Growth of Twin-Free and Low-Doped Topological Insulators on BaF₂(111)

We demonstrate the growth of twin-free Bi 2 Te 3 and Sb 2 Te 3 topological insulators by molecular beam epitaxy and a sizable reduction of the twin density in Bi 2 Se 3 on lattice-matched BaF 2 (111) substrates. Using X-ray diffraction, electron diffraction and atomic force microscopy, we systematically investigate the parameters influencing the formation of twin domains and the morphology of the films, and show that Se- and Te-based alloys differ by their growth mechanism. Optimum growth parameters are shown to result in intrinsically low-doped films, as probed by angle-resolved photoelectron spectroscopy. In contrast to previous approaches in which twin-free Bi 2 Se 3 films are achieved by increasing the substrate roughness, the quality of our Bi 2 Te 3 is superior on the flattest BaF 2 substrates. This finding indicates that, during nucleation, the films not only interact with the topmost atomic substrate layer but also with buried layers that provide the necessary stacking information to promote a single twin, an observation that is supported by ab initio calculations.