Development of High-Quality BaBiO3 Thin Films via Molecular Beam Epitaxy: Growth Mechanism, Structural Integration, and Functional Properties

Korte inhoud:Complex oxides offer a significantly versatile platform for studying different material excitations which can be employed in different technologies. Perovskite oxides with chemical formula ABO3 are of great interest due to their notable electronic, structural, and optical properties. BaBiO3 is an example of such perovskite oxides which has recently gained more research attention. According to density functional theory, n-type BaBi(O1-xFx)3 is expected to be a topological insulator. Alongside with its p-type superconductive counterpart (Ba1-xKxBiO3), a device concept based on the proximity effect is proposed, where topological superconductivity can be induced at the interface of the two materials. The topological superconductive states serve as a solid state host for Majorana bound states which can be used for realizing fault-tolerant quantum computations. Efforts are underway to measure the exotic Majorana states based on contacting different superconductive and topological insulating materials. However, BaBiO3 is considered advantageous in the sense that it can provide the two states based on the same parent material. This is the reason why studying this material in its pure form is important to be able to utilize it in applications based on the alloyed structures. Obtaining high quality epitaxial thin films of BaBiO3 with controlled composition is faced by multiple challenges. They comprise reaching stoichiometric levels for Ba and Bi, ensure the cations complete oxidation, and control the oxygen vacancies formation. Another challenge is to develop high crystalline quality interfaces between the material and the substrate and control the crystal orientation. As an outlook, possibility of alloying the structure should also be readily achievable. For these reasons, molecular beam epitaxy is chosen as a growth technique thanks to its co-deposition capabilities and ability to produce high quality thin films. In this thesis, the epitaxial growth of BaBiO3 on different substrates such as SrTiO3-buffered Si(001), SrTiO3(001), and MgO(001) is thoroughly studied. Detailed material characterization is presented based on different techniques such X-ray diffraction, X-ray photoemission spectroscopy, Rutherford bacscattering spectroscopy, Raman spectroscopy, spectroscopic ellipsometry, and transmission electron microscopy. Cationic stoichiometry is achieved using oxygen plasma by accessing the adsorption-controlled regime which enables the realization of a self-regulating growth window. The occurrence of ordered oxygen vacancies is investigated; and a molecular beam eptiaxy based method to reduce them by transforming BaBiO3-δ into BaBiO3 is established. Co-growth of BaBiO3(001) and BaBiO3(011) is observed; and pathways to suppress the BaBiO3(011) orientation are described. Moreover, the optical properties and the electronic structures of the different thin films are evaluated. Films with thickness of 4 nm or greater possess double oxidation states on the Bi sites, breathing phonon mode, and and optical band gap of around 2 eV. On the other hand, 3 nm thin film presents no evidence of these characteristics, suggesting a possible insulator to metal transition occurring for ultra-thin film regime. For future considerations, initial experimental results are presented in this thesis which shows that fluorine can be incorporated in BaBiO3 and BaBiO3-δ.

Jaar van publicatie:2025

Toegankelijkheid:Embargoed