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Large-area CVD Growth of Two-dimensional Transition

Metal Dichalcogenides and Monolayer MoS2 and WS2

Metal–oxide–semiconductor Field-effect Transistors

by

Pin-Chun Shen

M.S. Photonics and Optoelectronics

National Taiwan University, 2014

Submitted to the Department of Electrical Engineering and Computer Science

in Partial Fulfillment of the Requirements for the Degree of

Master of Science in Electrical Engineering

at the

Massachusetts Institute of Technology

June 2017

© 2017 Massachusetts Institute of Technology. All rights reserved.

Signature of author______________________________________________________________

Department of Electrical Engineering and Computer Science

May 12, 2017

Certified by____________________________________________________________________

Jing Kong

Professor of Electrical Engineering

Thesis Supervisor

Accepted by___________________________________________________________________

Leslie A. Kolodziejski

Professor of Electrical Engineering

Chair, Department Committee on Graduate Students

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Large-area CVD Growth of Two-dimensional Transition

Metal Dichalcogenides and Monolayer MoS2 and WS2

Metal–oxide–semiconductor Field-effect Transistors

by

Pin-Chun Shen

Submitted to the Department of Electrical Engineering and Computer Science

on May 12, 2017

in Partial Fulfillment of the Requirements for the Degree of

Master of Science in Electrical Engineering

Abstract

Two-dimensional semiconducting materials such as MoS2 and WS2 have been attractive for use in

ultra-scaled electronic and optoelectronic devices because of their atomically-thin thickness, direct

band gap, and lack of dangling bonds. Methods for large-area growth of 2D semiconducting

materials are needed to bring them to practical applications. This thesis aims to develop reliable

methods for growing high-quality monolayer MoS2 and WS2 by CVD and explore their intrinsic

electrical transport properties for electronic and optoelectronic device applications. The as-grown

monolayer MoS2 and WS2 exhibit n-type semiconducting behavior with excellent optical

properties. Various techniques are employed to characterize the CVD-grown materials, including

photoluminescence, UV-visible absorption, Raman spectroscopy, X-ray photoelectron

spectroscopy, and atomic force microscopy. Moreover, the electronic transport characteristics of

single-layer CVD-grown MoS2 and WS2 field-effect transistors with a back-gated configuration

are demonstrated.

Thesis supervisor: Jing Kong

Title: Professor of Electrical Engineering

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Table of Contents

Abstract .......................................................................................................................................... 3

Table of Contents .......................................................................................................................... 5

List of Figures ................................................................................................................................ 8

Chapter 1. Introduction ......................................................................................................... 11

1.1 Beyond Silicon: Opportunities in Two-dimensional Materials .............................................. 11

1.2 Significance of This Work .......................................................................................................... 13

Chapter 2. Physics of Transition Metal Dichalcogenides .................................................... 14

2.1 Atomically Thin Structure ......................................................................................................... 14

2.2 Electronic Structure ................................................................................................................... 14

2.3 Electron Transport and Scattering ........................................................................................... 16 2.3.1 Phonon Scattering ................................................................................................................. 16

2.3.2 Coulomb Scattering ............................................................................................................... 17

2.3.3 Surface Phonon Scattering and Roughness Scattering ......................................................... 17

2.4 Structural Defect-dependent Properties ................................................................................... 18

Chapter 3. CVD Growth of Monolayer MoS2 ...................................................................... 21

3.1 Monolayer MoS2 Growth ........................................................................................................... 21

3.2 Characterizations ....................................................................................................................... 22 3.2.1 Optical Microscopy Characterizations ................................................................................. 22

3.2.2 Raman Spectroscopy Characterizations ............................................................................... 23

3.2.3 Atomic Force Microscopy Characterizations ....................................................................... 25

3.3 Photoluminescence and Optical Absorption of MoS2.............................................................. 26 3.3.1 Photoluminescence ................................................................................................................ 26

3.3.2 UV-visible Absorption ........................................................................................................... 27

3.4 Defect Characterization of MoS2 ............................................................................................... 29 3.4.1 PL and Raman Intensity Mappings of MoS2 Triangular Domains ........................................ 29

3.4.2 PL Mappings of MoS2 Grain Boundaries .............................................................................. 29

3.5 Graphene/MoS2 In-plane Heterostructures ............................................................................. 31 3.5.1 Lateral Heterostructure Growth ........................................................................................... 31

3.5.2 Optical and AFM Images ...................................................................................................... 32

3.5.3 PL and Raman Intensity Mappings ....................................................................................... 33

Chapter 4. CVD Growth of Monolayer WS2 ........................................................................ 35

4.1 Monolayer WS2 Growth ............................................................................................................. 35

4.2 WS2 Growth Evolutions ............................................................................................................. 36

4.3 Raman Spectroscopy Characterizations .................................................................................. 38

4.4 Photoluminescence of WS2 ......................................................................................................... 39

4.5 X-ray photoelectron spectroscopy of WS2 ................................................................................ 40

Chapter 5. Monolayer MoS2 and WS2 Field-effect Transistors ......................................... 42

5.1 MoS2 Device Fabrication............................................................................................................ 42 5.1.1 TMD Transfer ........................................................................................................................ 42

5.1.2 E-beam Lithography, Electrode Deposition, and Lift Off ..................................................... 43

5.2 Electrical Transport Properties of MoS2 FETs with Ni contacts ........................................... 43

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5.3 Enhancement of Electron Mobility in MoS2 via a High-k Gate Dielectric ............................ 47

5.4 Electrical Transport Properties of WS2 FETs with Ni contacts ............................................. 49

Chapter 6. Conclusions and Future Work ........................................................................... 52

6.1 Conclusions ................................................................................................................................. 52

6.2 Future Work ............................................................................................................................... 53 6.2.1 Defect Characterization and Engineering ............................................................................ 53

6.2.2 Strategies to improve the electrical performance of CVD-grown 2D TMD transistors ....... 53

References .................................................................................................................................... 54

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List of Figures

Figure 2-1. (a) The lattice structure of monolayer MX2 and (b) the hexagonal planes of M and X

atoms [3, 7]. .................................................................................................................................. 14 Figure 2-2. Summary of electronic properties of TMD materials [8]........................................... 15 Figure 2-3. (a) Comparison for PL spectra of monolayer and bilayer MoS2. (b) The dependence of

MoS2 bandgap energy on its thickness. (c) Absorption spectra for monolayer and bilayer MoS2 [9].

....................................................................................................................................................... 15 Figure 2-4. Band structures for bulk, bilayer, and monolayer MoS2 and WS2 [10]. .................... 16 Figure 2-5. The dependence of carrier mobility in monolayer MoS2 on (a) temperature and (b)

carrier density (calculated from first-principles density functional theory). (c) The scattering

contributions from charged impurities and the total mobility of MoS2 due to the combined effect

[12, 13]. ......................................................................................................................................... 18 Figure 2-6. Typical defects in 2D TMD materials [24]. ............................................................... 19

Figure 2-7. (a) Formation energies of various point defects as functions of sulfur chemical potential.

Sulfur vacancy (Vs) has the lowest formation energy (~ 2 eV). (b) Schematic depiction of the

defect levels within MoS2 band gap [25]. ..................................................................................... 20 Figure 2-8. (a) Optical microscope image of a WSe2/MoS2 hetero-bilayer. (b) Normalized PL and

absorbance spectra for monolayer MoS2, WSe2, and their hetero-bilayers. (c) SEM images of

MoSe2/WSe2 heterostructures. (d) PL intensity map of a MoSe2/WSe2 lateral heterostructure

(Scale bars, 2 μm) [21-23]. ........................................................................................................... 20

Figure 3-1. (a) Molecular structure of PTAS and (b) PTAS aqueous solution. ............................ 22 Figure 3-2. (a) A schematically illustration of the CVD setup for growth of MoS2 films and (b) the

temperature profile used for monolayer MoS2 growth. ................................................................. 22 Figure 3-3. (a) Photograph of centimeter-scale monolayer MoS2 grown on SiO2/Si wafer. (b)

Optical images of the CVD-grown single-crystal triangular domain of MoS2, (c) the completely

continuous film of MoS2, and (d) the transition region between MoS2 triangular flakes and

continuous film. ............................................................................................................................ 23

Figure 3-4. Raman spectra of as-grown monolayer MoS2 (a) triangular flakes and (b) films

measured at different regions. ....................................................................................................... 24 Figure 3-5. AFM images of monolayer MoS2 (a) films and (b) triangular grains. (c) AFM step

height profile of a typical single-layer region. .............................................................................. 25 Figure 3-6. Room temperature photoluminescence of the as-grown monolayer MoS2 films. Raman

spectrum is also displayed in this figure in the unit of photon energy. An excitation wavelength of

