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@INPROCEEDINGS{Buchhorn:1053087,
      author       = {Buchhorn, Jonas and Jalil, Abdur Rehman and Grützmacher,
                      Detlev and Schäpers, Thomas},
      title        = {{O}ptimizing epitaxial growth of {B}i2{T}e3 layers on
                      sapphire towards high mobilities},
      reportid     = {FZJ-2026-01427},
      year         = {2025},
      abstract     = {Since the first proposal of creating Majorana bound states
                      at the interface of a strong topological insulator and a
                      superconductor [1], the interest in researching the
                      properties of three-dimensional topological insulators has
                      grown. Even though there were many different architectures
                      for fault-tolerant Majorana-based quantum bits presented
                      over the years [2-4], the two ingredients stayed the same,
                      being a three dimensional topological insulator nanowire and
                      a conventional s-wave superconductor. While there are
                      several methods to prepare topological insulator thin films,
                      molecular beam expitaxy promises to be the most scalable
                      approach of creating pristine interfaces with the
                      superconductor [5]. Therefore, in this work the method of
                      epitaxially preparing topological insulators is employed.
                      With various substrates available, we focused on sapphire
                      substrates and worked on improving the growth of Bi2Te3,
                      starting from optimizing the methods of chemical substrate
                      cleaning, concluding in a sequence of Piranha solution and
                      hydrofluoric acid etching. Further investigations were made
                      to find an optimal Bi/Te atomic flux ratio in a
                      Te-overpressure regime and a suitable substrate temperature,
                      finally leading to twin-defect free crystals. We were aiming
                      at high mobilities in cryogenic magnetotransport
                      measurements in van der Pauw geometries, performed only
                      hours after the crystal growth. All results are backed with
                      X-ray diffraction analysis, showing correlations between
                      crystal quality and electrical properties. Highest mobility
                      samples show indications of significant surface transport of
                      up to $40\%,$ by Hall voltage non-linearities and by
                      Shubnikov-de Haas oscillations revealing sheet carrier
                      densities below 1⋅1012 cm-2. However, mobility values
                      obtained by multi-channel Hall analysis and Dingle fits to
                      quantum oscillations do differ by an order of magnitude,
                      ranging from 2000 to 25000 cm2/Vs. This observation hints to
                      a more complex explanation than classical multi-channel
                      contributions from bulk and surface, like it was seen in
                      cleaved bulk crystals in the past [6]. As the next step we
                      plan to investigate the consistency of these anomalous
                      effects and high mobilities from milli-/micrometer scale
                      devices down to the nanometer regime, by ex-situ etching the
                      material or using selective area epitaxy. This work was
                      supported by JST within ASPIRE for rising scientists.[1] L.
                      Fu, C. L. Kane, Phys. Rev. Lett. 100 096407 (2010).[2] S.
                      Plugge et al., New J. Phys. 19 012001 (2017).[3] C. Schrade,
                      L. Fu, Phys. Rev. Lett. 121, 267002 (2018).[4] R. Aguado, L.
                      P. Kouwenhoven, Physics Today 73 (6), 44-50 (2020).[5] P.
                      Schüffelgen, D. Rosenbach, C. Li, et al., Nat. Nanotechnol.
                      14, 825-831 (2019).[6] D.-X. Qu et al., Science 329, 821-824
                      (2010).},
      month         = {Oct},
      date          = {2025-10-27},
      organization  = {International Workshop on Hybrid
                       Quantum Materials, Sciences, and
                       Technologies 2025, Matsue (Japan), 27
                       Oct 2025 - 29 Oct 2025},
      subtyp        = {After Call},
      cin          = {PGI-9 / PGI-10},
      cid          = {I:(DE-Juel1)PGI-9-20110106 / I:(DE-Juel1)PGI-10-20170113},
      pnm          = {5222 - Exploratory Qubits (POF4-522) / DFG project
                      G:(GEPRIS)491798118 - Magnetische topologische Isolatoren
                      für robuste Majorana Zustände (491798118) / DFG project
                      G:(GEPRIS)390534769 - EXC 2004: Materie und Licht für
                      Quanteninformation (ML4Q) (390534769)},
      pid          = {G:(DE-HGF)POF4-5222 / G:(GEPRIS)491798118 /
                      G:(GEPRIS)390534769},
      typ          = {PUB:(DE-HGF)24},
      url          = {https://juser.fz-juelich.de/record/1053087},
}