Appl. Phys. Lett. 102, 252108 (2013); https://doi.org/10.1063/1.4811410 102, 252108

© 2013 AIP Publishing LLC.

Large area single crystal (0001) orientedMoS2Cite as: Appl. Phys. Lett. 102, 252108 (2013); https://doi.org/10.1063/1.4811410Submitted: 10 April 2013 . Accepted: 23 May 2013 . Published Online: 28 June 2013

Masihhur R. Laskar, Lu Ma, Santhakumar Kannappan, Pil Sung Park, Sriram Krishnamoorthy, Digbijoy N.Nath, Wu Lu, Yiying Wu, and Siddharth Rajan

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Large area single crystal (0001) oriented MoS2

Masihhur R. Laskar,1 Lu Ma,2 Santhakumar Kannappan,3 Pil Sung Park,1

Sriram Krishnamoorthy,1 Digbijoy N. Nath,1 Wu Lu,1 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 Nanobio Materials and Engineering, Gwangju Institute of Science and Technology,Gwangju 500-712, South Korea

(Received 10 April 2013; accepted 23 May 2013; published online 28 June 2013)

Layered metal dichalcogenide materials are a family of semiconductors with a wide range of energy

band gaps and properties, the potential for exciting physics and technology applications. However,

obtaining high crystal quality thin films over a large area remains a challenge. Here we show that

chemical vapor deposition (CVD) can be used to achieve large area single crystal Molybdenum

Disulfide (MoS2) thin films. Growth temperature and choice of substrate were found to critically

impact the quality of film grown, and high temperature growth on (0001) oriented sapphire yielded

highly oriented single crystal MoS2 films. Films grown under optimal conditions were found to be

of high structural quality from high-resolution X-ray diffraction, transmission electron microscopy,

and Raman measurements, approaching the quality of reference geological MoS2.

Photoluminescence and electrical measurements confirmed the growth of optically active MoS2

with a low background carrier concentration, and high mobility. The CVD method reported here for

the growth of high quality MoS2 thin films paves the way towards growth of a variety of layered 2D

chalcogenide semiconductors and their heterostructures. VC 2013 AIP Publishing LLC.

[http://dx.doi.org/10.1063/1.4811410]

Molybdenum disulphide (MoS2) has received signifi-

cant attention recently for next generation electronics,1–5

optoelectronics,6 and sensor7,8 applications. The intrinsic 2-

dimensional nature of carriers in these chalcogenides semi-

conductors offers the advantages of superior vertical scaling

for a transistor topology9 and the prospect of creating heter-

ostructures that are not limited by out of plane bonding and

lattice mismatch, through techniques such as van der Waals

epitaxy. The potential of these materials for low-cost flexi-

ble or transparent electronics that could revolutionize tech-

nology is also very high.

Monolayer MoS2 device (transistor) results showing

excellent on-off ratio and high current density have been

obtained using flakes1,2 mechanically exfoliated from bulk

geological samples, which are valuable for demonstrating the

promise of MoS2 devices.10,11 However, exfoliation does not

allow control over thickness and area of the film, and may

not be suitable for large scale device fabrication. While previ-

ous approaches involving chemical vapor deposition

(CVD),12,13 sulfurization of molybdenum oxides,14,15 hydro-

thermal synthesis,16 and electrochemical synthesis17 led to

large area thin MoS2 the crystal quality of the layers grown

was significantly lower than that obtained from naturally

occurring MoS2, which remains the highest quality material

available to date. The low structural quality and polycrystal-

line nature, evident from Raman spectroscopy, X-ray diffrac-

tion, and transmission electron microscopy in these reports

led to low mobility which was orders of magnitude lower

than the mobility in relatively high-quality exfoliated sam-

ples. Methods to achieve large area metal dichalcogenide

films that use the conversion of bulk MoS2 into large scale

MoS2 film using laser trimming18 and liquid exfoliation19

exploit the high quality of geological MoS2 but have not

demonstrated area-specific and large area thin film synthesis.

In this paper, we show that chemical vapor deposition

on crystalline substrates can lead to large area oriented crys-

talline films with unprecedented high quality. The films

reported here have structural quality similar to the best qual-

ity geological samples available to date, and could enable a

variety of large area electronic and optoelectronic device

applications. The methods proposed can also be extended to

other layered chalcogenide materials, as well as lateral and

vertical heterostructures based on them.