532 nm was used for all PL and Raman spectra in this thesis. ..................................................... 27

Figure 3-7. Optical absorption spectra of as-grown monolayer MoS2 films on (a) mica, (b) fused

silica, and (c) c-plane sapphire. ..................................................................................................... 28 Figure 3-8. Intensity mappings of (a) PL emission wavelength and (b) Raman E2g and (c) A1g mode

frequencies of a representative as-grown MoS2 single crystal. .................................................... 30 Figure 3-9. (a) PL Intensity mapping for a region containing a linear-shape grain boundary and (b)

optical image of as-grown monolayer MoS2 film. The region corresponding to (a) are highlighted

by a blue dotted box. ..................................................................................................................... 30 Figure 3-10. Schematic illustration of the in-plane heterostructure of Graphene/MoS2 [32]. ...... 31

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Figure 3-11. (a)-(c) Typical optical images of as-grown in-plane heterostructure of

Graphene/MoS2. (d) The AFM image of the region highlighted by yellow circle in (a). (d) PL and

(e) Raman spectra of the grown MoS2 monolayer wrapping graphene. ....................................... 33 Figure 3-12. PL and Raman intensity mappings of the graphene/MoS2 junction corresponding to

the region highlighted by the yellow circle. .................................................................................. 34 Figure 4-1. Schematic illustrations of (a) the experimental setup and (b) the temperature profile

used for monolayer WS2 growth. .................................................................................................. 35 Figure 4-2. (a)-(d) Optical images of WS2 grains grown by different recipes. The WS2 monolayer

grown by recipe A is the focus of this chapter. (e) The AFM image of WS2 monolayer grown by

recipe A and (f) the thickness profile along the yellow line in the AFM image. .......................... 37

Figure 4-3. A schematic illustration of the CVD-grown WS2 evolution with growth time at 800 C.

....................................................................................................................................................... 38 Figure 4-4. Room-temperature Raman spectrum of as-grown WS2 flakes using a 532 nm laser

excitation. ...................................................................................................................................... 39 Figure 4-5. Room-temperature photoluminescence spectrum of as-grown monolayer WS2 (under

532 nm excitation). ....................................................................................................................... 40 Figure 4-6. Chemical composition analysis of as-grown WS2 using X-ray Photoelectron

Spectroscopy ................................................................................................................................. 41 Figure 5-1. Schematic process flow for fabrication of back-gate single-layer MoS2 field-effect

devices. The WS2 devices are fabricated using the same process flow. ....................................... 43 Figure 5-2. Schematic and optical image of the single-layer back-gate MoS2 EFT. Symmetrical 50

nm Ni contacts are defined on the CVD-grown monolayer MoS2 by e-beam lithography,

evaporation, and lift-off. ............................................................................................................... 44 Figure 5-3. Room temperature transfer characteristic for the single-layer MoS2 FET with Vds =

500 mV (L ~ 1 m and W ~ 10 m). Back-gate voltage is applied to the substrate. ................... 45

Figure 5-4. Ids-Vds characteristic for the MoS2 device acquired for different values of Vbg. ........ 46 Figure 5-5. Low-temperature Ids-Vbg transfer curve of the MoS2 FET acquired at 77 K. ............ 46 Figure 5-6. The Schematic of a back-gated MoS2 FET using a 25 nm HfO2/ 300 nm SiO2 hybrid

gate dielectric. ............................................................................................................................... 48

Figure 5-7. Transport properties of single-layer MoS2 FETs (L ~ 1 m and W ~ 6 m) on a

HfO2/SiO2 substrate. ..................................................................................................................... 48 Figure 5-8. Output characteristics (Ids-Vds) of the single-layer MoS2 FETs at different back-gate

voltages. ........................................................................................................................................ 49

Figure 5-9. Schematic and optical image of the single-layer back-gate WS2 FET. Symmetrical 50

nm Ni contacts are defined on the CVD-grown monolayer WS2 by e-beam lithography,

evaporation, and lift-off. ............................................................................................................... 50 Figure 5-10. (a) Saturation Ids-Vds behavior of a representative monolayer WS2 FET with Ni

contacts (L ~ 1 m and W ~ 15 m). (b) Output performance of the devices at small fields. ..... 50

Figure 5-11. Saturation Ids-Vds behavior of a representative monolayer WS2 FET with Ni contacts

(L ~ 1 m and W ~ 15 m). On/Off ratio of the device is larger than 106 at room temperature.

Inset: The Ids-Vds curve plotted in the logarithmic scale. .............................................................. 51

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Chapter 1. Introduction

1.1 Beyond Silicon: Opportunities in Two-dimensional Materials

Two-dimensional (2D) materials, a class of materials possessing ultimate limit of thinness

in vertical dimension, and representing the thinnest artificial materials in the universe, have

demonstrated themselves as a fertile ground for discovering exotic phenomena in condensed

matters and as a promising platform to push the frontier of semiconductor technology beyond the

Moore’s law.

Semiconducting materials, in particular silicon (Si), establish the foundation of modern

electronics. The configuration of conventional field-effect transistors (FETs) is composed of a

semiconducting channel contacted with source and drain electrodes and a gate electrode that can

create a vertical electric field coupling with the carriers on the channel material surface to control

the channel conductivity. Subsequent decrease in dimension of Si-based FETs, however, is

approaching limits of 5 nm gate lengths due to severe short channel effects such as increased direct

source-to-drain tunneling current, loss of gate control, and increased subthreshold swing (SS). New

device architectures and novel channel materials for next-generation FETs have therefore been

intensively searched. In this regard, 2D monolayer transition metal dichalcogenides (TMDs) come

into play because of their favorable electrostatic properties. Due to the atomically ultra-thin nature,

2D TMDs can approach the ideal effective screening length and can be operated beyond the

quantum capacitance limit, offering extremely high degree of gate electrostatic control for ideal

subthreshold swing (60 mV/decade) and low-power consumption. Moreover, most 2D TMDs

possess a higher band gap and heavier carrier effective masses than those of Si. These features are

advantageous to reduce direct source-to-drain leakage current and reach a high on/off current ratio

for ultra-scaled transistors [1].

As a member of 2D TMD material family, molybdenum disulfide (MoS2) has been

theoretically demonstrated to be superior to Si with respect to the sub-5 nm scaling limit [2]. The

first single-layer MoS2 transistor with a top-gated configuration was implemented in 2011. The

device showed n-type transfer characteristics with an excellent on/off ratio of ~108, a room-

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temperature field-effect mobility of > 200 cm2 V-1 s-1, and SS of 74 mV/decade [3]. A near ideal

SS of ~ 65 mV/decade has also been reached by MoS2 transistors using single-walled carbon

nanotube as the gate electrode [1], which confirms the great potential of MoS2 for low-power

electronic applications.

WS2, another representative of semiconducting TMDs, has also been a focus as next-

generation nanoelectronic and photonic materials. The large valence band splitting (~ 426 meV)

originating from strong spin-orbit coupling enables WS2 to be a perfect platform for realization of

spintronic and valleytronic devices. Theoretical simulations predict that the room-temperature

phonon-limited electron mobility in monolayer WS2 is over 1000 cm2 V-1 s-1, the highest value

among the 2D semiconducting TMDs. Recently, back-gated single-layer WS2 FETs sandwiched

between hexagonal boron nitride (hBN) films have been demonstrated, showing a high field-effect

mobility (~ 200 cm2 V-1 s-1) at room temperature with a high on/off ratio (~ 107) [4].

Despite the great progress in the electrical performance of TMD transistors as mentioned

above, the TMD materials used in those high-performance devices were obtained by mechanical

exfoliation methods. Such the mechanically exfoliated TMD flakes are typically few micrometers

in size with a random number of layers, limiting their application in commercial viable devices.

For practical applications, such TMD layered materials must be grown over large areas with good

electrical and optical properties. The development of synthesis methods for obtaining large-scale,

high-quality TMD materials is therefore of central importance.

Chemical vapor deposition (CVD) has been one of the most practical methods for large-

area growth of 2D materials because the precursors used for growth are human-friendly and the

process is cost-effective. The highest reported mobility of CVD-grown single-layer MoS2

transistors, however, has been below ~ 30 cm2 V-1 s-1 to date, which is substantially lower than the

theoretically predicted value (~ 410 cm2 V-1 s-1 at room temperature). Similarly, the field-effect

mobility of CVD-grown WS2 devices ranges from 1 to 10 cm2 V-1 s-1 [5, 6], showing significant

discrepancies between experiment and theory. Although the carrier transport properties in TMD

devices are strongly influenced by various extrinsic effects such as metal contacts, interface traps,

charged impurities, and dielectric environment, it is essential to improve the intrinsic properties of

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TMD materials from the synthesis side. Furthermore, the intrinsic defects and structural disorder

in TMD layered materials could also be a fertile ground to explore their undiscovered properties

and physical tunability. Thus, methods for large-scale growth of high-quality TMD monolayers, a

deep understanding of TMD structural defects, and strategies to liberate their remarkable carrier

transport properties are of vital importance to move the TMD field forward beyond Si technology.

