Hybridization gap approaching the two-dimensional limit of topological insulator Bi$_x$Sb$_{1-x}$

Bismuth antimony alloys (Bi$_x$Sb$_{1-x}$) provide a tuneable materials platform to study topological transport and spin-polarized surface states resulting from the nontrivial bulk electronic structure. In the two-dimensional limit, it is a suitable system to study the quantum spin Hall effect. In this work we grow epitaxial, single orientation thin films of Bi$_x$Sb$_{1-x}$ on an InSb(111)B substrate down to two bilayers where hybridization effects should gap out the topological surface states. Supported by a tight-binding model, spin- and angle-resolved photoemission spectroscopy data shows pockets at the Fermi level from the topological surface states disappear as the bulk gap increases from confinement. Evidence for a gap opening in the topological surface states is shown in the ultrathin limit. Finally, we observe spin-polarization approaching unity from the topological surface states in 10 bilayer films. The growth and characterization of ultrathin Bi$_x$Sb$_{1-x}$ alloys suggest ultrathin films of this material system can be used to study two-dimensional topological physics as well as applications such as topological devices, low power electronics, and spintronics.