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@PHDTHESIS{Ringkamp:1012784,
author = {Ringkamp, Christoph},
othercontributors = {Grützmacher, Detlev and Morgenstern, Markus},
title = {{M}olecular-beam epitaxy of topological insulator -
superconductor heterostructures on sapphire},
school = {RWTH Aachen University},
type = {Dissertation},
address = {Aachen},
publisher = {RWTH Aachen University},
reportid = {RWTH-2025-05170},
pages = {1 Online-Ressource : Illustrationen},
year = {2025},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University; Dissertation, RWTH Aachen University, 2025},
abstract = {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.},
cin = {134610 / 130000},
ddc = {530},
cid = {$I:(DE-82)134610_20140620$ / $I:(DE-82)130000_20140620$},
pnm = {EXC 2004: Matter and Light for Quantum Computing (ML4Q)
(390534769)},
pid = {G:(BMBF)390534769},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2025-05170},
url = {https://publications.rwth-aachen.de/record/1012784},
}
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