TY  - THES
AU  - Schüffelgen, Peter
TI  - Exploiting Topological Insulators for Majorana Devices and Physics via Molecular Beam Epitaxy
PB  - RWTH Aachen University
VL  - Dissertation
M1  - FZJ-2018-04800
SP  - 146
PY  - 2018
N1  - Dissertation, RWTH Aachen University, 2018
AB  - The prospect of fault-tolerant topological quantum computation (TQC) based on Majorana zero modes (MZM) motivated the fabrication of high quality hybrid structures comprised of three-dimensional (3D) topological insulators (TI) and s-wave superconductors (S). The work presented here, deals with the successive optimization of material growth and fabrication processes to pave the way towards scalable construction of hybrid devices for quantum computing applications. In order to protect the physical surfaces of Bi2-xSbxTe3 TIs grown via molecular beam epitaxy (MBE) from oxidation, thin films have to be capped in-situ, which makes device fabrication challenging. The simplest device to test whether or not a given combination of S and TI hosts desired Majorana excitations is a Josephson junction (JJ). JJs of first and second generation had a thin AlOx layer as capping. Ex-situ fabricated Nb electrodes showed a low transparency and only small dissipationless Josephson supercurrents could be induced. However, an attenuated first Shapiro step in transport experiments indicated signatures of possible Majorana contributions to the critical current. In order to reproduce and enhance these observations, novel fabrication techniques were established. Full fabrication of JJs under ultra-high vacuum (UHV) conditions via various stencil lithography techniques were pursuit in order to generate pristine S–TI interfaces. In addition, different superconductive materials (Al, Ti, Nb) were examined with regards to their suitability for in-situ defined electrodes. Fully in-situ fabricated junctions with Nb electrodes and stoichiometric Al2O3 capping showed a high interface transparency, large critical supercurrents and a fully suppressed first Shapiro step, indicating transport mediated by so-called Majorana bound states (MBS). Transmission electron microscopy (TEM) analysis confirmed a high S-TI interface quality and a fully capped weak link. These findings suggest that Nb as superconductive material in combination with the established stencil techniques is key towards hybrid devices based on Bi2-xSbxTe3 TIs. The fusion of stencil technique with selective area growth (SAG) allowed to combine networks of selectively grown topological nanostructures with superconductive electrodes of arbitrary geometry. This paves the way for flexible in-situ fabrication of scalable S-TI hybrid devices towards TQC applications.
LB  - PUB:(DE-HGF)11
UR  - https://juser.fz-juelich.de/record/851098
ER  -