1. University of Minnesota
EE 5171 Microelectronic Fabrication
2015 Project Paper
The synthesis of monolayer MoS2 thin film
Yi Ren

2. Introduction
Nowadays, the performance speed of cell phone, computer and other digital logic
electrical devices are becoming faster and faster. One of the basic reason of this performance
improvement is the increment of FET’s response speed. Modern digital logic is based on
silicon complementary metal oxide semiconductor (CMOS) technology[1]
. If the response
speed of each MOSFET increases, the performance speed of the whole system will increase.
This requires the FETs have short gate and high carriers mobility in the channel. Reducing
the channel length is one way to increase the response speed. However, the short gate will
cause many problems, such as threshold-voltage roll-off, drain-induced barrier lowering, and
impaired drain-current saturation, that reduce the performance of FET[2]
. By scaling theory,
reducing the thickness of the channel can reduce these short-channel effects above[3]
.
In order to reach the ultimate thickness of the channel, monolayer Molybdenum disulfide
(MoS2), which means the channel can be just one atomic layer thick, is applied in the 2-D
FET.
Properties of Molybdenum disulfide (MoS2)
The MoS2 is the family of transition metal dichalcogenides (TMD). One layer of Mo
atoms is sandwiched between two layers of S atoms by van der Walls interaction (Fig.1).

3. Fig.1 Three-dimensional schematic representation and top view of a typical MoS2 structure, with the
sulfur atoms (S) in yellow and the Molybdenum atoms (Mo) in blue[4]
.
MoS2 has many distinctive electronic, optical, and catalytic properties and is applied in
numerous areas such as, such as hydrodesulfurization catalyst, photovoltaic cell,
photocatalyst, nanotribology, lithium battery, and dry lubrication. But this paper will focus on
its application in 2-D FET. Monolayer MoS2 as a 2-D material has several advantages to be
the channel of 2-D FET:
1. One of the most important feature of monolayer MoS2 films is their atomic thickness
that allows easier control of channel charge by gate voltage and high degree of vertical
scaling that can reduce the short channel effects[5]
.
2.Compared to pristine graphene with no bandgap, monolayer MoS2 is a direct gap
semiconductor with a bandgap of 1.8 eV[6,7]
.
3. Compared to traditional semiconducting materials such as Si and Ge, the surface of

4. monolayer MoS2 or 2-D materials is free of dangling bonds. The absence of dangling bonds
reduces the surface roughness scattering and interface traps resulting in low density of
interface states on the semiconductor−dielectric interface[7]
.
So far, there are two major strategies that have been employed to obtain monolayer
TMDs: one is the chemical or mechanical exfoliation from bulk crystals; the other is the
bottom-up growth method.
Exfoliation Approaches
1. Mechanical exfoliation. In a typical mechanical exfoliation process, appropriate thin
TMD crystals are first peeled off from their bulk crystals by using adhesive scotch tape.
These freshly cleaved thin crystals on scotch tape are brought
into contact with a target substrate and rubbed by using tools such as plastic tweezers to
further cleave them. After the scotch tape is removed, monolayer and multilayer TMD
nanosheets are left on the substrate[8]
. Another way is using a fresh surface of a layered
crystal to rub against another surface, which left a variety of flakes attached to it. Among
these flakes, people can find the monolayer TMD nanosheet[9]
.
2. Chemical exfoliation. The commonly used method is the lithium-based intercalation
that was first found by Morrison et al[10]
. The MoS2 powder is first soaked in a n-butyl
lithium solution in hexane for a least 48 hours, in a dry box containing an argon atmosphere.
In this step, lithium will intercalate into the interval of MoS2. Then, the MoS2 is removed,
washed repeatedly in hexane, dried and sealed in a vial, still in the dry box under argon
atmosphere. The lithium-MoS2 solution is then reacting with water without contact with air.

5. Upon contact with the water copious gas follows out and the MoS2 powder formed a highly
opaque suspension in the water. The suspension was ultrasonicated during the reaction to
assist in the exfoliation. The reaction between the water and the intercalated lithium forms
hydrogen gas between the layers, and the expansion of this gas tends to separate the MoS2
layers. That’s why this method called chemical exfoliation. Finally, the exfoliated MoS2 can
be deposited on the surface of alumina powder from the suspension.
After exfoliated the TMD nanosheets from the bulk material, people can use the optical
contrast, Raman spectrum (fig.2) and AFM to distinguish the layer numbers[8, 9]
.
Figure 2. Typical landscape of MoS2 atomic layers on SiO2 substrates. The intensity of the Raman
E1
2g mode is correlated to the number of layers[11]
.
The exfoliation approaches can produce high-quality and micrometre-sized monolayers
and are commonly used in research and proof-of-concept device fabrication to obtain the
monolayer TMDs[4,11]
. However, mechanical cleavage is not suitable for large-scale
production due to the absence of layer number controllability. The number of monolayers are
in a great minority among accompanying thicker flakes[9]
. On the other hand, chemical
exfoliation of monolayer TMDs may unavoidably alter the lattice structure of thin TMD

