Properties of Barium Bismuthate Layers on Si: Molecular Beam Epitaxy Study

With slowing down of Moore’s law, related to scaling of integrated circuits, alternative technologies such as quantum computing require research efforts for pushing the limits of new generation of electronics. A promising approach to realize fault-tolerant quantum computations is to build qubits that are intrinsically protected against quantum errors thanks to the topological nature of the used materials. Such computations are realized when Majorana fermions are detected and manipulated. Topological superconductors are the perfect solid state material system for hosting Majorana bound states. Equivalently, an interface between a superconducting and a topological insulating layer is also expected to host such states, based on the proximity effect.<br/>Barium bismuthate (BBO) is a relevant material system because when hole-doped with potassium, it is a superconductor with a critical temperature of 29.8 K. In addition, with the spin-orbit coupling of bismuth considered, fluorine-doped BBO is predicted to be a topological insulator according to density functional theory (DFT) studies.<br/>The work presented is related to material development and characterization of the parent compound utilizing oxide molecular beam epitaxy (MBE) process. In our work, we integrate BaBiO_x on Si(001) substrate, using an epitaxially grown strontium titanate SrTiO₃ single-crystalline buffer layer. It is demonstrated that the epitaxy of BaBiO_x is only achieved in excess of oxygen plasma because of the volatile nature of bismuth molecules reaching the substrate. Due to the high vacuum environment in the oxide chamber of the MBE machine that can reach below 10^-8 torr as base pressure, ordered oxygen vacancies are created within the perovskite layers, forming a mixture of brownmillerite and perovskite phases. The mixed phase is demonstrated based on transmission electron microscopy (TEM) images. A method, in function of oxide MBE process parameters, to retrieve fully perovskite phase (with no ordered oxygen vacancies) is presented.<br/>Due to the breathing distortion within its lattice, barium bismuthate is a Peierls insulator with an optical band gap of 1.96 eV. Optical conductivity is measured using spectroscopic ellipsometry, showing a peak centred around 2 eV. The Raman response of BBO observed at 570 cm^-1 is attributed to the breathing distortion of BiO₆ octahedra, which is in resonance with a laser wavelength of 633 nm. In our studies, thickness dependence of the presence of the breathing distortion is studied. In the upcoming study, a light will be shed on the band structure of fluorine-doped by measuring it using angle-resolved photoemission spectroscopy (ARPES) to detect the existence of topological states.