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@PHDTHESIS{Korte:172850,
      author       = {Korte, Stefan},
      title        = {{L}adungstransport durch {G}raphenschichtenund
                      {G}a{A}s-{N}anodrähte untersucht mit einem
                      {M}ultispitzen-{R}astertunnelmikroskop},
      volume       = {90},
      school       = {RWTH Aachen University},
      type         = {Dissertation},
      address      = {Jülich},
      publisher    = {Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag},
      reportid     = {FZJ-2014-06284},
      isbn         = {978-3-89336-990-4},
      series       = {Schriften des Forschungszentrums Jülich. Reihe
                      Schlüsseltechnologien/ Key Technologies},
      pages        = {96 S.},
      year         = {2014},
      note         = {Dissertation, RWTH Aachen University, 2014},
      abstract     = {This work describes the use of the combination of a
                      scanning electron microscope (SEM) and a multitip scanning
                      tunneling microscope (STM) with four tips as a nanoprober.
                      Electrical measurements on graphene layers and freestanding
                      gallium arsenide (GaAs) nanowires were conducted.
                      Four-probe-measurements are necessary to measure the
                      resisitvity of such one- and two-dimensional conductors. Due
                      to unknown voltage drops at contacts that carry currents,
                      additional contacts have to be employed for current-free
                      potential measurements. Therefore, the multitip scanning
                      tunneling microscope with its four individually controllable
                      tips has been upgraded with extended electronics, enabling
                      us to use it as a flexible nanoprober. Graphene layers on
                      insulating SiO$_{2}$ and hexagonal boron nitride (h-BN),
                      prepared by mechanical exfoliation, were contacted with the
                      multitip STM. Tunneling current could not be used as
                      feedback when approaching the first tip. Therefore, a
                      contrast change in the SEM image upon contacting a graphene
                      flake with a tip was used. Once contacted, flakes were
                      scanned by RTM and electrical measurements were conducted.
                      Graphene transferred to h-BN showed bubbles, wrinkles and
                      contaminations. Still, STM images of clean areas revealed a
                      moiré pattern, proving that the atomically thin graphene
                      lay flat on the atomically flat h-BN surface. Four point
                      measurements of these samples showed a poor conductivity of
                      1/$\sigma$ = 16$^{k \Omega} /box$ and a low field effect
                      mobility of $\mu$ = 300$^{cm^{2}}$/Vs. The reason for this
                      might be the contaminations from the transfer process, as
                      well as effects from prolonged irradiation with electrons
                      from the SEM. Freestanding p-doped GaAs nanowires, grown by
                      metal-organic vapor-phase-epitaxy in the
                      vapor-liquid-solid-growth mode, in a process with two
                      temperature steps, were contacted with the multitip STM.
                      Using three tips as well as the substrate as contacts, four
                      point measurements were performed. It showed that elastic
                      deformation of these flexible nanowires has no significant
                      influence on their conductivity. The high spatial resolution
                      of the combination of a SEM with a multitip STM made it
                      possible to record resistance profiles of freestanding
                      nanowires by performing four point measurements along a
                      nanowire. The main segment of the nanowires, grown at
                      400$^{\circ}$C for better crystal quality exhibits a
                      resisitivity of a few $^{k\Omega}$/$_{\mu m}$, in agreement
                      with literature values. The nanowire base, grown at
                      450$^{\circ}$C to facilitate better nucleation, shows an
                      increased resisitvity of several $^{M\Omega}$/$_{\mu m}$.
                      The resistance of the nanowire base is relevant especially
                      for future opto-electronical components based on
                      freestanding nanowires and thus has to be understood.
                      Comparing profiles of nanowires grown by an identical
                      process on different substrates showed that the substrate is
                      not the cause of the increased resistance. From the measured
                      resistivities the dopant concentrations, as well as the
                      thickness of the space charge layer at the surface of the
                      GaAs nanowires were calculated. The nanowire segments grown
                      at 400$^{\circ}$C have a dopant concentration of roughly
                      10$^{19}$cm$^{-3}$, those grown at 450$^{\circ}$C about 2
                      $\cdot$10$^{17}$ cm$^{-3}$. In the base the space charge
                      layer poses a considerable constriction to the conduction. A
                      qualitative explanation for the temperature dependence of
                      the dopant concentration is given.},
      keywords     = {Dissertation (GND)},
      cin          = {PGI-3},
      cid          = {I:(DE-Juel1)PGI-3-20110106},
      pnm          = {141 - Controlling Electron Charge-Based Phenomena
                      (POF3-141)},
      pid          = {G:(DE-HGF)POF3-141},
      typ          = {PUB:(DE-HGF)3 / PUB:(DE-HGF)11},
      url          = {https://juser.fz-juelich.de/record/172850},
}