a stronger tendency of Sr segregation (in growth direction) in the 113-phase compared to the 214-phase is unveiled. Moreover, detailed high-resolution STEM imaging and spectroscopy of PLD-grown NdNiO3 epitaxial layers on [011]-oriented NdGaO3 substrate experimentally show a structural re-orientation from the [011]-phase (α-phase) to the [101]-phase (β-phase), which could be understood within the framework of DFT+U calculations. The calculations further unveil enhanced NiO6 octahedral breathing distortions for tensile strained α- and β-phases of NdNiO3