Composition-dependent structural transition in epitaxial Bi1-xSbx thin films on Si(111)

Bismuth-antimony alloys (Bi1-xSbx) 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 Bi1-xSbx 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), Bi1-xSbx 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 Bi1-xSbx 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 associated with surface states in Bi(001) was not observed in the Bi1-xSbx(012) films, suggesting that the (012) orientation may significantly reduce surface-state transport. Growth on a Bi(001) buffer layer may prevent this orientation transition. Finally, we demonstrate that Sb alloying improves the continuity and quality of nanoscale Bi1-xSbx(012) films in the thickness regime expected for the black phosphorus allotrope, suggesting a promising route to large-area growth of puckered-layer two-dimensional Bi1-xSbx, which will be necessary to harness its unique electronic properties in practical applications.