Optimization of Epitaxial Graphene Growth for Quantum Metrology

The electrical quantum standards have played a decisive role in modern metrology,
particularly since the introduction of the revised International System of Units (SI) in
May 2019. By adapting the basic units to exactly defined natural constants, the
quantized Hall resistance (QHR) standards are also given precisely. The Von Klitzing
constant RK = h/e
2
(h Planck's constant and e elementary charge) can be measured
precisely using the quantum Hall effect (QHE) and is thus the primary representation
of the ohm. Currently, the QHR standard based on GaAs/AlGaAs heterostructure has
succeeded in yielding robust resistance measurements with high accuracy <10−9
.
In recent years, graphene has been vastly investigated due to its potential in QHR
metrology. This single-layer hexagonal carbon crystal forms a two-dimensional electron
gas system and exhibits the QHE, due to its properties, even at higher temperatures.
Thereby, in the future the QHR standards could be realized in more simplified
experimental conditions that can be used at higher temperatures and currents as well as
smaller magnetic fields than is feasible in conventional GaAs/AlGaAs QHR.
The quality of the graphene is of significant importance to the QHR standards
application. The epitaxial graphene growth on silicon carbide (SiC) offers decisive
advantages among the known fabrication methods. It enables the production of largearea graphene layers that are already electron-doped and do not have to be transferred
to another substrate. However, there are fundamental challenges in epitaxial graphene
growth. During the high-temperature growth process, the steps on the SiC surface
bunch together and form terraces with high steps. This so-called step-bunching gives
rise to the graphene thickness inhomogeneity (e.g., the bilayer formation) and extrinsic
resistance anisotropy, which both deteriorate the performance of electronic devices
made from it.
In this thesis, the process conditions of the epitaxial graphene growth through a socalled polymer-assisted sublimation growth method are minutely investigated. Atomic
force microscopy (AFM) is used to show that the previously neglected flow-rate of the
argon process gas has a significant influence on the morphology of the SiC substrate and
atop carbon layers. The results can be well explained using a simple model for the
thermodynamic conditions at the layer adjacent to the surface. The resulting control
option of step-bunching on the sub-nanometer scales is used to produce the ultra-flat,
monolayer graphene layers without the bilayer inclusions that exhibit the vanishing of
the resistance anisotropy. The comparison of four-point and scanning tunneling
potentiometry measurements shows that the remaining small anisotropy represents the
ultimate limit, which is given solely by the remaining resistances at the SiC terrace steps.
Thanks to the advanced growth control, also large-area homogenous quasi-freestanding
monolayer and bilayer graphene sheets are fabricated. The Raman spectroscopy and
scanning tunneling microscopy reveal very low defect densities of the layers. In
addition, the excellent quality of the produced freestanding layers is further evidenced
by the four-point measurement showing low extrinsic resistance anisotropy in both
micro- and millimeter-scales.
ABSTRACT
ii
The precise control of step-bunching using the Ar flow also enables the preparation of
periodic non-identical SiC surfaces under the graphene layer. Based on the work
function measurements by Kelvin-Probe force microscopy and X-ray photoemission
electron microscopy, it is shown for the first time that there is a doping variation in
graphene, induced by a proximity effect of the different near-surface SiC stacks. The
comparison of the AFM and low-energy electron microscopy measurements have
enabled the exact assignment of the SiC stacks, and the examinations have led to an
improved understanding of the surface restructuring in the framework of a step-flow
model.
The knowledge gained can be further utilized to improve the performance of epitaxial
graphene quantum resistance standard, and overall, the graphene-based electronic
devices. Finally, the QHR measurements have been shown on the optimized graphene
monolayers. In order to operate the graphene-based QHR at desirably low magnetic
field ranges (B < 5 T), two known charge tuning techniques are applied, and the results
are discussed with a view to their further implementation in the QHR metrology