Samples were grown by sulfurization of e-beam evapo-

rated Mo films (see Fig. S1 in supplementary information20

for more details). Our experiments indicate that growth tem-

perature and substrate were critical in determining film qual-

ity. We describe here characteristics of samples grown at

temperatures 500 �C, 700 �C, 900 �C, and 1100 �C, referred to

as sample A, B, C, and D, respectively. From optical micros-

copy, each of these films was found to have specular reflec-

tion indicating smooth MoS2 coverage and absence of any

remaining metal on the surface. Atomic force micrographs of

the films (Fig. 1) show the evolution of surface morphology

as a function of growth temperature. At relatively low temper-

atures sample A showed smooth surface morphology (rms

roughness¼ 0.30 nm) with small grains that we attribute to

the polycrystalline nature of the film. For sample B, the grain

size was found to be larger, while the surface roughness

(3.5 nm) was higher than sample A. In sample C, thin sheet of

MoS2 was found to cover the entire surface, and the morphol-

ogy revealed hexagonal symmetry. We believe this originates

from the hexagonal basal plane symmetry of the MoS2 crystal

itself, suggesting that the layers grown are oriented along

(0001) or c-axis of the MoS2 unit cell. In sample D, MoS2

a)Author to whom correspondence should be addressed. Electronic mail:

rajan@ece.osu.edu. Tel.: þ1(614) 247 7922. Fax: þ1 (614) 292 7596.

0003-6951/2013/102(25)/252108/5/$30.00 VC 2013 AIP Publishing LLC102, 252108-1

APPLIED PHYSICS LETTERS 102, 252108 (2013)

flakes several microns in lateral size and tens of nanometer

thick were formed, indicating the significant mass transport

takes place during the growth. MoS2 flakes (sample D) with

sharp edges and corners were found on the surface with sides

parallel to step-edge of sapphire substrate which is an indica-

tion that the layers perhaps are grown epitaxially.

Further detailed analysis of the AFM image of sample C

shown in Figs. S3(a) and S3(b)20 reveals that the step edges

are 0.56 nm thick, approximately equal to one atomic mono-

layer of MoS2.1 Spiral growth features similar to those seen

in other material systems21 were also evident. The white dots

in the image (Fig. S3(a)20) are due to excess sulfur droplets

which can be removed with a chemical clean (Fig. S2(b)20).

All the samples show room temperature photolumines-

cence (PL). The PL spectra of sample C (Fig. S3(c)20) show a

peak at 570 nm (2.17 eV), and an additional shoulder at

700 nm (1.77 eV). These peak positions are at a higher energy

than reported earlier for multilayer exfoliated films22 and

CVD single-layer films,23 indicating that the optical proper-

ties of the CVD films reported here are probably different

from those cases, and further investigation is needed to

understand these phenomena better. An optical image shows

the film is continuous and specular over a 2 in. length scale

(Fig. S3(d)20). The width of the film (or substrate) was lim-

ited in our experiments by diameter of the quartz tube (1 cm),

but could be scaled up.

Samples were characterized using Raman spectroscopy

(Renishaw, 514 nm laser and 60 mW power). The

characteristic Raman peaks for the E12g and A1g vibration

modes appear at 382 cm�1 and 407 cm�1 for all samples, and

their separation D¼ 25 cm�1 corresponds to the bulk MoS2

films.24 In Fig. 2(a), the Raman peaks observed from samples

A and B taken under identical conditions are shown. In the

case of the lowest growth temperature (500 �C) sample A, the

intensity of E12g peak was lower than the A1g peak, whereas

the opposite was seen in bulk geological MoS2. This is similar

to the ratio of the Raman peaks from previous reports on

CVD,12,13 and indicative of low structural quality in the film.

In sample B, which was grown at a higher temperature, how-

ever, the overall intensity of the peaks increased and the rela-

tive ratio, of the E12g the A1g peaks were similar to high-

quality exfoliated MoS2. Samples C and D, which were grown

at even higher temperatures, showed similar ratios (i.e., E12g

peak greater than A1g), but with intensity that was 50 times

higher than sample B. This suggests that structural quality in

these films was significantly higher than samples A or B and

the high growth temperature is critical for the formation of

high quality MoS2. In Fig. 2(b), Raman measurements of sam-

ple C and thick flakes from a commercial SPI MoS2 wafer are

compared. With equal laser power (1% laser power), the width

and intensity of the peaks are almost identical. The full width

at half maximum (FWHM) of the Raman peak can be corre-

lated with the quality of the film, and as expected (Fig. 2(c))

the FWHM decreases with increasing temperature of sulfuri-

zation, approaching that of commercial MoS2. The intensity

ratios of E12g and A1g peaks as a function of growth

FIG. 1. Atomic force micrographs of (a) sample A (500 �C), (b) sample B (700 �C), (c) sample C (900 �C), (d) sample D (1100 �C) samples. Scan area and

height data scales are varied to display surface features clearly.