6. layers or introduce extrinsic defects during the exfoliation process and thus require a post
treatment to reconstruct the structure of monolayer TMDs[12]
.
Bottom-up approaches
By comparing with the exfoliation approaches, bottom-up approaches have much better
layer number controllability and large area uniformity[11]
. These synthetic approaches include
chemical vapor deposition and thermolysis.
1. Sulfurization of metal (or metal oxide) thin film. The reaction mechanism for this
method can be simply understood as a direct chemical reaction. It is direct to think about
depositing S atoms on Mo metal to get large area monolayer. Zhan et al[13]
. used Mo film that
was deposited on a SiO2/Si wafer and followed by thermal annealing in sulfur vapours to
produce MoS2 film (Fig.3). The direct sulfurization of Mo metal thin film provides a quick
and easy way to access atomically thin MoS2 layers on insulating substrates. But the
presence of unreacted metal impurities cause the obtained MoS2 layers show a low on/off
current ratio.
Figure 3. (a) Illustration of introducing sulfur to the Mo thin film which was predeposited
on the SiO2/Si substrate. (b) The structure of MoS2 on the substrates, where black and yellow atoms
represent Mo and S, respectively[11]
.

7. An alternate way is replace the metal by metal oxide[14]
. In the report by Lin et al. MoO3
thin layer was firstly prepared by thermal evaporation on sapphire substrates. Then, MoO3
was reduced to MoO2 or other reduced Mo forms in a H2/Ar environment at 500 o
C. Next,
the sample was annealed in a sulfur-rich environment at 1000 o
C, which leads to the
formation of a wafer-scale MoS2 thin layer (fig.4). A FET fabricated with MoS2 thin layers
that derived from this method in a bottom gate geometry shows n-type behaviour with on/off
current ratio ~ 105
and the field-effect electron mobility of the device is about 0.8 cm2
V-1
s-1
that are comparable with those obtained from the mechanical exfoliated MoS2 thin film.
Figure 4. Schematic illustration for the synthesis of MoS2 layers by MoO3 sulfurization[14]
.
The thickness of metal and metal oxide thin films directly influence the thickness of
obtained TMD thin film in this direct chemical reaction. One of the limitations of
these two methods above is they are hard to control the thickness of pre-deposited metal
oxide or metal thin film which limits the film uniformity in wafer-scale.
Recently, the application of atomic layer deposition (ALD) provides a solution to this
limitation. The ALD process can provide the atomically TMD thin film with good thickness
controllability and wafer-scale uniformity. Song et al[15]
. demonstrated that the number of
WS2 layers can be controlled by tuning the number of

8. cycles of ALD WO3 (fig.5).
Figure 5. Schematic illustration of the synthetic procedure for the ALD-based WS2 nanosheet[15]
.
So far, the problem is finding the suitable precursors to form the metal oxide on the substrate
and the substrate is also limited by the property of the metal and metal oxide; for example,
the chemically inert and hydrophobic substrates may not be able to efficiently initiate the
growth of WO3 by ALD[11]
.
There are also some bottom-up approaches that grow the monolayer MoS2 on other
substrate like Mica[16]
and graphene[17]
. The basic processes are both CVD process but with
different precursors.
2. Thermal decomposition of thiosalts.The basic idea is the thermolysis of the
precursor that contains Mo and S atoms. Liu et al’s group reported a two-step thermolysis
process of ammonium thiomolybdate which was able to produce highly crystalline and
large-area MoS2 thin sheets on a variety of insulating substrates[18]
. The (NH4)2MoS4 thin
film was firstly prepared by the dip-coating method and carried into a quartz tube chamber
with the Ar/H2 flow. Temperature was elevated to 500 o
C to efficiently remove the residual
solvents, NH3 molecules and other by-products dissociated from the precursors. For the
second step, temperature was raised to 1000 o
C and the sulfur vapours were introduced. the
addition of sulfur during the second annealing process removes the oxygen-containing

9. defects and improves the crystallinity of MoS2 thin film (fig.6). The transistor devices
fabricated with MoS2
Figure 6. Schematic illustration of the two-step thermolysis process for the synthesis of MoS2 thin
layers on insulating substrates[18]
.
thin layers using a bottom gate geometry exhibit n-type behaviours with a significantly
improved on/off current ratio which is about 105
and the field-effect electron mobility is up to
6 cm2
V-1
s-1
. However, synthesis of large area TMDs by this method is still challenging due
to the technical limitations to get uniform and ultra-thin ammonium thiomolybdate film.
Conclusion
To date, the novel MoS2 FETs, such as single layer MoS2 FET made by Radisavljevic1,
B. et al[7]
and fully 2-D materials built FET made by Tania Roy et al[19]
. Both of them use
mechanical exfoliation to get the monolayer MoS2. Because so far, it is the easiest and
cheapest way to get high quality and micrometer size monolayer nanosheets for the research
and novel device fabrication. However, this method can not generate large-area monolayer or
be used on commercial production. On the other hand, the CVD technique has shown a high
potential to generate the large-area or wafer-scale monolayer TMD with high quality, uniform

10. thickness and good electronic properties. But there are still many technical problems need to
be overcome. I believe the future development of monolayer TMD nanosheet synthesis can
definitely make the electrical device into new era.
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