Appl. Phys. Lett. 107, 193503 (2015); https://doi.org/10.1063/1.4934941 107, 193503

© 2015 AIP Publishing LLC.

Transferred large area single crystal MoS2field effect transistorsCite as: Appl. Phys. Lett. 107, 193503 (2015); https://doi.org/10.1063/1.4934941Submitted: 30 May 2015 . Accepted: 13 October 2015 . Published Online: 10 November 2015

Choong Hee Lee, William McCulloch , Edwin W. Lee, Lu Ma, Sriram Krishnamoorthy, Jinwoo Hwang,Yiying Wu, and Siddharth Rajan

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Transferred large area single crystal MoS2 field effect transistors

Choong Hee Lee,1,a) William McCulloch,2,b) Edwin W. Lee II,1,b) Lu Ma,2

Sriram Krishnamoorthy,1 Jinwoo Hwang,3 Yiying Wu,2 and Siddharth Rajan1,a)

1Department of Electrical and Computer Engineering, The Ohio State University, Columbus, Ohio 43210, USA2Department of Chemistry and Biochemistry, The Ohio State University, Columbus, Ohio 43210, USA3Department of Materials Science and Engineering, The Ohio State University, Columbus, Ohio 43210, USA

(Received 30 May 2015; accepted 13 October 2015; published online 10 November 2015)

Transfer of epitaxial, two-dimensional (2D) MoS2 on sapphire grown via synthetic approaches is a

prerequisite for practical device applications. We report centimeter-scale, single crystal, synthe-

sized MoS2 field effect transistors (FETs) transferred onto SiO2/Si substrates, with a field-effect

mobility of 4.5 cm2 V�1 s�1, which is among the highest mobility values reported for the trans-

ferred large-area MoS2 transistors. We demonstrate simple and clean transfer of large-area MoS2

films using deionized water, which can effectively avoid chemical contamination. The transfer

method reported here allows standard i-line stepper lithography process to realize multiple devices

over the entire film area. VC 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4934941]

Transfer of large-area, two-dimensional (2D) materials

onto desired target substrates are attractive for thin film flexible

and transparent electronics on a wide range of substrates. Due

to the weak interactions between atomic layers in 2D materi-

als, isolation of single layer or multilayer films can be achieved

using various methods such as mechanical exfoliation using

scotch tape1,2 or polydimethylsiloxane (PDMS) stamp,3,4 elec-

trochemical bubbling,5,6 or polymethylmethacrylate (PMMA)

or PDMS-meditated metal etching.7,8 For instance, very large

area (30-in.) graphene devices have been transferred to poly-

ethylene terephthalate (PET) substrates by removal of the Cu

substrates used for graphene growth.9 Though graphene-based

devices have shown large mobility and high cut-off frequency

with high flexibility, they are not applicable for power or logic

devices since graphene has zero energy band gap.

In contrast, 2D transition metal dichalcogenides (TMDs)

are attractive due to the intrinsic bandgap, absence of dangling

bonds, and high flexibility.10,11 Recently, top-gated mechani-

cally exfoliated MoS2 field effect transistors (FETs) with

high-k gate dielectrics showed a high on/off ratio of �108 and

a field-effect mobility of 200 cm2 V�1 s�1.12 Chang et al.reported flexible back-gated transistors based on mechanically

exfoliated MoS2 with a mobility of 30 cm2 V�1 s�1.13 The

high-k dielectric for these devices was deposited on polyimide

(PI) substrates. While these results demonstrate the intrinsic

properties of MoS2 devices, they are not suitable for manufac-

turing environments, since mechanical exfoliation method

results in small flakes on the scale of tens of microns.

Sulfurization14,15 or chemical vapor deposition

(CVD)16,17 growth is an attractive method to realize electronic

devices and circuits on a larger scale. We have previously

reported large-area (�0.8 cm � 1 cm, �10 nm thick) single-

crystalline MoS2 films sulfurized on sapphire with a space-

charge limited mobility higher than 150 cm2 V�1 s�1.18 X-ray

diffraction (XRD) measurements of the large area film grown

on sapphire clearly confirmed the single crystallinity and

epitaxial registry of (0002)-oriented MoS2 on c-plane sap-

phire.18 Jeon et al. demonstrated direct CVD-grown trilayer

MoS2 on oxygen plasma treated SiO2/Si with an electron mo-

bility of 15.6 cm2 V�1 s�1.16 In this work, we report transfer

of large-area, single-crystal, few-layer MoS2 films onto SiO2/

Si substrates using a transfer technique and demonstrate a

back-gated transistor array using standard i-line stepper lithog-

raphy to realize multiple devices over the entire film area. We

achieve film transfer using a transfer method where only

deionized (DI) water was used to initiate the film transfer.