FIG. 2. Raman spectra of (a) sample A (500 �C) and sample B (700 �C) taken under identical conditions. The peak intensity ratio E12g/A1g is less than 1 for

sample A and greater than 1 for sample B, confirming higher growth temperature leads to improved film quality. (b) Raman spectra obtained from sample C

(900 �C) and a thick (>100 nm) flake from a bulk geological MoS2 show similar peak intensity and peak sharpness. Note that the peak intensity for sample C

is approximately 50 times higher than for samples A or B. (c) Raman peak FWHM as a function of growth temperature. Increasing temperature leads to lower

FWHM associated with improved structural quality. Inset: Peak ratio for E12g and A1g peaks for samples A, B, C, and D. The corresponding values for bulk

MoS2 is indicated by an arrow symbol.

252108-2 Laskar et al. Appl. Phys. Lett. 102, 252108 (2013)

temperature for samples A-D, and for a commercial bulk geo-

logical sample, are shown in the inset of Fig. 2(c). Samples C

and D exhibit a ratio >1, similar to bulk MoS2 indicating simi-

lar structural quality from Raman spectroscopy. The Raman

measurements, therefore, indicate that samples grown at high

temperatures have similar characteristics to geological high

quality MoS2.

X-ray diffraction measurements of the MoS2 confirm the

trends evident from the Raman spectra. High resolution x-2hscans of the four investigated samples A, B, C, D, and for

commercial (geological) bulk MoS2 are compared in Fig. 3.

In the reference bulk sample, a sharp (0002) diffraction peak

was observed at 2h¼ 14.5�, corresponding to the (0002) dif-

fraction condition. Higher order peaks for the MoS2 and the

(0006) diffraction peak of the sapphire substrate (blue) were

observed. In the case of the CVD grown thin layers, while

sample A showed no clear diffraction peaks, the lowest order

peak (0002) peak with 2h¼ 14.5 was clearly visible in the

spectra for sample B. In the case of samples C and D, higher

order peaks are evident from the figure, indicating long-range

crystalline order in these samples. It is evident from these

results that increasing temperature improves structural quality

and crystal orientation of the MoS2. Triple axis X-ray rocking

curve measurements [Figs. S4(a) and S4(b) (Ref. 20)] about

the (0002) reflection condition for sample C were done to

assess the sample quality and the FWHM was found to be 25

arc sec (compared with 42 arc sec for exfoliated samples, not

shown here) confirming that the films are highly oriented with

negligible mosaicity (tilt). The films grown were too thin to

observe any off-axis peaks that would have helped to deter-

mine the epitaxial relation with the sapphire substrate, or the

twist mosaicity.

HR-TEM measurements revealed ordered crystalline

MoS2, confirming the XRD measurements. Fig. 4 shows the

growth direction of MoS2 is along C [0001] direction of sap-

phire substrate, and the ordering of the MoS2 layers with uni-

form periodic atom arrangement over the entire region.

Selected area electron diffraction (SAED, Fig. S520) pattern

taken from the MoS2/Sapphire interface indicated that the

CVD grown MoS2 is single crystalline. The epitaxial rela-

tionship between the basal planes of MoS2 and sapphire

could not be determined from this measurement. However,

though there were few slightly rotated regions, no amor-

phous region were observed confirming the crystalline nature

of the MoS2 over a large area.

To investigate the effect of substrate on the MoS2

growth, films were grown on SiO2/Si substrates with growth

conditions identical to sample C, which as discussed earlier

in this paper, had excellent structural quality. The XRD spec-

tra of the films grown on 300 nm SiO2/p-Si substrates did not

show any peaks, indicating that they were either polycrystal-

line or amorphous, matching reports for lower temperature

CVD growth of MoS2 on SiO2.13 We conclude that the hex-

agonal symmetry of the basal plane in sapphire plays an im-

portant role in determining the crystalline quality and

orientation of the overgrown MoS2 layers. Other crystalline

substrates such as GaN and ZnO, or other 2D materials, could

also be viable candidates for high quality MoS2 growth.

The transport properties of these films were investigated

using transmission line measurements (TLM) by depositing

Ti/Au contacts using photolithography, metal evaporation,

and lift-off. Current voltage (I-V) characteristics of sample C

for different contact spacing (L) are shown in Fig. 5. The I-V

characteristics, shown in a log-log scale, display (Fig. 5 inset)

two different regimes with dependence I a V at lower current

density, and I a V2 at higher current density. The transition of

the voltage dependence of current from linear to quadratic is a

characteristic feature of space charge limited transport,25 sug-

gesting that the MoS2 has relatively low background carrier

density, a desirable property for field effect transistors, and

that current is carried through injection of electrons from the

contacts into a nominally insulating semiconductor.