1.2 Significance of This Work

This thesis aims to develop reliable methods for growing high-quality monolayer MoS2 and WS2,

explore their intrinsic electrical transport properties for electronic and circuit applications, and

characterize the structural defects and homogeneity in the CVD-grown 2D TMD materials. This

thesis is organized as follows: Chapter 2 offers an overview of 2D TMD materials, in particular,

their intrinsic electronic and optical properties and structural defects. Chapter 3 illustrates a CVD

synthesis method for large-area growth of high-quality monolayer MoS2 films and graphene/MoS2

in-plane heterostructures. Various techniques are employed to characterize the CVD-grown

monolayer MoS2, including Raman spectroscopy, atomic force microscopy, photoluminescence,

and absorption spectra. Chapter 4 demonstrates a simple CVD method for monolayer WS2 growth

and discusses the growth mechanism. X-ray photoelectron spectroscopy, Raman spectroscopy,

atomic force microscopy, and photoluminescence are performed on the as-grown WS2 monolayer.

Chapter 5 explores the electrical transfer characteristics of the single-layer MoS2 and WS2

triangular grains grown by the proposed CVD methods. Single-layer MoS2 and WS2 FETs with a

back-gated configuration using nickel (Ni) as source and drain contacts are demonstrated. The

enhancement of MoS2 electron mobility enabled by a high-k dielectric substrate is also presented.

Chapter 6 provides a summary of our findings and discusses the future work for improving the

performance of TMD transistors and for further study of 2D TMD defect characterizations and

engineering.

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Chapter 2. Physics of Transition Metal Dichalcogenides

2.1 Atomically Thin Structure

Two-dimensional transition metal dichalcogenides (2D TMDs) which are only three-atom-thick

are a class of materials with layered structures of the form X-M-X (MX2), where M is a transition

metal element from group IV, V, and VI and X is a chalcogen (S, Se, and Te). The layered structure

is constructed by a hexagonal plane of metal atoms which separates two hexagonal planes of

chalcogen atoms with covalent bonds, as shown in Figure 2-1.

Figure 2-1. (a) The lattice structure of monolayer MX2 and (b) the hexagonal planes of M

and X atoms [3, 7].

These layered materials may exhibit semiconducting, metallic, or superconducting properties,

depending on the selection of the transition metal elements. Figure 2-2 summarizes the electronic

properties of some representative TMD materials.

2.2 Electronic Structure

The electronic structures and properties of TMD materials vary with their thickness. For instance,

semiconducting TMDs such as molybdenum disulfide (MoS2), tungsten disulfide (WS2), and

tungsten diselenide (WSe2) exhibit a transition from direct bandgap to indirect bandgap as the

thickness varies from single layer to multilayer. As an example, monolayer MoS2 has a direct

bandgap of 1.8 eV, while bulk MoS2 possesses an indirect bandgap at 1.2 eV. The direct band gap

results in a sharp photoluminescence emission at ~ 1.8 eV from monolayer MoS2. Figure 2-3

clearly shows that the absorption, photoluminescence (PL), and bandgap energy of MoS2 highly

depend on its thickness. Single-layer MoS2 exhibits a much stronger PL emission than that of

bilayer MoS2 and the MoS2 bandgap decreases as its thickness increases.

(a) (b)

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Figure 2-2. Summary of electronic properties of TMD materials [8].

Figure 2-3. (a) Comparison for PL spectra of monolayer and bilayer MoS2. (b) The

dependence of MoS2 bandgap energy on its thickness. (c) Absorption spectra for monolayer

and bilayer MoS2 [9].

The electronic band structures of monolayer, bilayer, and bulk MoS2 and WS2 calculated by first

principles are presented in Figure 2-4. The influence of layer number on band structure is due to

the quantum confinement effect and the change in orbital hybridization between d orbitals of Mo

atoms and pz orbitals of S atoms. Unlike graphene that the lattice is all occupied by carbon atoms,

the A and B sublattices of in MoS2 (WS2) lattice structure are occupied by Mo (W) atoms and a

pair of S atoms (Figure 2-1 (b)). The difference between A and B sublattices results in the lift of

the decency at K (K)’ points in the Brillouin zone and creates a desirable bandgap in MoS2 (WS2).

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Figure 2-4. Band structures for bulk, bilayer, and monolayer MoS2 and WS2 [10].

2.3 Electron Transport and Scattering

In 2D TMD materials, transport and scattering of carries are confined in the plane of the materials.

There are four mechanisms that affect the mobility of carriers [11]: (i) acoustic and optical phonon

scattering, (ii) Coulomb scattering caused by charged impurities, (iii) surface interface phonon

scattering, and (iv) roughness scattering.

2.3.1 Phonon Scattering

Crystal deformation in 2D TMDs results in polarization fields that interact and scatter electrons.

As temperature increases, carrier mobility is increasingly influenced by phonon scattering. Figure

2-5 (a) and (b) illustrate the dependence of MoS2 carrier mobility on temperature and carrier

density. At low temperature (< 100 K), the acoustic phonon scattering dominates, while the optical

phonon scattering dominates at higher temperature. The maximum mobility of MoS2 that can be

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reached at room temperature is limited to ~ 410 cm2 V-1 s -1, mainly due to the optical phonons.

Similar values are expected for other monolayer semiconducting TMDs.

2.3.2 Coulomb Scattering

Coulomb scattering in 2D TMDs originates from the charged impurities that randomly distribute

within the layer or on the surface. The Coulomb scattering plays a dominant role for low-

temperature carrier transport properties, as shown in Figure 2-5 (c) [12]. In device engineering, the

carrier concentration can be controlled through adding ionic impurities, while the mobility is also

decreased due to the Coulomb scattering effect. The performance of devices is therefore

significantly affected by the doping level of the materials. Theoretical calculations shows that the

impurity scattering dominates over phonon scattering when the impurity concentration reaches ~

5 x 1011 cm-2 [13], which is considered as heavily doping.

2.3.3 Surface Phonon Scattering and Roughness Scattering

In metal-oxide-semiconductor field-effect transistors, 2D TMD materials are placed on a dielectric

material such as SiO2. The surface roughness of SiO2 would cause ripples or wrinkles on the 2D

TMD materials, which may also contribute to scattering and thus reduce carrier mobility. The

effects of interface phonon scattering and roughness scattering have been experimentally observed

in GaAs-based quantum wells [14], graphene placed on SiO2, and freely suspended graphene [15,

16].

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Figure 2-5. The dependence of carrier mobility in monolayer MoS2 on (a) temperature and

(b) carrier density (calculated from first-principles density functional theory). (c) The

scattering contributions from charged impurities and the total mobility of MoS2 due to the

combined effect [12, 13].

2.4 Structural Defect-dependent Properties

Defects in TMD layered materials can be classified as zero-dimensional, one-dimensional, and

two dimensional defects, as shown in Figure 2-6. Zero-dimensional defects are the most abundant

defects in TMDs, including point defects, dopants, or non-hexagonal rings. One-dimensional

defects contain grain boundaries, edges, and in-plane heterostructures. Layer stacking of different

TMDs, wrinkling, folding, and scrolling are assigned to two dimensional defects. Structural

defects in the crystal lattices of TMDs can significantly change their physical and chemical

properties. For example, sulfur vacancies, the most common defects in chemically synthetic and

mechanically exfoliated MoS2 monolayers due to the lowest formation energy of these defects,

introduce unpaired electrons into the lattice, resulting in a n-doping effect on the material. These

sulfur vacancies create additional density of states within the band gap (Figure 2-7), and further

alter the electrical transport properties of MoS2. Most MoS2 devices show a n-type transfer

characteristic due to the plenty of sulfur vacancies in the material. A p-type transfer behavior has

also been reported from sulfur-rich (or molybdenum-deficient) MoS2 and Nb-doped MoS2. In

addition to electronic properties, optical properties of TMDs are also strongly affected by structural

defects. Tunable photoluminescence emissions can be achieved by doped TMDs such as MoxW1-

xS2 and MoSxSe2-x [17], which is promising for LED and display applications. Bi-sulfur vacancies

(a) (b) (c)

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generated by plasma irradiation in exfoliated MoS2 monolayer induce a PL peak with an energy

lower than the band gap value [18]. As 1D defects in TMDs, visible light emissions from the edges

of CVD-grown WS2 single-crystalline domain show similar or higher intensities compared to the

interior regions [19, 20]. Vertical and in-plane heterostructures of TMD materials also enable new

excitonic transitions. For example, mechanically stacked MoS2/WSe2 hetero-bilayers show a

strong PL emission at 1.50 ~ 1.56 eV (Figure 2-8 (a) and (b)) [21], originating from strong

interlayer coupling of charge carriers between two single-layer TMDs. Moreover, in parallel

stitched heterostructures of MoS2/WS2 and MoSe2/WSe2, their in-plane interfaces can emit visible

light with an energy laying between the band gap values of the two materials and the emission

intensity from the in-plane junctions is stronger than those from both sides [22, 23], as shown in

Figure 2-8 (c) and (d).

