Molecular Beam Epitaxy of La2-xSrxCuO4 Films and Heterostructures
Since 1986, the study of high-temperature superconductivity (HTS) in cuprates has revealed a massive amount of discoveries, such as pseudogap, charge density wave, d-wave superconductivity, etc. These novel states of matter trigger even more unknowns in fundamental science and inspire enormous emergent applications. This dissertation presents our research on the archetypical La2-xSrxCuO4 (LSCO) thin films and heterostructures. Specifically, the research has been driven by several fundamental questions. For example, can we create c-axis Josephson junctions for scientific research and superconductor-based quantum computation? What controls the fundamental behaviors of interface superconductivity? To answer those questions, high-quality crystals are required. Here we utilized and improved the oxide atomic-layer-by-layer molecular beam epitaxy (ALL-MBE) technique to grow atomically smooth cuprate films and heterostructures to answer the proposed research questions. The main results are presented as follows. First, we improved the ALL-MBE growth in several ways to enhance the film quality significantly. Specifically, we studied the thermal annealing of oxide substrates and developed treatment methods for LaSrAlO4(LSAO) and SrTiO3(STO) substrates. Ramp-up rate and annealing temperature are found to be the most critical parameters. We then studied the synthesis of LSCO thin films via the ALL-MBE system. A detailed recipe for the growth of LSCO thin films on LSAO substrates is presented. Unique reflection high energy electron diffraction (RHEED) pattern features are observed in LSCO films. A strategy to monitor the film growth and maintain the correct stoichiometry is developed based on the real-time RHEED feedback. We also investigated the power and stability of ozone oxidation and compiled empirical post-annealing procedures suitable for various doping levels. Substrates and LSCO films were evaluated using atomic force microscopy (AFM) and RHEED. The results indicate that they are atomically perfect with high crystallinity. Mutual inductance (MI) tests reveal that the LSCO films are uniform over the whole sample area with a sharp superconducting transition. Second, LSCO heterostructures and superlattices have been synthesized to study the HTS c-axis Josephson junction and interfacial superconductivity. The method to probe the superconducting dead layer number near the interface is introduced using a series of superlattices. At the LSCO-LSAO interface, MI and transport measurements imply that the first two LSCO layers that are near the LSAO exhibit a substantial suppression of superconductivity, resulting in a barrier that is five layers thick in total. And an overdoped LSCO protective layer is found to be effective against carrier depletion in superconducting layers. Within LSAO barriers, a thickness of 2 unit-cells of LSCO interface superconductor is synthesized. The superconducting transition of the sample is tunable with doping and demonstrates the highest transition temperature of 34 K.