The MoS2 samples were prepared by depositing a 5-nm-

thick Mo layer on sapphire substrates by sputtering (AJA

Orion RF/DC Sputter Deposition Tool). The samples were

subsequently sulfurized with 8.0 mg of MoS2 powder in a

furnace at 1100 �C for 4.5 h and then cooled down to room

temperature at a rate of 0.5 �C/min. A heavily doped p-type

Si wafer with 20-nm-thick SiO2 layer was used as the back-

gate. Transfer of the MoS2 films was initiated by first placing

a PDMS stamp in the corner of the sample and detaching a

small part of the MoS2 layer to create an air gap between the

MoS2 layer and the sapphire substrate, which allowed DI

water to penetrate the MoS2/sapphire interface. The sample

was then dipped into the DI water slowly to separate the

MoS2 layer from the substrate. Once the MoS2 film floated

on the DI water surface, it was picked up using the SiO2/Si

wafer and dried overnight in air. Water molecules trapped at

the interface between MoS2 and SiO2 were removed by

annealing the sample at 110 �C in a vacuum chamber with a

base pressure of 10�6 Torr for 30 min.

Ti/Au/Ni metal stack was then deposited as an Ohmic

contact, defined using i-line stepper lithography. The device

areas were covered with patterned photoresist and mesa iso-

lated using BCl3/Ar (3:5) inductively coupled plasma/reac-

tive ion etching (ICP-RIE) at 30 W, at an etch rate of 2 nm/

min. The crystalline quality of the transferred MoS2 films

was evaluated using X-ray diffraction (Bruker, D8 Discover)

and Raman spectra (Renishaw) with a 10 mW laser at

514 nm. The surface morphology of the samples was exam-

ined by atomic force microscopy (AFM) (Veeco Instrument,

a)Authors to whom correspondence should be addressed. Electronic

addresses: lee.5107@osu.edu and rajan.21@osu.edub)W. McCulloch and E. W. Lee contributed equally to this work.

0003-6951/2015/107(19)/193503/4/$30.00 VC 2015 AIP Publishing LLC107, 193503-1

APPLIED PHYSICS LETTERS 107, 193503 (2015)

DI 3000). Electrical characterization was done using Agilent

B1500 parameter analyzer.

The transfer process of MoS2 films is schematically illus-

trated in Figure 1(a). The film transfer method uses the surface

energy difference between the MoS2 film (hydrophobic) and

sapphire substrate (hydrophilic), and was reported recently by

Gurarslan et al.19 The authors coated polymer on top of MoS2

film and poked at the edge of the film to initiate the transfer so

that polymer/MoS2 film can be lifted off by the water penetra-

tion. Here, we used a similar method, but the MoS2 was not

coated with a carrier polymer to avoid any polymer residue on

the MoS2 surface after transfer. In addition, we used a PDMS

stamp to initiate the separation of the film from the sapphire

substrate. First, a PDMS stamp was pressed down in the cor-

ner of the MoS2 film. The region of the film in contact with

PDMS was exfoliated due the adhesion force of the PDMS

stamp. Then, the sample was tilted and slowly dipped into DI

water. While the sample was dipped, water seeped into the

MoS2/sapphire interface and mildly lifted off the entire film.

Figure 1(b) shows the floating MoS2 film in the DI water.

Even without a carrier film coated on MoS2, �8 mm � 5 mm

of continuous, large-area MoS2 film was detached from sap-

phire and floated in the DI water. Since the film was not cov-

ered with a carrier polymer, the MoS2 film fragmented into

several pieces during transfer. Nevertheless, the individual

pieces were large enough with dimensions ranging from

hundreds of microns to several millimeters in length of

continuous MoS2 layer after transfer (Figure 1(c)). Fully fabri-

cated MoS2 transfer length method (TLM) structures with dif-

ferent spacing (from 2 lm to 25 lm) on SiO2 are shown in

Figure 1(d) (scale bar¼ 100 lm).

Figure 2(a) shows the comparison of the XRD spectra of

as-grown MoS2 on sapphire and MoS2 films transferred to

SiO2/Si substrates. The XRD spectra clearly show the (0002)

family of planes for transferred MoS2 without the sapphire

peak. The XRD of transferred MoS2 showed fairly similar

peak intensities and positions compared to the as-grown

sample, indicating that the quality of the large-area MoS2

film is well maintained after the transfer process. This mea-

surement confirms that relatively large areas of coherent

MoS2 films were transferred.