Figure 2-6. Typical defects in 2D TMD materials [24].

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Figure 2-7. (a) Formation energies of various point defects as functions of sulfur chemical

potential. Sulfur vacancy (Vs) has the lowest formation energy (~ 2 eV). (b) Schematic

depiction of the defect levels within MoS2 band gap [25].

Figure 2-8. (a) Optical microscope image of a WSe2/MoS2 hetero-bilayer. (b) Normalized PL

and absorbance spectra for monolayer MoS2, WSe2, and their hetero-bilayers. (c) SEM

images of MoSe2/WSe2 heterostructures. (d) PL intensity map of a MoSe2/WSe2 lateral

heterostructure (Scale bars, 2 μm) [21-23].

(a) (b)

(a) (b)

(c) (d)

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Chapter 3. CVD Growth of Monolayer MoS2

In this chapter, a synthesis method based on chemical vapor deposition (CVD) for growing high-

quality continuous films of monolayer MoS2 is demonstrated. Various optical and surface

characterizations including photoluminescence (PL), optical absorption, Raman spectroscopy, PL

and Raman intensity mappings, and atomic force microscope (AFM) are employed to characterize

the quality and homogeneity of the CVD-grown MoS2 films and the MoS2 single-crystal domains.

This proposed method offers a high reliability for growth of large-area high-quality MoS2 and can

be applied to various substrates such as mica, fused silica, and sapphire.

3.1 Monolayer MoS2 Growth

In this work, the monolayer MoS2 films were synthesized by chemical vapor deposition (CVD)

method. Here, 300 nm SiO2/Si wafers were used as substrates for MoS2 growth. Perylene-3,4,9,10-

tetracarboxylic acid tetrapotassium salt (PTAS) as shown in Figure 3-1 (a) and (b) were used as

seeding molecules. Figure 3-2 (a) schematically illustrates our experimental setup. At the center

of the CVD furnace, three wafers were faced down and placed directly above a crucible containing

15 mg of molybdenum oxide (MoO3) precursor. The upstream and downstream wafers were coated

with PTSA molecules, while the central substrate is a cleaned, bare SiO2/Si wafer for MoS2 film

growth. Another crucible that contains 20 mg of sulfur (S) powder was put at the inlet of the CVD

furnace. 15 sccm Argon (Ar) was introduced into the CVD system as a carrier gas. The system

was then heated to 625°C at a rate of 30 °C min−1, and the MoS2 monolayer films were grown on

the central substrate at 625°C for 3 min under atmospheric pressure. Finally, the system was

naturally cooled down to room temperature. Figure 3-2 (b) schematically shows the temperature

profile of the growth process.

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Figure 3-1. (a) Molecular structure of PTAS and (b) PTAS aqueous solution.

Figure 3-2. (a) A schematically illustration of the CVD setup for growth of MoS2 films and

(b) the temperature profile used for monolayer MoS2 growth.

3.2 Characterizations

3.2.1 Optical Microscopy Characterizations

Three-atom-thick semiconducting MoS2 films deposited on SiO2/Si wafers are blue-green in color

which exhibit apparent contrast with the purple of the substrates as shown in Figure 3-3 (a). The

continuous area of the MoS2 thin films grown in this work is typically ~ 1 cm x 1cm (limited by

the dimension of the 1-inch quartz tube of our CVD system). As shown in Figure 3-3 (b),

triangular-shape domain of MoS2 single crystals can be found at the edges of the continuous region.

The largest size of the single triangular grains is ~ 50 μm. Figure 3-3 (c) shows optical microscopy

images of the typical as-grown monolayer MoS2 film. The film is completely continuous with high

uniformity. With optimized process conditions, regions of multilayer MoS2 or particle clusters are

(b) (a)

(a) (b)

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not observed in our MoS2 films. The transition from single-layer grains to continuous films is

presented in Figure 3-3 (d).

Figure 3-3. (a) Photograph of centimeter-scale monolayer MoS2 grown on SiO2/Si wafer. (b)

Optical images of the CVD-grown single-crystal triangular domain of MoS2, (c) the

completely continuous film of MoS2, and (d) the transition region between MoS2 triangular

flakes and continuous film.

3.2.2 Raman Spectroscopy Characterizations

To confirm the layer thickness of the as-grown MoS2 thin films and flakes, Raman spectroscopy

is employed. As shown in Figure 3-4 (a), the CVD-grown MoS2 triangular grains exhibit two

characteristic Raman modes: the out-of-plane vibration of S atoms (A1g) at ~ 405.5 cm-1 and the

in-plane vibration of S and Mo atoms (E12g) at ~ 385 cm-1. The frequency difference of Raman

shifts between A1g and E12g modes (∆) can be related to the thickness of MoS2. Our deposited MoS2

triangular domain shows a ∆ value of ~ 20.5 cm-1, which evidences the existence of single-layer

MoS2. The ratio between intensities of the A1g and E12g modes can be linked to the doping levels

(a) (b)

(c) (d)

24

in MoS2 [26]. The results indicate that our CVD-grown MoS2 can be analogous to the case of less

doping. Additionally, since the widths of the Raman peaks are narrow, it can be inferred that there

does not exist high structural disorders in our MoS2 samples [27]. Figure 3-4 (b) displays the

Raman spectra performed at different locations in the continuous regions of the deposited MoS2

films. All of the regions measured show a similar Raman profile with the monolayer MoS2

characteristic of ∆ ~ 20 cm-1. No apparent variations of Raman peak intensity and ∆ are observed.

One can notice that both the positions of A1g and E12g modes at point C are slightly shifted toward

lower frequencies while maintains a ∆ value of ~ 20 cm-1. Since the Raman frequencies are strain-

sensitive, one can infer that the shifts at point C may originate from the strain of grain boundaries

near point C. A typical image of the grain boundaries in the as-grown MoS2 films is shown in the

following section 3.2.3. Overall, the Raman spectroscopy analysis indicates that the as-grown

films of monolayer MoS2 possess highly spatial uniformities.

Figure 3-4. Raman spectra of as-grown monolayer MoS2 (a) triangular flakes and (b) films

measured at different regions.

(a) (b)

25

3.2.3 Atomic Force Microscopy Characterizations

Figure 3-5 shows the AFM images of the grown MoS2 films. Grain boundaries can be clearly seen

in Figure 3-5 (a), indicating our MoS2 films are formed from merging different grain domains. The

average grain size is few micrometers large. Compared with the MoS2 films grown by MOCVD

[28], the grain size of our CVD-grown MoS2 films is much larger. A thin-film material with a

larger grain size is typically expected to have better performance on its intrinsic electrical transport

properties, which is critical to device and circuit applications. The electrical properties of the CVD-

grown MoS2 are evaluated in chapter 5. Figure 3-5 (b) and (c) illustrate a thickness of 0.75 nm for

a typical as-grown MoS2 triangular domain, which corresponds to single-layer thickness plus a

van der Waals gap.

Figure 3-5. AFM images of monolayer MoS2 (a) films and (b) triangular grains. (c) AFM step

height profile of a typical single-layer region.

(a) (b)

(c)

26

3.3 Photoluminescence and Optical Absorption of MoS2

3.3.1 Photoluminescence

In addition to Raman spectroscopy, the photoluminescence (PL) of MoS2 exhibits a high

dependence on its thickness. Due the 2D confinement, monolayer MoS2 possesses a direct band

gap of at least 1.8 eV and allows strong PL emission, while multilayer or bulk MoS2 has an indirect

band gap smaller than 1.8 eV and the emission intensity is thus much weaker. Figure 3-6 shows

the photoluminescence spectrum acquired at room temperature of our CVD-grown MoS2 films.

