Fluorine doping of barium bismuthate by SF6 plasma

Topological insulators (TIs) are materials with an insulating bulk and topologically protected conductive surface states. Many potential applications can be found in the literature for this class of materials. However, the small bandgap and the poor stability of the TIs extensively studied in the literature, such as Bi2Se3 and Bi2Te3, restrict their potential. One of the proposed alternatives is barium bismuthate (BBO), an oxide perovskite that is predicted to be a TI with a relatively large bandgap of about 0.7 eV. Despite the potential of this material, accessing its TI phase requires increasing the Fermi energy by ∼2 eV, and, to our knowledge, this has not yet been achieved experimentally. One proposed way to shift the Fermi energy is to substitute a part of the oxygen atoms in its structure with fluorine. In this study, we investigated the low-temperature fluorination of MBE-grown BBO thin films using SF6 plasma as a fluorine source. We began our study by using XRD and AFM to identify the processing conditions that allowed us to preserve the quality of the film. Then, we confirmed the incorporation of fluorine into the exposed thin film by XPS and SIMS. Since oxygen deficient films are expected to be easier to alloy, we also compared these results with the fluorination of oxygen-deficient BBO3−x films. Finally, we used DFT to demonstrate that the fluorination of BBO3 is an energetically favorable process, supporting our observations.