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@PHDTHESIS{Belete:834237,
author = {Belete, Melkamu Adgo},
othercontributors = {Lemme, Max C. and Waser, Rainer},
title = {{T}wo dimensional materials-based vertical heterojunction
devices for electronics, optoelectronics and neuromorphic
applications},
school = {Rheinisch-Westfälische Technische Hochschule Aachen},
type = {Dissertation},
address = {Aachen},
publisher = {RWTH Aachen University},
reportid = {RWTH-2021-09736},
pages = {1 Online-Ressource : Illustrationen},
year = {2021},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University; Dissertation, Rheinisch-Westfälische Technische
Hochschule Aachen, 2021},
abstract = {Advancement of digital microelectronics relied for decades
on the classical scaling philosophy guided by the famous
evolutionary trend known as “Moore’s law”. However, a
slowdown of this relentless device scaling is becoming
inevitable due to fundamental physical limits. This led to
the rise of a new strategy called “more than Moore
(MtM)” that targets on integrated circuit functionality
diversification by promoting novel non-digital (analog)
applications such as radio-frequency (RF) electronics, power
management systems, optoelectronics, sensors, micro/nano
electromechanical systems, next generation computing
systems, etc. This strategy requires new device concepts and
novel materials outperforming conventional ones. Novel
two-dimensional (2D) materials such as graphene and
molybdenum disulfide (MoS2) are suit- able for MtM
applications due to their unique structural, electrical and
optical properties. In line with the MtM goals, this thesis
investigates vertical hybrid devices based on graphene, MoS2
and their heterostruc- tures integrated into conventional 3D
silicon (Si) for applications to- ward RF electronics,
optoelectronics and neuromorphic computing. The fabrication
schemes used here are scalable and semiconductor process
technology compatible. The primary milestone in this thesis
has been the investigation of the potential of MoS2 as the
emitter diode of graphene-base hot electron transistors
(GBTs). GBTs are promising devices for high-speed analog
electronics and they have a vertical architecture comprising
a Si-emitter, a graphene-base and a metal-collector, each
isolated by a thin barrier. Maximizing the performance of
GBTs requires efficient hot-electron injection across the
emitter-base-barrier. Theory suggests that this can be
realized by using semiconducting barrier materials that form
low energy barriers to promote thermionic emission. MoS2 is
a good candidate in this regard owing to its semiconducting
behavior with a bandgap and electron affinity values
enabling band alignments providing a small barrier with
respect to the Si-emitter. Hence, “Si/MoS2 using
capacitance-voltage (C-V) and conductance-voltage (G-V)
techniques. The static dielectric constant of MoS2 is
obtained from the measured C- V data. Measurements under
electric-field stress, verified by analytical simulations,
have indicated the presence of interface states and mo- bile
negative ions in MoS2. This observation was further
supported by time-of-flight secondary ion mass spectroscopy
analysis that showed hydroxyl ions (OH−) possibly
originating from catalytic water splitting by MoS2.
Furthermore, transmission electron microscopy studies reveal
the structural properties of the film including its
polycystallinity with vertically aligned layers. Next,
charge carrier transport proper- ties were investigated
across “n+-Si/MoS2/Graphene” vertical heterojunction
diodes analogous to the emitter diodes of GBTs. Analyses of
the measured temperature dependent I-V data in corroboration
with analytical models confirmed that the electron transport
across the n+- Si/MoS2 heterojunction barrier is dominated
by thermionic emission. This fulfils the prerequisites for
using MoS2 as the emission barrier of GBTs.The thesis also
includes experiments on the “Si/MoS2/Metal” vertical
heterojunctions for memristive switching. Static (DC)
current-voltage (I-V) and resistive switching (RS)
characterizations including endurance and state-retention
tests demonstrate the memristive functionality of the
devices. The switching tests exhibit a bipolar and
nonvolatile RS behavior with encouraging endurance and state
retention for at least 140 DC switching cycles and 2500
seconds, respectively. Controlled C-V, G-V and switching
measurements in ambient and vacuum conditions, elucidated by
analytical simulations, suggest that the observed RS
behavior is due to electric field-driven movements of the
mobile OH- ions along the vertical MoS2 layers and their
influence on the potential barrier at the Si/MoS2 interface.
In addition, electro-optical characterizations, in
particular I-V measurements with and without white light
illumination and spectral responsivity (SR) measurements,
were carried out on vertical “n+-Si/MoS2/Graphene”
heterojunction diodes, which exhibit broadband optical
sensitivity. The SR data feature multiple peaks in the
ultraviolet and visible regions indicating that the measured
photocurrent is mainly due to excitations in the MoS2. In
addition, an infrared response is observed for energies
below the Si and MoS2 bandgaps. This may be attributed to
absorption in the graphene and/or inter-layer transitions in
a staggered band alignment or absorptions via midgap states
in the MoS2 bandgap. In conclusion, the work and findings in
this thesis can serve as a guideline for integrating 2D
materials and their heterostructures into the existing Si
platform to create hybrid heterojunction devices for
potential electronic, optoelectronic and neuromorphic
applications.},
cin = {618710},
ddc = {621.3},
cid = {$I:(DE-82)618710_20170609$},
pnm = {Graphene Flagship Core Project 2 / Quantum Engineering for
Machine Learning / GIMMIK : Graphenprozessierung auf 200mm
Wafern für mikroelektronische Anwendungen / Verbundprojekt:
Neuro-inspirierte Technologien der künstlichen Intelligenz
für die Elektronik der Zukunft - NEUROTEC - / Scalable MoS2
based flexible devices and circuits for wireless
communications / Skalierbare MoS2-basierte flexible
Bauelemente und Schaltkreise für drahtlose Kommunikation /
DFG project 255449811 - SPP 1796: High Frequency Flexible
Bendable Electronics for Wireless Communication Systems
(FFLexCom) (255449811)},
pid = {G:(EU-Grant)785219 / G:(EU-Grant)829035 /
G:(DE-82)BMBF-03XP0210F / G:(DE-82)BMBF-16ES1134 /
G:(GEPRIS)407080863 / G:(GEPRIS)255449811},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2021-09736},
url = {https://publications.rwth-aachen.de/record/834237},
}
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