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@PHDTHESIS{Just:820616,
author = {Just, Sven},
othercontributors = {Voigtländer, Bert and Morgenstern, Markus},
title = {{D}isentangling parallel conduction channels by charge
transport measurements on surfaces with a multi-tip scanning
tunneling microscope},
school = {RWTH Aachen University},
type = {Dissertation},
address = {Aachen},
publisher = {RWTH Aachen University},
reportid = {RWTH-2021-05732},
pages = {1 Online-Ressource : Illustrationen, Diagramme},
year = {2021},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University; Dissertation, RWTH Aachen University, 2021},
abstract = {Within this thesis, both position-dependent charge
transport measurements with a multi-tip scanning tunneling
microscope (STM) are performed, and theoretical models for
describing these measured data are developed. Only a
combination of both allows for actually disentangling
multiple current transport channels present in parallel, in
order to reveal the physical properties of the investigated
systems, i.e. the conductivity of the individual channels.
In chapter 2, the instrumental setup for the multi-tip STM
is shown in general and the specific methods used for tip
positioning are discussed. An introduction into the theory
of distance-dependent four-point resistance measurements is
given in chapter 3. Here, the relations between four-point
resistance and conductivity influenced by the chosen probe
geometry are discussed for both a pure two-dimensional and a
pure three-dimensional system. Furthermore, also anisotropic
conductance in two dimensions is considered. Chapters 4-7
depict actual measurements with the multi-tip STM on
different sample systems, as semiconductors and topological
insulators. First, in chapter 4 the conductivity of the
Si(111)-(7x7) surface and the influence of atomic steps of
the underlying substrate are investigated. In order to
interpret the measured resistances, a 3-layer model is
introduced which allows for a description by three parallel
conductance channels, i.e. the surface, the space charge
region and the bulk. Such a model enables to extract a value
for the surface conductivity from the measurements.
Moreover, by a measurement of the conductance anisotropy on
the surface, the conductivity of a single atomic step can be
disentangled from the conductivity of the step-free
terraces. In chapter 5, the 3-layer model is extended to an
N-layer model in order to model the strongly depth-dependent
conductivity of the near-surface space charge region in
semiconductors in a more precise way. In order to
demonstrate the universal applicability of the N-layer
model, it is used to extract values for the surface
conductivity of Ge(100)-(2x1) and Si(100)-(2x1)
reconstructions from data already published in the
literature, but not evaluated in terms of the surface
conductivity. Chapter 6 depicts a further combined
experimental and theoretical approach in order to reveal
parallel conductance channels in topological insulators thin
films, i.e. the interface channel at the boundary to the
substrate and the interior of the film itself, which are
both in parallel to the transport channel through the
topological surface states at top and bottom surface of the
film. From measurements on specific surface reconstructions,
the conductivity of the interface channel can be revealed,
while the interior of the thin film is approached by band
bending calculations in combination with results from
angle-resolved photoemission spectroscopy measurements
(ARPES). Here, it turns out that in the thin-film limit the
charge carrier concentration inside the film is only
governed by the position of the Fermi level at the surface,
as it is revealed by ARPES, but not influenced by the actual
dopant concentration inside the film material which is
usually unknown, thus allowing for a reliable estimate of
the film conductivity. Finally, in chapter 7, the weak
topological insulator $Bi_14Rh_3I_9$ is investigated by
means of scanning tunneling spectroscopy and scanning
tunneling potentiometry, in order to reveal the presence and
the transport properties of the one-dimensional edge state
at step edges on the dark surface. From the spectroscopy and
thermovoltage measurements, it turns out that the
topological channel can indeed be found at step edges of the
2D TI-layer, as it has been already reported in literature,
and additionally at artificially created scratches into the
surface. However, it is not located directly at the Fermi
energy, and thus cannot substantially contribute to current
transport. Additionally, it turns out that the surrounding
so-called dark surface is very conductive itself due to
unintentional surface doping, as deduced from potentiometry
with applied transport field and distance-dependent
four-point measurements. Both facts prevent to directly
reveal the transport properties of the edge channels for the
studied $Bi_14Rh_3I_9$ crystals, but nevertheless in
principle the depicted measurement method should be capable
of revealing direct transport through an edge channel on
more sophisticated samples.},
cin = {134110 / 130000},
ddc = {530},
cid = {$I:(DE-82)134110_20140620$ / $I:(DE-82)130000_20140620$},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2021-05732},
url = {https://publications.rwth-aachen.de/record/820616},
}
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