High-mobility 3D topological insulator nanoplatelets on hBN sheets

Topological insulators (TIs) have attracted immense attention during the past few years due to the fact that they are insulators in the bulk, but manifest conducting helical states at their boundaries [1]. In contrast to the quantum Hall state, these boundary states originate from strong spin-orbit coupling without any external magnetic field, such that time reversal symmetry is conserved and accordingly backscattering is forbidden. The unique combination of dissipation-less charge transport channels with intrinsic spin polarization renders TIs of strong interest for spintronic applications. However, while theory predicts TIs to be perfect bulk insulators, their experimental charge transport behavior is dominated by point defects (e.g., Se vacancies). Such defects are commonly introduced during the growth of thin films of three-dimensional (3D) TIs such as Bi2Se3, Bi2Te3, and Sb2Te3, which leads to strong bulk doping in these compounds. This leads to a quasi-metallic conduction channel parallel to the surface channels, rendering it difficult to unequivocally assign charge transport features to the topological protected surface states. Strategies to minimize the contribution of the bulk transport include compensation doping, alloying of differently doped TIs, or increasing the surface to bulk ratio by using ultrathin samples. However, doping or alloying often results in a drastic decrease in carrier mobility due to the introduction of structural defects. One promising strategy to improve the structural quality involves epitaxial growth of TI thin films on appropriate ultrathin crystal sheets. Thus far, epitaxial growth on graphene has been demonstrated to significantly reduce the defect density, although the electrically conductive graphene is not suitable as an underlying substrate if the grown TIs are to be characterized by charge transport experiments.