Scanning transmission electron microscopy (STEM)

was used to determine the structural quality and crystallinity

of the transferred MoS2 film. For STEM characterization,

MoS2 films were transferred directly to Cu-mesh TEM grids

using the same method described above. The high angle an-

nular dark field (HAADF) image in Figure 2(b) confirms that

the quality of the film is retained after the transfer. Selected

area electron diffraction (SAED) patterns were also acquired

from many areas of the sample. All patterns consistently

showed single crystal diffraction patterns, such as the one

shown in Figure 2(b) inset.

AFM image (5 lm � 5 lm) shown in Figure 3(a) indi-

cates that the surface morphology (RMS roughness of

1.23 nm) of the transferred MoS2 film is comparable to that

of the as-grown film (approximately 1 nm). The thickness of

transferred MoS2 film on SiO2 measured by AFM shown in

Figure 3(b) was approximately 10 nm which corresponds to

about 15 layers of MoS2.

Figure 4(a) shows the Raman spectra of MoS2 films

with in-plane (E12g) and out-of-plane (A1g) vibrational

modes at 381 cm�1 and 406 cm�1, respectively, for both the

as-grown and the transferred samples. A small Si peak at

FIG. 1. (a) Schematic of the large-area

MoS2 transfer using DI water. First, an

air gap was created by placing a

PDMS stamp in the corner of the sam-

ple. Then, the sample was carefully

dipped into the DI water. (b)

Centimeter scale MoS2 film floating in

the DI water. (c) The film was trans-

ferred to SiO2/Si wafer and (d) TLM

structures were fabricated on the film.

FIG. 2. (a) XRD spectra of the as-grown and transferred MoS2 films show-

ing identical peak positions and similar peak intensities. The clear (0002)

diffraction peak can be seen for transferred MoS2 sample. (b) Representative

HAADF STEM image and SAED pattern (inset) of free standing MoS2

layer.

FIG. 3. (a) AFM image of transferred MoS2 layer on SiO2/Si wafer with

RMS of 1.23 nm. (b) Cross-sectional plot of the transferred MoS2 layer on

SiO2 showing thickness of 10 nm.

193503-2 Lee et al. Appl. Phys. Lett. 107, 193503 (2015)

520 cm�1 is also detected after transfer in the Raman spectra.

Similar peak separations of �25 cm�1 for as-grown and

transferred MoS2 films reveals that transfer process main-

tains the quality of the film. We also evaluated the film qual-

ity by mapping the peak intensity ratio (E12g: A1g) on the

transferred MoS2 films over 100 lm2 area (Figure 4(b)). The

variation of the peak intensity ratio falls within in a narrow

range of 1.0–1.3, indicating that the film is quite uniform

across the measured area, and the crystalline quality remains

after transfer, which consistent with the XRD data already

illustrated in Figure 2.

Figure 5(a) shows current-voltage (I-V) characteristics

in TLM structures with spacing varying from 3 lm to 5 lm

on as-grown and transferred MoS2 devices. The as-grown

sample was cleaved in half, and the MoS2 from one of the

cleaved samples was transferred from sapphire to another

identical sapphire substrate. The current density of trans-

ferred MoS2 devices was 85% of the current density of the

as-grown devices. A space charge mobility of 98 cm2 V�1

s�1 was obtained after transfer, which is comparable to the

space charge mobility value of �120 cm2 V�1 s�1 of as-

grown MoS2 already shown from our previous work.18

Details of space charge mobility extraction were reported

earlier.18 Figure 5(b) shows the output characteristics (IDS-

VDS) of the MoS2 transistor with Ti/Au/Ni electrodes. Linear

dependence of IDS on VDS clearly indicates Ohmic behavior

of Ti contacts at low drain bias. From TLM measurements

(supplementary material, Figure S1),30 a contact resistance

of 170 X mm and sheet resistance of 270 kX=sq were

extracted at a low drain bias (VGS¼ 0 V). The device shows

a typical n-channel MOS field effect transistor (FET) behav-

ior from the transfer curve (sweep from depletion to accumu-

lation) with an on/off ratio of �105 (Figure 5(c)). Channel

pinch-off was also observed at a gate bias of �8 V. A nega-

tive threshold voltage of �0.9 V was extracted by extrapola-

tion from the linear region (VDS¼ 0.1 V), shown in Figure

5(d), indicating depletion mode operation. Using the expres-

sion, l ¼ dIDS=dVGSðL=WCOXÞ, where L (2 lm) and W(100 lm) are the channel length and width, respectively, and

