% IMPORTANT: The following is UTF-8 encoded.  This means that in the presence
% of non-ASCII characters, it will not work with BibTeX 0.99 or older.
% Instead, you should use an up-to-date BibTeX implementation like “bibtex8” or
% “biber”.

@PHDTHESIS{Perla:911176,
      author       = {Perla, Pujitha},
      title        = {{G}rowth and characterization of {I}n{A}s nanowire-based
                      {J}osephson junctions},
      school       = {RWTH Aachen},
      type         = {Dissertation},
      publisher    = {RWTH Aachen University},
      reportid     = {FZJ-2022-04488},
      pages        = {126 p},
      year         = {2022},
      note         = {Dissertation, RWTH Aachen, 2022},
      abstract     = {This work delves into the growth mechanism as well as
                      structural and electrical characterization of InAs nanowires
                      (NWs) for Josephson junctions. The superconductors used in
                      this case are aluminum and niobium. Josephson junctions are
                      an essential component of a superconducting qubit. This work
                      describes the evolution of the Josephson junctions within
                      the state-of-the-art and achieving higher transparency of
                      the semiconductor/superconductor interfaces. The first part
                      of the work deals with the optimization of the selective
                      area growth. This method offers greater control of the
                      growth of nanowires and provides higher uniformity.
                      Parameters such as temperature, indium growth rate, and
                      arsenic beam equivalent pressure (BEP) have been optimized
                      to achieve a growth yield of $95\%,$ using a 20 nm thick
                      silicon dioxide mask on a Si(111) substrate. Eventually, the
                      InAs nanowires are grown and optimized for diameters of
                      70-80 nm and lengths of 4-5 μm. Additional experiments have
                      been performed to dope the InAs nanowires with tellurium. In
                      the case of Josephson junctions, they offer a huge asset,
                      with a doping range of 1× $10^18$ $cm^−3$ to 1× $10^19$
                      $cm^−3.$ An increase in the conductance of these nanowires
                      is observed with increased doping and thereby an enhanced
                      critical current of the Josephson junctions. Moreover, Te
                      doping has shown an impact on the diameter and the length of
                      the nanowires, since it is a surfactant. Atom probe
                      tomography investigations performed on these nanowires show
                      additional (211) lateral facets, that shift the hexagonal
                      structure of the InAs nanowire to a partly dodecagon
                      structure at Te doping concentrations greater than
                      $1×10^19$ $cm^−3.$ Furthermore, the transparency of the
                      InAs/superconductor interface has been tuned. A defect-free
                      interface and a smooth film of a superconductor is
                      apre-requisite for a high-quality Josephson junction, since
                      this ensures a good coupling between the materials. A
                      complete in-situ method has been adopted, to grow Al and Nb,
                      onto the nanowires, thereby eliminating, any possible
                      exposure of the semiconductor surface to the ambient. To
                      achieve defect-free semiconductor/superconductor interfaces,
                      a brief degassing step is introduced to the nanowires before
                      the growth of the superconducting metals such as aluminum or
                      niobium. This process, ensured enhanced transparency between
                      the materials, thereby strengthening the coupling, by that
                      improving the proximity effect. To be brief, the proximity
                      effect induces Cooper pairs into the semiconductor, i.e. it
                      turns the NW partly into a superconductor. Furthermore, the
                      growth parameters of the metals evaporated are optimized to
                      produce a smooth and defect-free interface and are
                      investigated systematically. Lastly, the in-situ approach is
                      expanded to encompass the fabrication of Josephson junctions
                      at ultra-high vacuum conditions and to include other
                      superconducting and capping materials in the process. The
                      substrates made for this purpose have been prepared in such
                      a way that two nanowires grow in a square trench at 90° to
                      the planes of the trench. The growth windows for the NW
                      growth are meticulously and selectively placed in such a way
                      that one NW shadows the other during the metal evaporation,
                      thus, causing a junction on the latter wire. The
                      superconductors used in this process are optimized to create
                      smooth and defect-free layers. In the case of aluminum, the
                      growth of the metals is found to depend more on the
                      temperature than on the angle of deposition. In contrast,
                      for Nb, the angle of evaporation has a huge effect on the
                      smoothness of the film. The investigations presented in
                      these sections include transmission electron microscopy and
                      corresponding low-temperature electrical measurements. This
                      shadow approach, increased the metal evaporation angles onto
                      the nanowires, from 30° to 87°, thus causing smooth and
                      defect-free layers. This has also been shown to increase the
                      interface transparency, between the NW and the
                      superconductors. Lastly, this platform has also been used to
                      demonstrate the growth of complex NW networks and multiple
                      Josephson junctions.},
      keywords     = {nanowires (Other)},
      cin          = {PGI-9},
      cid          = {I:(DE-Juel1)PGI-9-20110106},
      pnm          = {5222 - Exploratory Qubits (POF4-522)},
      pid          = {G:(DE-HGF)POF4-5222},
      typ          = {PUB:(DE-HGF)11},
      doi          = {10.18154/RWTH-2022-08606},
      url          = {https://juser.fz-juelich.de/record/911176},
}