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@PHDTHESIS{Schffelgen:851098,
      author       = {Schüffelgen, Peter},
      title        = {{E}xploiting {T}opological {I}nsulators for {M}ajorana
                      {D}evices and {P}hysics via {M}olecular {B}eam {E}pitaxy},
      school       = {RWTH Aachen University},
      type         = {Dissertation},
      reportid     = {FZJ-2018-04800},
      pages        = {146},
      year         = {2018},
      note         = {Dissertation, RWTH Aachen University, 2018},
      abstract     = {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.},
      cin          = {PGI-9},
      cid          = {I:(DE-Juel1)PGI-9-20110106},
      pnm          = {524 - Controlling Collective States (POF3-524)},
      pid          = {G:(DE-HGF)POF3-524},
      typ          = {PUB:(DE-HGF)11},
      url          = {https://juser.fz-juelich.de/record/851098},
}