COX¼ 0:17lFcm�2 ðCOX ¼ e0er=d; e0¼ 8:85�10�14 Fcm�2,

er ¼ 3:9, d¼ 20nm), a field-effect mobility value of 4.5cm2

V�1 s�1 was extracted at VDS¼0.1V, which is two orders of

magnitude higher than the previous reports for transferred

large area films on SiO2.19 A field effect mobility of 6.8cm2/

Vs was extracted from the reverse VGS sweep (accumulation

to depletion) in the hysteresis measurements (Figure S2).30

Output characteristics of multiple multilayer MoS2 FETs

with different channel lengths are shown in Figure S3.30

The low field effect mobility measured could be due to

the interface states at the SiO2/Si interface and poor dielectric

screening due to the lower permittivity of SiO2.20 In the case

of FETs made using exfoliated MoS2 (which are typically

small area) on SiO2/Si, there is a wide range of reported field-

effect mobility values (0.1–30 cm2 V�1 s�1)21–23 due to the

strong dependence of mobility on the interface states between

SiO2 and MoS2. The mobility values reported in this work fall

within the range of reported mobility values for mechanically

exfoliated MoS2 transistors, highlighting the viability of the

transfer process for producing large-area devices.

The transconductance, gm ¼ dIDS=dVGS, was found to

be 39 lS/mm for VGS¼ 4.9 V and VDS¼ 0.1 V. This value is

lower than state-of-art MoS2 devices with high-k dielectric24

but can be improved by substituting the SiO2 with high-k

dielectrics such as Al2O3 or HfO2.12,25 The charge density of

MoS2 at VGS¼ 0 V was estimated ðn2D ¼ COXðVGS � VTÞÞto be 9.7� 1011 cm�2, which is bit higher than the extracted

charge density (2� 1011 cm�2) from the as-grown MoS2 film

in our previous work.

A subthreshold swing (SS) of 1.2 V/decade was meas-

ured at VGS¼�6 V, VDS¼ 0.1 V, and the trap density at

the interface between the MoS2 and SiO2 was extracted

to be �2.1� 1013 eV�1 cm�2 using the expression,

SS ¼ lnð10Þð1þ qDit=COXÞkT=q, where Dit is the interface

trap density. This value is similar to reported value of the

interface trap density (�1.6� 1013 eV�1 cm�2) of direct

CVD grown MoS2 on SiO2.26 Such large subthreshold swing

(>1 V/decade) was also reported previously.27,28 To verify

the interface trap density, we measure the hysteresis (supple-

mentary material, Figure S2),30 defined as the VTH shift, by

sweeping VGS at VDS¼ 0.1 V. VTH shift of 3.4 V was

observed, and trap density of 3.7� 1012 cm�2 was extracted

from the expression, Qit ¼ DVTHCOX. We attribute the inter-

face states to air/water vapor trapped at the interface during

the transfer process, even though the sample was annealed in

the vacuum. Because of the absence of out-of-plane bonds in

FIG. 4. (a) Raman spectra for as-grown and transferred MoS2 film. The

transferred MoS2 sample retains similar peak position and intensity ratio.

(b) Contour map of Raman peak intensity ratio of E12g and A1

g for trans-

ferred MoS2 film over 100 lm2 area.

193503-3 Lee et al. Appl. Phys. Lett. 107, 193503 (2015)

2D crystals, interface traps are from broken bonds of 3D

crystal surface and not from the 2D semiconductor itself.

Therefore, the interface trap density is strongly affected by

the dielectric material.

In conclusion, we demonstrate a benign process for trans-

fer of centimeter scale multilayer MoS2 films onto SiO2/Si

substrates without using a carrier polymer. This transfer pro-

cess circumvents any chemical contamination of the MoS2

films. X-ray and Raman measurements confirm that the

crystalline quality of the films is maintained after transfer. A

field-effect mobility of 4.5 cm2 V�1 s�1 was measured in the

transferred MoS2/SiO2/Si back-gated transistor. The growth

and transfer technique demonstrated here could enable large-

area flexible devices as well as synthetic heterostructures con-

sisting of MoS2 and other 3D semiconductors.29

We acknowledge the support from Air Force Office of

Scientific Research (Dr. Kenneth Goretta) under Contract

No. FA9550-15-1-0294.

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details.

FIG. 5. (a) I-V measurement from

MoS2 device transferred from sapphire

to another identical sapphire substrate

showing 15% reduction in current. (b)

Output characteristics (IDS-VDS) from

the MoS2 transistor device of 2 lm

channel length. VGS ranges from �8 V

to 8 V, with 4 V step. (c) Transfer char-

acteristics (IDS-VGS) of the MoS2 de-

vice with different VDS. (d) Transfer

characteristics in linear scale at

VDS¼ 0.1 V, and extracted threshold

voltage is �0.9 V.

193503-4 Lee et al. Appl. Phys. Lett. 107, 193503 (2015)