The MoS2 films exhibit a band gap at ~1.84 eV, which further confirms that the as-grown MoS2 is

single-layer. The PL emission profile of MoS2 depends on the relative contributions of A and B

direct excitonic transitions. In our MoS2 films, the intense A excitonic emission at ~1.84 eV (674

nm) can be clearly observed, which is assigned to the direct transition from the conduction band

minimum to the uppermost valence band maximum at the K valley in the Brillouin zone. The B

exciton, corresponding to the direct transition from the conduction band minimum to the lower

valence band maximum at the K valley, however, is not observed. That suggests most of the

photogenerated holes relax to the upper valence band before recombination. It has been reported

that the B excitonic peak intensity is tunable at higher excitation densities [29]. At a high excitation

density, the occupation of hole at the upper valence band increases, so the available states for the

photogenerated hole relaxation from lower to upper valence band reduce, which is called state-

filling effects.

Besides the direct band gap characteristic, since the intensity of PL emission is also

significantly associated with the defects in the materials and interface traps between the deposited

materials and substrates, the PL intensity in principle can be an indicator of the quality of the as-

grown materials. It is worth mentioning that the ratio of PL to Raman peak intensities (IPL/IA1g) of

our MoS2 is ~ 100, suggesting an excellent optical quality. The full-width-half-maximum (FWHM)

of the PL emission peak is as small as ~ 55 meV (~ 20 nm), which is comparable to or even smaller

than that of monolayer MoS2 grown on sapphire substrates [30, 31]. The results evidence that high-

quality CVD-grown MoS2 films are deposited on SiO2/Si substrates via the proposed growth

method.

27

Figure 3-6. Room temperature photoluminescence of the as-grown monolayer MoS2 films.

Raman spectrum is also displayed in this figure in the unit of photon energy. An excitation

wavelength of 532 nm was used for all PL and Raman spectra in this thesis.

3.3.2 UV-visible Absorption

To investigate the pristine behavior of MoS2 optical absorption, we directly grown MoS2 films on

various transparent substrates, including mica, sapphire, and fused silica. Thanks to the high

transparency of these substrates, we can easily perform optical absorption on those CVD-grown

samples without PMMA transfer processes. Therefore, the potential contaminations such as

PMMA and KOH residuals can be avoided. The absorption spectrum of the MoS2 grown on mica

(Figure 3-7) shows three pronounced peaks in the wavelength range from 350 to 750 nm. MoS2

films deposited on fused silica and sapphire also display the same absorption characteristic. The

A and B excitonic absorptions at 670 nm (~ 1.85 eV) and 620 nm (~ 2.00 eV) originate from

transitions from the highest-laying spin-split valence bands to lowest conduction bands. The C

absorption peak at ~ 445 nm is ascribed to van Hove singularity of monolayer MoS2. The existence

of van Hove singularity indicates strong light-matter interactions in MoS2 and could be used to

enhance the performance of photonic and photovoltaic devices.

28

Figure 3-7. Optical absorption spectra of as-grown monolayer MoS2 films on (a) mica, (b)

fused silica, and (c) c-plane sapphire.

(a) (b)

A

B

C

A

B

C

A

B

C (c)

29

3.4 Defect Characterization of MoS2

3.4.1 PL and Raman Intensity Mappings of MoS2 Triangular Domains

To more qualitatively characterize the uniformity of our CVD-grown MoS2, the intensity mappings

of PL emission wavelength and Raman mode frequencies (A1g and E2g) are performed, as shown

in Figure 3-8. The as-grown MoS2 grain does not show obvious local PL variations. The PL

quenches associated with micro-scale defects such as adlayers, cracks and other physical damages

are not observed in the MoS2 single-crystal domain, indicating a high spatial homogeneity of the

MoS2 triangular domain.

3.4.2 PL Mappings of MoS2 Grain Boundaries

Figure 3-9 (a) shows the PL mapping of the CVD-grown MoS2 film with a focus on a region

containing linear-shape grain boundary (highlighted by a blue dotted square in Figure 3-9 (b)). An

enhancement of PL emission is observed at the grain boundary (the yellow region in Figure 3-9

(a)). The enhanced PL emission could be caused by the inhomogeneous strain at the boundary

which alters the electronic structure of the local MoS2 and leads to an increased exciton binding

energy. Additionally, higher PL intensities are also observed at some locations rather than the grain

boundaries in the MoS2 film. The enhanced PL at those points could be interpreted as carrier

inhomogeneity at those locations. Lattice defects such as vacancies or impurities could contribute

to extra carriers that can recombine with the photogenerated carriers, resulting in stronger PL

emissions at those locations.

To further confirm the charge inhomogeneity in the CVD-grown MoS2, we also tried to perform

conductive atomic force microscopy (C-AFM) on our CVD MoS2 samples. However, since the

CVD-grown MoS2 generally shows an insulating behavior as no gate voltage is applied, it has been

difficult to attain meaningful signals from that measurement. More efforts will be made to

characterize the lattice defects or charge inhomogeneity in the CVD-grown TMDs. Other strategies

such as introducing extrinsic doping and employing scanning gate microscopy (SGM) will be

considered for our future study of TMD structural defect characterizations.

30

Figure 3-8. Intensity mappings of (a) PL emission wavelength and (b) Raman E2g and (c) A1g

mode frequencies of a representative as-grown MoS2 single crystal.

Figure 3-9. (a) PL Intensity mapping for a region containing a linear-shape grain boundary

and (b) optical image of as-grown monolayer MoS2 film. The region corresponding to (a) are

highlighted by a blue dotted box.

(a) (b) (c)

MoS2

E2g

MoS2

A1g

MoS2

PL

(b) (a)

31

3.5 Graphene/MoS2 In-plane Heterostructures

Heterostructures formed by 2D materials embrace rich physics and offer opportunities to create

new device architectures with multifunctionality and high performance. For example, hybrid

structures based on semiconducting monolayer TMDs can serve as building blocks for flexible p-

n junction devices, high-speed electronics, and optoelectronics. Vertical van der Waals

herterostructures can be achieved by layer-by-layer stacking of 2D materials using mechanical

transfer. However, van der Waals herterostructures in horizontal direction (in-plane junction) can

only be formed by growth. The growth of atomically clean and sharp interfaces of in-plane

herterostructures remains challenging. In this section, a two-step growth method for large-area

growth of metal-semiconductor graphene/MoS2 herterostructures is demonstrated. The lateral

graphene/MoS2 heterojunction is schematically shown in Figure 3-10.

Figure 3-10. Schematic illustration of the in-plane heterostructure of Graphene/MoS2 [32].

3.5.1 Lateral Heterostructure Growth

First, SiO2/Si substrates are cleaned by piranha solution (a 3:1 mixture of concentrated sulfuric

acid with hydrogen peroxide) for 1 h. Next, the mechanically exfoliated graphene flakes are

transferred onto the piranha-treated SiO2/Si substrate. After that, the graphene/SiO2/Si samples are

annealed in a H2 (100 sccm)/Ar (300 sccm) environment at 350C for 3 h for removing the residue

of scotch tape. The graphene/SiO2/Si samples then serves as the growth substrate for CVD MoS2

deposition. The MoS2 growth process follows the same steps described in section 3.1. The growth

patterns of MoS2 are dominated by the hydrophile and hydrophobe of the local areas on the growth

substrates. The mechanism of the PTAS molecule diffusion during the growth allows MoS2 to

Graphene MoS2

32

grow only on the hydrophilic SiO2 surface, while MoS2 deposition on the hydrophobic graphene

surface is significantly suppressed. As a result, a heterostructure with graphene wrapped by

monolayer MoS2 can be obtained.

3.5.2 Optical and AFM Images

The typical configuration of the CVD-grown lateral graphene/MoS2 heterostructures is shown in

Figure 3-11 (a)-(c). The in-plane interfaces between graphene and MoS2 are distinguishable by the

contrast difference. The graphene flakes are encircled by CVD-grown single-layer MoS2

(identified by PL and Raman spectra as shown in Figure 3-11 (e) and (f)). The AFM image (Figure

3-11 (d)) shows that a continuous MoS2 film is grown on the hydrophilic SiO2 region and are well

stitched with the graphene flake. The boundaries between MoS2 and graphene are sharp with an

overlap height ~ 2 nm, as shown in the AFM step profile. Moreover, there are no MoS2 clusters or

adlayers observed on the graphene surface, revealing the selective growth nature of MoS2.

33

Figure 3-11. (a)-(c) Typical optical images of as-grown in-plane heterostructure of

Graphene/MoS2. (d) The AFM image of the region highlighted by yellow circle in (a). (d) PL

and (e) Raman spectra of the grown MoS2 monolayer wrapping graphene.

