Composition-dependent structural transition in epitaxial thin films on Si(111)

Bismuth-antimony alloys (Bi 1− x Sb x) are topological insulators between 7 and 22% Sb in bulk crystals, with an unusually high conductivity suitable for spin-orbit torque applications. Reducing the thickness of epitaxial Bi 1− x Sb x films is expected to increase the maximum band gap through quantum confinement, which may improve isolation of topological surface-state transport. Like Bi (001) on Si (111), Bi 1− x Sb x has been predicted to form a black phosphoruslike allotrope with unique electronic properties in nanoscale films; however, the impact of Sb alloying on both the bulklike and nanoscale crystal structures on Si (111) is currently unknown. Here we demonstrate that the allotropic transition in ultrathin epitaxial Bi 1− x Sb x films on Si (111) is suppressed above 8–9% Sb, resulting in an unexpected (012) orientation within the topologically insulating regime. The metallic temperature-dependent conductivity …