While the well-known Mott Guirney law for space charge

transport in semiconductors, I ¼ 9e0erlV2=8L3 , is applicable

to bulk transport where 1-dimensional electrostatics are valid,

it is not applicable in the present case where the field is

applied laterally. We, therefore, follow previous work26,27 on

the analysis of space charge transport in thin films with lateral

contact geometry. The current density (in A/cm) in this case

is given by I ¼ 2e0erlV2=pL2, where the current still has

quadratic dependence on the voltage, as in the Mott-Guirney

equation, but the dependence on the distance between the con-

tacts is now modified from I a V2/L3 to I a V2/L2 due to the

thin film geometry. Our analysis shows that indeed, the de-

pendence of the current density in the space-charge region

varies as V2/L2 (thin film transport) rather than V2/L3 (bulk

transport). Based on this equation, we estimate independently

from each of the curves, a high field-effect mobility

12 6 2cm2/Vs (Table I of supplementary material20) in the

MoS2 films. Using the mobility extracted from the quadratic

region and conductivity from the linear region, a carrier

FIG. 3. (a) Wide angular range X-ray diffraction spectra for samples A, B,

C, and D and bulk MoS2. The diffraction peak for (0002) reflection condi-

tion for MoS2 can be seen distinctly for sample C (900 �C) and sample D

(1100 �C), suggesting the films are c-plane oriented.

252108-3 Laskar et al. Appl. Phys. Lett. 102, 252108 (2013)

density �1016 cm�3 was estimated. Gated measurements

showed that the current increased as the gate voltage was

made more positive, suggesting that the conduction is n-type.

Based on previous band structure calculations,28 the effective

mass of electrons and holes in MoS2 are not expected to be

significantly different, and, therefore, electron and hole mobil-

ity may be expected to be similar in magnitude.

The mobility reported here is higher than previous reports

of CVD grown MoS2,12–15 but the estimate from space charge

injection is likely to be much lower than the low-field mobil-

ity that would be applicable if degenerate gases were induced

in these films. First, space charge injection takes place at rela-

tively high fields where the mobility is usually lower in most

semiconductors due to increased energy dissipation mecha-

nisms such as optical phonon emission. Second, due to the

low background charge, screening, which plays a very impor-

tant role in mitigating charged defect related scattering28 in

doped and degenerate carrier gases, is expected to be mini-

mal. Since the film is thin, we expect that remote charge scat-

tering can degrade mobility significantly especially when

there is little or no screening of the impurity scattering poten-

tial. In the future, degenerate 2D gases or higher density gases

through doping in these films could be expected to have

higher mobility due to the screening effects.

In conclusion, large area (0001) oriented crystalline

MoS2 films with structural quality comparable to bulk geo-

logical samples were achieved using sulfurization of Mo.

We find that the use of crystalline substrates and suitable

growth conditions allows for synthesis of high quality crys-

talline films with mobility significantly higher than previous

results. The method could be extended to synthesize other

members of the layered metal dichalcogenide family, such as

TiS2, WS2, and HfS2, many of which are promising for elec-

tronic and optoelectronic applications. Depositing constitu-

ent metals on top of each other, or adjacent to each other

could enable vertical and lateral heterostructures, and inclu-

sion of other elements could be achieved by changing the

composition of the metal film or the sulfur precursor, ena-

bling alloys and doping. Epitaxial growth of 2D layered

materials on technologically important hexagonal symmetry

semiconductors such as GaN, ZnO, and SiC could also

expand the functionality of these semiconductors. The

growth techniques described here fill a critical gap in taking

these materials from the laboratory to real applications that

need large-area high quality.

W.L. and S.R. acknowledge funding from the NSF

NSEC (CANPD) Program (EEC0914790); S.R. acknowl-

edges support from NSF ECCS Grant ECCS-0925529. W.L.

acknowledges support from NSF Grants (ECCS0824170 and

CMMI0928888). L.M. and Y.W. acknowledge the support

from NSF (CAREER, DMR-0955471). M.L. was supported

by the OSU NSF MRSEC CEM Seed Program. TEM work

was supported by the WCU program at GIST through a

MEST (Korea) Grant. M.L. thanks Craig Polchinski and

Sanyam Bajaj for AFM measurements, and R. C. Myers and

T. Kent for using their photoluminescence setup.

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