Pushing a topological insulator to its ultrathin limit

In the field of condensed matter physics, there is a never-ending interest to find new materials that host unique properties. The implementation of these newly found materials in the field of quantum computing might present a solution to the current challenges that the semiconductor industry is facing regarding the increasing demand for computational power. By incorporating topological insulators in this field, one would be able to utilise their topologically protected surface states, where the electron's propagation direction and spin are coupled by spin-momentum locking. Limiting the distance between opposite topological surfaces causes their wave functions to overlap and hybridise, giving rise to one-dimensional edge states. Only in this limit the forbidden backscattering imposed by the spin-momentum locking results in quantised, dissipationless transport.

In light of our final goal, to push our material towards this low-dimensional limit, we aim to optimise the deposition of thin layers of the ternary chalcogenide three-dimensional topological insulator (Bi_0.4Sb_0.6)₂Te₃ by molecular beam epitaxy. On the one hand, this thesis aims to create the perfect platform to look for signatures of the quantum spin Hall effect. For this purpose we established a theoretical framework regarding the binary three-dimensional topological insulator materials. Experimentally, we investigated the influence of dissimilar InP (111)A, Al₂O₃ (001) and SrTiO₃ (111) substrates on the morphological and structural properties of the (Bi_0.4Sb_0.6)₂Te₃films. We found that the combination of vicinal Al₂O₃ (001) substrates and a post-anneal treatment results in high-crystalline, single domain films with a homogeneous height distribution. On the other hand, this thesis explores experimental methods such as angle-resolved photoemission spectroscopy, scanning tunnelling spectroscopy and magneto-transport measurements to investigate the electronic properties and verify the topological properties of (Bi_0.4Sb_0.6)₂Te₃ in its ultrathin limit.