Since 2004, the discovery of graphene with its one-atom thickness and outstanding
carrier mobility, has initiated tremendous amount of research works in two-dimensional (2D)
materials. However, the absence of an intrinsic band gap limits graphene in its use for logic
electronics. Molybdenum disulfide (MoS
2), a member of 2D transition metal dichalcogenides
(TMDCs), is regarded as one of the alternative candidates beyond graphene. With its layer
dependent properties and existence of a semiconducting band gap, 2D MoS
2 shows the
potential to be the building blocks for future electronics. In 2011, exfoliated monolayer MoS
2
field effect transistors (FETs) with dielectric engineering were first demonstrated with an
enhanced mobilities of 217 cm
2/V.s and good immunity to the short chann...[
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Since 2004, the discovery of graphene with its one-atom thickness and outstanding
carrier mobility, has initiated tremendous amount of research works in two-dimensional (2D)
materials. However, the absence of an intrinsic band gap limits graphene in its use for logic
electronics. Molybdenum disulfide (MoS
2), a member of 2D transition metal dichalcogenides
(TMDCs), is regarded as one of the alternative candidates beyond graphene. With its layer
dependent properties and existence of a semiconducting band gap, 2D MoS
2 shows the
potential to be the building blocks for future electronics. In 2011, exfoliated monolayer MoS
2
field effect transistors (FETs) with dielectric engineering were first demonstrated with an
enhanced mobilities of 217 cm
2/V.s and good immunity to the short channel effect.
Afterwards, a numbers of prototypes were also demonstrated for nano-electronics such as
integrated circuits (ICs) and logical operators based on exfoliated 2D MoS
2. However, in
manufacturing, micromechanical exfoliation is not a reliable production method of MoS
2 as
the size and thickness of the products are uncontrollable. Recently, chemical vapor deposition
(CVD) has been demonstrated as the most promising synthesis method, achieving a
reasonable electrical performance compared to that of natural MoS
2 crystals. However, it still
remains challenging to produce high quality and continuous MoS
2 film up to wafer scale,
meaning that the development of MoS
2 based electronics is still in its early stage.
In this thesis, selectable CVD growth of MoS
2 isolated triangular islands and large area
continuous thin film on SiO
2/Si substrates were performed independently in a home-built
furnace system using MoO
3 and sulfur as precursors, by controlling the timing of sulfur
vaporization and the substrates positions. Hydrofluoric acid (HF) substrate pre-treatment was
found to improve the adhesion and modulate the size of the MoS
2 triangular islands. MoS
2
triangular islands are monolayers with a grain size of up to ~70 μm and with outstanding PL
signals, whereas the MoS
2 continuous thin film was composed of small grain size MoS
2
nanosheets, with smooth surface morphology and large covered area over centimeters.
Natural MoS
2 samples prepared by micromechanical exfoliation were also characterized and
fabricated as field effect transistors (FETs) as the references. Back gated FETs based on
as-grown MoS
2 triangular islands demonstrated comparably higher current densities and
estimated mobilities of up to ~ 16 cm
2/V.s due to their single crystal nature; while MoS
2
continuous thin film FETs showed lower mobilities of up to ~ 9 cm
2/V.s. Both the CVD MoS
2
devices showed a remarkable current on/off ratio of up to more than 10
8 and clear current
saturation. Thanks to the high-k environment, passivation with a 50 nm Al
2O
3 over-layer
deposited by atomic layer deposition (ALD) of MoS
2 triangular islands back-gated FETs
experiences less hysteresis and a significant increase in current densities of up to few times.
Also, the corresponding top gated device with the Al
2O
3 as the gate dielectric showed an
improved subthreshold slope, compared to those of back gated devices and enhanced current
densities. Finally, simple MoS
2/p-GaN heterostructures were prepared by depositing natural
MoS
2 by micromechanical exfoliation and CVD-growing of MoS
2 continuous thin film on
p-GaN/Si substrates correspondingly. A clear current rectifying effect with a rectification
ratio of up to ~40 was shown in the fabricated natural MoS
2/p-GaN heterostructure devices,
while the heterostructure devices based on CVD-grown MoS
2 continuous thin film on p-GaN
experienced abnormal reverse rectification, which may be due to the changes of their intrinsic
properties during the CVD growth and transport direction issue of MoS
2.
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