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TY  - THES
AU  - Ringkamp, Christoph
TI  - Molecular-beam epitaxy of topological insulator - superconductor heterostructures on sapphire
PB  - RWTH Aachen University
VL  - Dissertation
CY  - Aachen
M1  - RWTH-2025-05170
SP  - 1 Online-Ressource : Illustrationen
PY  - 2025
N1  - Veröffentlicht auf dem Publikationsserver der RWTH Aachen University
N1  - Dissertation, RWTH Aachen University, 2025
AB  - Recent concepts in fault-tolerant quantum computing are based on use of Majorana fermionlike states. These states are proposed to exist at the edges of a p-wave superconductor nanowires. Over ten years ago, the discovery of topological surface states in topological insulators (TIs) such as Bi2Te3, Sb2Te3, and Bi2Se3 offered a new pathway towards the artificial creation of p-wave-like superconductivity. Molecular beam epitaxy (MBE) is an already established method for the controlled growth of these materials and is the center of this thesis. In this work, c-plane sapphire substrate was used for the controlled epitaxy of binary(Bi2Te3 and Bi2Se3) and ternary (Bi2-xSbxTe3 and Bi2Te3-ySey) topological insulators. Furthermore, Fermi level tuning was explored through Cd-doping of Bi2Se3, which resulted in CdBi2Se4 septuple layers for higher Cd incorperation. The stacking of Bi2Se3 and Sb2Te3into superlattices also has been tested. Both approaches were investigated to determine their suitability for enhancing the control over the band gap alignment towards the Fermi level, but did not show yet the necessary improvements. To progress from the plain growth, selective area epitaxy on sapphire was adopted for the standard TI materials to prevent degradation of crystal quality during fabrications processes and exposure to ambient conditions. Hall measurements demonstrate improved transport properties when switching from Si(111) to sapphire. In the next step, the shadow bridge technique on sapphire has been implemented, which enables in-situ fabrication of Josephson junctions. Nb and Ta were explored as potential superconductors (SC) for TI-SC Josephson junctions. To circumvent the typical interface problems of the TI-SC heterostructures, the introduction of Nb into the Bi2Se3 matrix was also studied to investigate intrinsic superconductivity in a TI material. I observed a change into a misfit layered structure of BiSe and NbSe2,which resulted in a transition into a superconducting phase with a two-fold anisotropy in magneto-transport measurements. This is already a necessary sign towards non-trivial superconductivity and paves the way for further investigations. This thesis demonstrates that utilizing sapphire as a platform for growing and fabricating TI-based devices is a viable way to improve their performance. Sapphire also allows for further material combinations for either TI-SC heterostructures or even intrinsic topological superconductors, increasing the possibilities of finding a suitable platform for creating Majorana-like states which can be harnessed then towards quantum computing.
LB  - PUB:(DE-HGF)11
DO  - DOI:10.18154/RWTH-2025-05170
UR  - https://publications.rwth-aachen.de/record/1012784
ER  -