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

Bismuth-antimony alloys (${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}$) 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 ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}$ 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), ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}$ 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 ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}$ 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 associated with surface states in Bi(001) was not observed in the ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}(012)$ 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 ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}(012)$ films in the thickness regime expected for the black phosphorus allotrope, suggesting a promising route to large-area growth of puckered-layer two-dimensional ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}$, which will be necessary to harness its unique electronic properties in practical applications.