3.5.3 PL and Raman Intensity Mappings

The spatial homogeneity of the graphene/CVD-grown MoS2 in-plane heterosturctures can be

further confirmed by the mappings of the PL and Raman intensities. Figure 3-12 shows that there

is no MoS2 PL signal on the graphene surface, which evidences that the monolayer MoS2 is grown

only outside the graphene flakes. The mapping images suggest that sharp and well-stitched

Monolayer MoS2 Film

Graphene

Monolayer MoS2

Graphene

Monolayer MoS2 Film

Graphene

(a) (b) (c)

(e) (f)

MoS2

Graphene (d)

34

boundaries are formed at the junctions of MoS2 and graphene, with no cracks or tears. Interestingly,

the PL emission at the MoS2/graphene junction exhibits much a stronger intensity than that at other

MoS2 areas, indicating an inhomogeneous carrier distribution or a higher photogenerated carrier

recombination rate at the edges. The unique enhancement of light-matter interactions at the in-

plane junction of MoS2/graphene heterostructures could play an important role in designing novel

solar cells, light-emitting diodes, and broad-spectrum photosensors.

Figure 3-12. PL and Raman intensity mappings of the graphene/MoS2 junction

corresponding to the region highlighted by the yellow circle.

(d) MoS2 PL (e) MoS2 E2g (f) MoS2 A1g

(b) Graphene G band (c) Graphene G’ band Monolayer MoS2

Graphene flake

(a)

35

Chapter 4. CVD Growth of Monolayer WS2

4.1 Monolayer WS2 Growth

In this section, a simple CVD growth method is demonstrated. The synthesis of monolayer WS2 is

performed in a quartz tube furnace at atmospheric pressure. Figure 4-1 schematically illustrates

the CVD experimental setup. At the center of the furnace, ~10 mg of tungsten trioxide (WO3)

powder is directly sprayed onto a piece of bare SiO2/Si wafer (1.6 cm x 1.6 cm), which serves as

a plate for carrying the WO3 precursor. A pre-cleaned Si/SiO2 (300 nm) substrate for WS2 growth

is positioned face-up 1 cm away the upstream wafer containing WO3. A crucible containing Sulfur

(S) powder is placed upstream, 1 cm away from the edge of the furnace heating zone. The

optimized distance between the S crucible and the WO3-contained wafer is ~ 18 cm. Prior to

synthesis, 1000 sccm of Argon (Ar) is employed to purge the quartz tube for 5 min. To grow

monolayer WS2, the furnace temperature is quickly ramped to 800 °C at a rate of 39 °C/min and

then hold the temperature at 800 °C for 5 min. A continuous 50 sccm Ar flow is used during the

growth process. Finally, the furnace temperature is naturally cooled down to room temperature.

Figure 4-1. Schematic illustrations of (a) the experimental setup and (b) the temperature

profile used for monolayer WS2 growth.

(a)

(b)

36

4.2 WS2 Growth Evolutions

In our experimental setup, the optimized condition for monolayer WS2 growth is at 800 °C for 5

min as described above. To further reveal the evolution of WS2 growth with growth time and

temperature, we compare the WS2 grains grown by various recipes, namely, recipe A: 800 °C for

5 min, recipe B: 775 °C for 15 min, and recipe C: 800 °C for 15 min. First, it is instructive to

investigate the effect of temperature. By comparing the optical images of WS2 grown by recipe A

and B, one can clearly see that a lower growth temperature generally results in a smaller grain

domain. The average size of WS2 single domain grown at 800 °C using recipe A is ~ 40 m and

the largest size found is ~ 65 m, as shown in Figure 4-2 (a). On the other hand, recipe B (775°C)

shows a relatively small domain size ~ 5 m. Furthermore, a longer growth time (15 min for both

recipe C and B) is favorable for multilayer WS2 growth. Compared to recipe A, both recipe B and

C lead to thicker WS2 grains. Note that the domain size grown by recipe C is larger than that of

recipe B, which again suggests that higher temperatures would be beneficial to large-domain WS2

growth. Next, we investigate the effect of growth time. A recipe D using the same temperature of

800 °C instead for 1 min is used for making a comparison with recipe A. The recipe D leads to

monolayer WS2 but with a small average grain size of ~ 3 m, revealing the existence of monolayer

WS2 deposited on SiO2/Si substrates at the very beginning of growth process. Based on the results,

we conclude the evolution of WS2 growth: at the beginning of 800 °C, nucleation sites (probably

WO3) are first absorbed onto the substrates. The nucleation sites continue to grow and then become

monolayer triangular domain within 1 min. After that, the lateral growth mechanism dominates in

the following 5 min, meaning that the small WS2 monolayers start to grow laterally and eventually

enlarge to ~ 40 m. After the 5th minute of the growth process at 800 °C, the vertical growth

mechanism starts to take over the WS2 growth, meaning that the rate of vertical growth become

higher than that of lateral growth. As a result, the WS2 grains grown at either 775 °C or 800 °C for

15 min shows a nature of multilayer. Figure 4-2 (e) and (f) shows the AFM image for the WS2

tringle grown at 800 °C, where the thickness of ~ 1 nm is consistent with the monolayer WS2

thickness plus a van der Waals gap. The sharp height profile at the edge of the WS2 tringle indicates

that there are active sites at the WS2 edges where the precursors are absorbed and then undergo

chemical reactions. The active sites at the edges are responsible for the lateral enlargement of WS2

grains. There are also particles absorbed on the WS2 surface, which may suggest that the adlayer

37

or multilayer WS2 start to grow around the moment of the 5th minute. Figure 4-3 schematically

summaries the growth dynamics of WS2 grown at 800 °C.

Figure 4-2. (a)-(d) Optical images of WS2 grains grown by different recipes. The WS2

monolayer grown by recipe A is the focus of this chapter. (e) The AFM image of WS2

monolayer grown by recipe A and (f) the thickness profile along the yellow line in the AFM

image.

(a) Recipe A: 800C for 5min

(b) Recipe B: 775C for 15min (c) Recipe C: 800C for 15min

(d) Recipe D: 800C for 1min

(e)

(f)

The Largest WS2 grain grown by recipe A

38

Figure 4-3. A schematic illustration of the CVD-grown WS2 evolution with growth time at

800 C.

4.3 Raman Spectroscopy Characterizations

Raman spectroscopy using an excitation wavelength of 532 nm is employed to characterize the

thickness of the CVD-grown WS2. As shown in Figure 4-4, two Raman peaks located at ~ 350 and

~ 417 cm-1 are observed. The 350 cm-1 peak is attributed to the 2LA (longitudinal acoustic) mode

merged with the E12g modes. The LA phonon vibrational mode, as a function of crystalline disorder,

arises from in-plane collective movements of atoms in the lattice, while the E12g is optical mode

and originates from the in-plane vibration of S and W atoms. On the other hand, the 417 cm-1

Raman peak is the out-of-plane vibration A1g characteristic of WS2. It has been reported that not

only the frequency difference (∆) of E12g and A1g peaks, but also the peak intensity ratio of 2LA to

A1g of WS2 is highly sensitive to its thickness. For single-layer WS2 grown on SiO2 at an excitation

wavelength of 514 nm, the height of the 2LA peak is roughly 2 times that of the A1g peak (I2LA/IA1g

~ 1 for bilayer and smaller than 1 for three or more layers). Our-grown WS2 shows I2LA/IA1g ~ 2.5

with a ∆ smaller than 67 cm-1 under 532 nm excitation, which evidences that our CVD-grown

material is monolayer WS2.

39

Figure 4-4. Room-temperature Raman spectrum of as-grown WS2 flakes using a 532 nm

laser excitation.

4.4 Photoluminescence of WS2

For bulk WS2, there are two direct transitions at the K point in the Brillouin zones due to the

splitting of the valence band. These two transitions are assigned to A (1.95 eV) and B (2.36 eV)

excitons, respectively, and have been experimentally detected by absorption spectroscopy. On the

other hand, this splitting of the valence band for a monolayer WS2 is absent, which means only

one direct electronic transition is expected to be observed from optical spectroscopy. Figure 4-5

represents the PL spectrum of as-grown WS2 samples (with 532 nm laser excitation). The WS2

samples exhibits a single strong PL emission at ~ 1.97 eV (~ 629 nm), which is consistent with the

direct band gap property of monolayer WS2. Also, PL FWHM can be an indicator of sample quality.

A smaller FWHM in principle suggests a higher quality. Our CVD-grown WS2 shows a PL FWHM

of ~ 51 meV, which is comparable or even narrower compared to those previously reported CVD-

grown and exfoliated monolayer WS2 on SiO2/Si substrates. This result demonstrates a high optical

quality in our CVD-grown monolayer WS2. The electrical transport quality of as-grown WS2 is

performed in chapter 5.

