Growth Optimisation of Bismuth Ultra-Thin Films by Molecular Beam Epitaxy

Bismuth (Bi), a well-established semimetal, exhibits unique electronic properties such as low effective mass, low carrier density, and the potential to host topological phases, including higher-order topological phase and quantum spin Hall phase. Bi ultra-thin films are essential for studying strain effects on electronic properties and topology, as strain relaxes with increasing thickness. In this study, we optimise growth conditions to achieve high-quality Bi ultra-thin films (<5 nm) of the (001) hexagonal phase, on GaSb(111)B substrates using molecular beam epitaxy. Structural characterisation was performed using X-ray diffraction and X-ray reflectivity, while morphological characterisation was conducted via atomic force microscopy. A two-step growth process was employed: a 2-bilayer seed layer was grown at 20°C, followed by additional Bilayers at elevated temperatures. Growth temperature variation (20°C–300°C) revealed that roughness reduces as growth temperature increases. Bi flux variation in the range of 3.3x10⁻⁸ mbar ~ 9.9x10⁻⁸ mbar revealed that the triangular morphology, characteristic of Bi's rhombohedral crystal structure, varies with the Bi flux. Bi films with large terraces of the triangular morphology showed Laue fringes from X-ray diffraction peaks from the Bi(001)H phase, indicating a well-defined interface between the substrate and the film. Magnetotransport measurements revealed mixed carriers and weak antilocalization, likely due to topological surface states or Bi's strong spin-orbit coupling. These findings lay the groundwork for future studies on topological phase transitions in strained Bi ultra-thin films, with potential applications in spintronics and quantum information technologies.