40

Figure 4-5. Room-temperature photoluminescence spectrum of as-grown monolayer WS2

(under 532 nm excitation).

4.5 X-ray photoelectron spectroscopy of WS2

To characterize the chemical composition and further estimate the potential structural defects of

our CVD-grown WS2, X-ray photoelectron spectroscopy (XPS) is performed. Figure 4-6 reveals

the XPS spectra of the as-grown WS2 samples. Three characteristic XPS peaks of WS2 at binding

energies 33.0 eV, 35.2 eV, and 37.8 eV corresponding to W4f7/2, W4f5/2, and W5p3/2 core energy

levels, respectively, are observed for tungsten (W) atom. The W4f7/2, which represents the 4+

valence state, shows a dominant contribution and it indicates the WO3 (6+) precursor is sufficiently

sulfurized even without employing H2 in our experimental setup. A S2p doublet is also observed,

confirming the grown materials are WS2.

The stoichiometry of the as-grown WS2 monolayer can be calculated by

[𝑾]

[𝑺]=

𝛌𝑺𝟐𝒑

𝝀𝑾𝟒𝒇×

𝝈𝑺𝟐𝒑(𝒉𝝊)

𝝈𝑾𝟒𝒇(𝒉𝝊)×

𝑰𝑾𝟒𝒇

𝑰𝑺𝟐𝒑 (4-1)

where σS2p(hν) and σW4f(hν) are photo-ionization cross sections of the 2p and 4f core level of S

and W, respectively, and λS2p and λW4f are inelastic mean free paths of the photoelectrons with

41

kinetic energies corresponding to the S and W core levels, respectively. The values of these

abovementioned parameters can be obtained from literatures. Accordingly, the [W]/[S] ratio is

estimated to be ~ 0.6, suggesting ~ 20% sulfur vacancies in the CVD-grown monolayer of WS2.

Since the existence of sulfur vacancies, we expect that our grown WS2 monolayer is a n-type

semiconducting material. The n-type behavior of as-grown monolayer WS2 is demonstrated by

field-effect devices in chapter 5.

Figure 4-6. Chemical composition analysis of as-grown WS2 using X-ray Photoelectron

Spectroscopy

42

Chapter 5. Monolayer MoS2 and WS2 Field-effect Transistors

For field-effect transistors (FETs) based on TMDs such as MoS2 and WS2, the electrical contacts

can significantly influence the device performance. In general, the electrical performance of TMD

devices is limited by Schottky barriers at the metal/TMD interface. Therefore, the realization of

ohmic contacts on TMD materials plays an important role for improving the performance of TMD

devices. In this chapter, CVD-grown MoS2 and WS2 FETs with Nickel (Ni) contacts are explored.

Their electrical transport characteristics are discussed.

5.1 MoS2 Device Fabrication

5.1.1 TMD Transfer

The CVD-grown monolayer MoS2 grown on a SiO2/Si substrate is transferred using a KOH wet

transfer method to SiO2/Si(p++) substrates, which also serve as back gates for field-effect

transistors. Poly-methylmethacrylate (PMMA, 950k 4.5% dissolved in Anisole) is spin-coated

onto the CVD-grown monolayer MoS2 samples. The stack is then placed in a KOH solution and

the solution is heated up to 85 C. The PMMA/MoS2 stack is able to be separated from the substrate

and remains floating once the SiO2 layer is etched away. The PMMA/MoS2 film is then transferred

into distilled water using a glass slide for 20 min to remove the KOH residues. It is worth

mentioning that because the KOH residues would damage the surface of the SiO2/Si(p++) substrates,

leading to current leakages and the potassium ion (K+) left between SiO2 and MoS2 may cause the

threshold voltage shifts and significant hysteresis of electrical transport, the above-mentioned

distilled water rinse step is repeated for at least three times. After that, the distilled water rinsed

PMMA/MoS2 film is transferred onto the SiO2/Si(p++) substrates and is then baked at 80 C for 10

min and 130 C for another 10 min. The bake steps can remove moisture and improve the adhesion

between MoS2 and the substrates. Finally, the MoS2/SiO2/Si(p++) sample is immersed in acetone

for 1 min and then the sample is annealed at 350 C for 3 h in a mixture of argon (300 sccm) and

hydrogen (100 sccm) environment.

43

5.1.2 E-beam Lithography, Electrode Deposition, and Lift Off

In this section, the detailed process flow for fabrication of bottom-gate monolayer MoS2 field-

effect transistors is described. Overall process flow is depicted in Figure 5-1.

Figure 5-1. Schematic process flow for fabrication of back-gate single-layer MoS2 field-effect

devices. The WS2 devices are fabricated using the same process flow.

5.2 Electrical Transport Properties of MoS2 FETs with Ni contacts

A schematic depiction and optical microscope image of a representative single-layer MoS2 field-

effect transistor (FET) are shown in Figure 5-2. 50 nm pure Nickel (Ni) contacts for source and

drain were deposited directly on MoS2 by e-beam evaporation. All electrical measurements were

carried out in a vacuum probe station (~ 2.5 x 10-4 torr).

44

Figure 5-2. Schematic and optical image of the single-layer back-gate MoS2 EFT.

Symmetrical 50 nm Ni contacts are defined on the CVD-grown monolayer MoS2 by e-beam

lithography, evaporation, and lift-off.

Figure 5-3 shows the typical current (Ids) and back-gate voltage (Vbg) measurements at a drain-to-

source bias Vds = 0.5 V for the single-layer MoS2 FETs. The MoS2 transistor we fabricated

behaviors as a n-type channel device with a narrow hysteresis window at room temperature. A

large hysteresis gap of MoS2 FETs has been often observed regardless of the device structures and

the number of layers. The larger hysteresis represents the more instability of the devices. The origin

of the hysteresis behavior could be associated with interface impurities between MoS2 channel and

the oxide layer, the gaseous and/or water molecules absorbed from the environment, and the

intrinsic defects in MoS2. Our MoS2 FETs exhibit small hysteresis behavior in vacuum, indicating

an intrinsically high-quality MoS2 monolayer is grown. The threshold voltage (VT) of the device

can be obtained by the linear extrapolation method, suggesting the VT is ~ -7.5 V. Figure 5-4 shows

the Ids-Vds curves at different Vbg values. The linear dependence of the channel current on drain-

source bias indicates that the Ni contacts are ohmic. Based on the assumption of linear charge

dependence on the gate voltage overdrive, the carrier density in MoS2 channel surface is estimated

by

𝑛 ≈ 𝐶𝑜𝑥

𝑞 (𝑉𝑔𝑠 − 𝑉𝑇) (5-1)

where Vgs = Vbg (Vs = 0 V) and Cox is the oxide capacitance of 1.15 x 10-8 F/cm-2 in our case. The

carrier densities of MoS2 channel are in the order of ~ 1012 cm-2 at various back gate bias from 0

45

to 30 V. The field-effect mobility can be extracted from the data as shown in Figure 5.3 using the

expression

= 𝒈𝒎𝟏

𝑽𝒅𝒔

𝑳

𝑾𝑪𝒐𝒙 (5-2)

where L and W are the channel and the width of the device and gm (= dIds/dVgs) is the

transconductance. The peak room-temperature field-effect mobility of the MoS2 FETs with Ni

contacts is ~ 5.2 cm2 V-1 S-1.

Figure 5-3. Room temperature transfer characteristic for the single-layer MoS2 FET with

Vds = 500 mV (L ~ 1 m and W ~ 10 m). Back-gate voltage is applied to the substrate.

46

Figure 5-4. Ids-Vds characteristic for the MoS2 device acquired for different values of Vbg.

Low-temperature of 77 K transport characteristic of the monolayer MoS2 FETs was also performed,

as shown in Figure 5.5. A n-type characteristic with Ion/Ioff of at least 105 is obtained. The threshold

voltage VT is shifted to ~17.5 V at 77 K.

Figure 5-5. Low-temperature Ids-Vbg transfer curve of the MoS2 FET acquired at 77 K.

47

5.3 Enhancement of Electron Mobility in MoS2 via a High-k Gate Dielectric

In this section, we explore the effect of high-k gate dielectrics on the field-effect mobility of MoS2.

A high-k gate dielectric Hafnium(IV) oxide (HfO2) of 25 nm is introduced between MoS2 and SiO2

by electron beam physical vapor deposition. The structure of the resultant MoS2 devices is shown

in Figure 5.6. The dielectric constant of HfO2 is ~ 25 with a bandgap of 5.8 eV [33], which is much

higher than that of SiO2 (εSiO2 ~ 3.9). As a result, the gate capacitance Cg of the hybrid HfO2/SiO2

dielectric for the MoS2 FETs is ~ 11.3 nF/cm2, calculated by considering two capacitances (CSiO2

and CHfO2) are connected in series. Figure 5.7 represents the typical electrical transport properties

(Ids-Vbg) of the monolayer MoS2 transistors with the hybrid gate dielectric. The MoS2 devices with

HfO2/SiO2 hybrid dielectric show n-type conduction and exhibit one order of magnitude higher

current density compared to the MoS2 devices using bare SiO2 dielectric at the same gate biases (~

0.9 µA/µm for the MoS2/SiO2 device and ~ 7.2 µA/µm for the MoS2/HfO2/SiO2 device at Vbg =

30 V). Since the gate capacitance of the hybrid HfO2/SiO2 substrate is only ~ 1.2% smaller than

the bare SiO2 substrate, a similar value of carrier density (n = CgVbg) is expected when those two

devices are biased at the same gate voltage. According to the definition of current density (J =

nqµE), the increased current density indicates an enhanced carrier mobility in the HfO2/SiO2

dielectric environment. We extract the field-effect mobility from the data presented in Figure 5-7.

The HfO2-based MoS2 FETs show an improved carrier mobility reaching ~ 18.2 cm2 V-1 S-1 at

room temperature. The reasons for the enhanced field-effect mobility could be linked to the

reduction of Coulomb scattering due to the strong screening effect of high k dielectric. Output

characteristics (Ids-Vds) at various back-gate voltages ranging from -20 V to 20 V for the HfO2-

based MoS2 devices are presented in Figure 5-8. The linear Ids-Vds characteristics indicates the

contacts between Ni and MoS2 are ohmic. Theoretical study has revealed that the Ni/MoS2

interface exhibits a lower vertical Schottky barrier height for electrons, compared to the Au/MoS2

and Pt/MoS2 interfaces [34].

48

Figure 5-6. The Schematic of a back-gated MoS2 FET using a 25 nm HfO2/ 300 nm SiO2

hybrid gate dielectric.

Figure 5-7. Transport properties of single-layer MoS2 FETs (L ~ 1 m and W ~ 6 m) on a

HfO2/SiO2 substrate.

49

Figure 5-8. Output characteristics (Ids-Vds) of the single-layer MoS2 FETs at different back-

gate voltages.

5.4 Electrical Transport Properties of WS2 FETs with Ni contacts

Figure 5-9 displays a schematic depiction and the top view optical microscopy image of a

representative single-layer WS2 channel contacting 50 nm thick e-beam evaporated Ni electrodes

as source (S) and drain (G) on a SiO2(300 nm)/Si(p+) substrate, which serves as a back gate (G).

All the electrical characteristics of the devices were investigated under vacuum (~ 3.6 x 10-4 torr).

Output characteristics of Ids-Vds at various back-gate voltages (-35 V to 0 V) for the single-layer

WS2 FET are depicted in Figure 5-10. The linear dependence of the current on source-drain biases

at small fields indicates that the contact between Ni and WS2 is ohmic. The symmetry of the current

with respect to the origin at positive and negative bias voltages (Figure 5-10(b)) further evidences

the ohmic nature of the contacts. As is typical of long channel MOSFETs, the single-layer WS2

FET exhibits current saturation at higher drain biases due to the formation of depletion region on

the drain side (pinch-off), as shown in Figure 5-10 (a). The transfer characteristics (Ids-Vbg) of

single-layer WS2 FET with Ni contacts are presented in Figure 5-11. The WS2 device shows n-

type behavior with channel current Ids = 60 A (4 A/m) at back-gate voltage Vbg = 60 V and

bias Vds = 2 V. It is worth mentioning that there is no hysteresis behavior observed in the WS2

FET, which indicates the monolayer WS2 channel is intrinsically high-quality and clean Ni/WS2

50

and oxide/WS2 interfaces are achieved. The on/off ratio is larger than 106 at room temperature.

The field-effect mobility can be estimated using Equation 5-2. The peak mobility of the single-

layer WS2 FETs reaches ~ 7 cm2 V-1 S-1 at room temperature.

Figure 5-9. Schematic and optical image of the single-layer back-gate WS2 FET. Symmetrical

50 nm Ni contacts are defined on the CVD-grown monolayer WS2 by e-beam lithography,

evaporation, and lift-off.

Figure 5-10. (a) Saturation Ids-Vds behavior of a representative monolayer WS2 FET with

Ni contacts (L ~ 1 m and W ~ 15 m). (b) Output performance of the devices at small

fields.

(a) (b)

51

Figure 5-11. Saturation Ids-Vds behavior of a representative monolayer WS2 FET with Ni

contacts (L ~ 1 m and W ~ 15 m). On/Off ratio of the device is larger than 106 at room

temperature. Inset: The Ids-Vds curve plotted in the logarithmic scale.

52

Chapter 6. Conclusions and Future Work

6.1 Conclusions

In summary, we have demonstrated a method for growing large-area monolayer MoS2 films

on SiO2/Si substrates via chemical vapor deposition. The as-grown monolayer MoS2 films display

high-quality optical properties with semiconducting properties and a direct bandgap of ~1.84 eV.

The ratio of PL and Raman peak intensities is as high as ~ 100 and the FWHM of the PL emission

peak is ~ 55 meV, which is comparable to or even smaller than that of previous reported monolayer

MoS2 grown on sapphire. The results of Raman characterization and PL intensity mapping indicate

the monolayer MoS2 is uniform in micro-scale. For the in-plane graphene/MoS2 heterostructures,

the MoS2 PL emission from the in-plane junction is much stronger than that from other areas. This

PL enhancement at the heterojunctions could be utilized to explore novel optoelectronics. The

AFM images suggest that the large-area MoS2 films are formed from merging different domains

and the size of each domain is a few microns large. The single-layer CVD-grown MoS2 FETs

using nickel as contacts show n-channel behavior, with a peak room-temperature field-effect

mobility of ~ 5.2 cm2 V-1 S-1. An enchantment of room-temperature field-effect mobility reaching

18.2 cm2 V-1 S-1 is achieved by introducing a hybrid HfO2/SiO2 gate dielectric in the MoS2 FETs.

The linear dependence of the current on source-drain biases indicate that the Ni contacts are ohmic.

Furthermore, we have developed a simple method for synthesizing monolayer WS2 based

on CVD. The monolayer characteristics are confirmed by Raman spectroscopy and AFM

characterization. The photoluminescence from the WS2 samples and the electrical transport

properties indicate the as-grown monolayer WS2 is semiconducting and possesses a direct bandgap

at ~ 1.97 eV. The single-layer WS2 transistors with Ni contacts exhibit n-type conduction and a

peak room-temperature field-effect mobility of ~ 7 cm2 V-1 S-1. Current saturation behavior is

observed at higher source-drain biases.

It is noteworthy that the large-area MoS2 growth method presented in this thesis is

applicable not only to SiO2/Si substrates, but also to other substrates such as mica, fused silica,

and sapphire.

53

6.2 Future Work

The electronic properties of 2D TMD materials are highly dominated by their structural defects.

Consequently, comprehensive investigation and understanding of the defects in 2D TMDs are

needed. The future work is twofold: defect characterization and engineering and electrical

performance improvement.

6.2.1 Defect Characterization and Engineering

(i) Characterization: employ TEM to investigate various structural defects such point

defects, grain boundaries, and heterohuctions in 2D TMDs, and utilize scanning gate

microscopy (SGM) to characterize the local charge in-homogeneities in 2D TMDs.

(ii) Understand defects: understand the defect-dependent properties of 2D TMDs. For

example: what kind of defects would enable MoS2 to show p-type semiconducting

behavior.

(iii) See defects: develop characterization methods to study the dynamics of defect

formation.

(iv) Control defects: introduce defects of specific types into desired locations and control

the defect density.

(v) Remove defects: Convert defective TMDs into perfect crystalline materials.

6.2.2 Strategies to improve the electrical performance of CVD-grown 2D TMD transistors

(i) High-κ dielectrics: introduce HfO2 or ZrO2 into our TMD FET architectures, to enhance

the gate electrostatic control.

(ii) Clean transfer: replace KOH transfer with HF transfer, which might reduce the

Coulomb scattering induced by potassium ion (K+) and develop PMMA-free transfer

process.

(iii) Lower contact resistance: explore 2D/2D ohmic contacts such as VS2/MoS2 and

Graphene/WS2 heterostructures.

(iv) Reduce interface traps: Since h-BN is flat and does not possess dangling bonds, it can

be used to encapsulate CVD-grown TMDs. The surface/interface phonon scattering

and roughness scattering are expected to be mitigated.

54

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