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Wenjing Zhang, Jing-Kai Huang, Chang-Hsiao Chen, Yung-Huang
Chang,
Yuh-Jen Cheng, and Lain-Jong Li*
High-Gain Phototransistors Based on a CVD MoS2Monolayer
DOI: 10.1002/adma.201301244
Graphene, one of the most promising two-dimensional mate-rials,
has great potential in various applications,
includingnanoelectronics, optoelectronics, energy harvesting, and
bio-sensing.[1] The linear dispersion of the Dirac electrons
makesgraphene-based broadband optical modulators[2] and
ultrafasthigh frequency photodetectors[3] possible. However, the
zerobandgap of graphene limits its applications in
nanoelectronicsand optoelectronics. With this consideration, the
molybdenumdisulfide (MoS2) monolayer recently emerged with a
directbandgap of 1.8 eV.[4] The large direct bandgap, high
electronmobility (200 cm2 V1 s1), excellent current ON/OFF
ratio(108), low subthreshold swing (74 mV dec1), and highquantum
luminescence efficiency of monolayered MoS2makeit a new hot
research topic,[57]especially in the field of
optoelec-tronics.[810]A phototransistor made from a monolayer of
MoS2has shown a high photoresponsivity (7.5 mA W1) and a
shortresponse time (50 ms).[11] Also, the wavelength
dependentphotosensitivity varies with the thickness of MoS2.
[12]Moreover,the phototransistors based on multilayers of MoS2
exhibit abroad spectral response from ultraviolet to
infrared.[13]All theseproperties indicate that the MoS2 layers are
promising as newbuilding blocks for optoelectronic
devices.[1419]
With a high surface-to-volume ratio, the
photoelectricalresponses of the ultrathin monolayer of MoS2could be
affectedby the adsorbates in ambient air. It has been demonstrated
thatthe electrical characteristics of a mechanically exfoliated
MoS2monolayer can be remarkably changed after exposure to
triethyl-amine or acetone.[20]Recently, we have successfully
synthesizedlarge area mono- and few-layer MoS2by chemical vapor
deposi-tion methods.[2123]The photoluminescence, electron
mobility,and current ON/OFF ratio of chemical vapor deposited
(CVD)MoS2are comparable with that of mechanically exfoliated
MoS2flakes in a backgate transistor structure.[5,6,21,22] In this
contri-bution, we find that the photoresponsivity and photogain
wereup to 2200 A W1and 5000, respectively, for a CVD MoS2mon-olayer
at room temperature in a high vacuum. The shallow
trapped long-range Coulomb potentials, which could be fromthe
charged impurities at the MoS2/substrate interface, drasti-cally
affect the recombination of the photogenerated carriers,accounting
for the observation of the persistent photocurrentat room
temperature. The adsorbates from ambient air interactstrongly with
MoS2, resulting in the change of the out-of-planeRaman A1g mode,
the enhancement of carrier scattering, andthe decrease in mobility,
photoresponsivity, and photogain. Thestudy of the photocurrent
decay time reveals that the adsorp-tion/desorption processes of the
adsorbates assist the recombi-nation of the photogenerated
carriers, causing the decrease ofthe decay time.
The large-area MoS2 monolayer was directly grown on a300 nm
SiO2/Si substrate by the vapour-phase reaction of MoO3and sulfur
powders in a hot-wall CVD system as described inour previous
work.[22]Figure1a displays a typical Raman spec-trum taken for the
CVD MoS2monolayer, where the energy dif-ference between the Raman
A1gand E
12gmodes (=A1g E12g)
is 18.2 cm1, indicating that it is a monolayer.[21,22,24]Figure
1bshows the typical photoluminescence (PL) spectrum of a
MoS2monolayer film, which consists of two peaks at 657 (1.89
eV)
Figure 1. a) The Raman spectrum for the CVD MoS2monolayer on
a300 nm SiO2/Si substrate. The frequency separation between A1g
andE12gpeaks is 18.2 cm
1. b) The photoluminescence spectrum for the CVDMoS2 monolayer.
c) Schematic diagram of the phototransistor devicestructure based
on a MoS2monolayer. The source/drain electrodes werecomposed of 10
nm of Ti and 80 nm of Au. d) The top-view optical micro-scopy of
the device, where the source and drain metals are comb-shaped.
370 380 390 400 410 420Ramanintensity(a.u.)
Raman shift (cm-1
)
A1g
E1
2g
(a)
500 550 600 650 700 750
B1
PLintensity(a.u.)
Wavelength (nm)
A1
Si Raman
MoS2Raman
(b)
Dr. W. Zhang, J.-K. Huang, Dr. C.-H. Chen,Dr. Y.-H Chang, Dr.
L.-J. LiInstitute of Atomic and Molecular SciencesAcademia
SinicaTaipei 10617, TaiwanE-mail: [email protected]
Dr. J.-Y. ChengResearch Center for Applied SciencesAcademia
Sinica, Taipei, 11529, Taiwan
Adv.Mater.2013, 25, 34563461
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decrease of the electron mobility, photoresponsivity, and
gain.On the other hand, the adsorbates assist photocurrent
relaxa-tion and decrease the photocurrent decay times. In our
experi-ments, a low contact resistance was realized between
MoS2andthe Ti/Au electrodes, promising the operation of the
transistorby modulation of the channel resistance. The ohmic-like
con-
tact makes the effects from disorders or extrinsic
impuritiesmore pronounced on the optoelectronic properties of
MoS2,causing an obvious PPC. The temperature dependence of thePPC
shows that the shallow long-range Coulomb potentialsmay dominate
the trap centers of the photogenerated carriers.Our findings
suggest that a fast response and high gain CVDMoS2phototransistor
could be realized by improving the MoS2/substrate interface, and
the CVD MoS2has a great potential inoptoelectronic
applications.
Experimental Section
Sample Preparation: Large-area MoS2films were synthesized on
SiO2/Si
substrates in a hot-wall furnace. Prior to the growth, the
SiO2/Si substrateswere cleaned with standard Piranha solutions.
High purity MoO3 (99%,Aldrich) and S powder (99.5%, Alfa) were
placed in two separate Al 2O3crucibles, and the substrates were
placed face down on the upper sideof the MoO3power. The MoS2
samples were fabricated by annealing at650 C for 15 min with a
heating rate of 15 C min 1and under a N2flow(1 sccm) at ambient.
Standard photolithography was used to define thedevices contacted
by interdigitated Ti (10 nm)/Au (80 nm) electrodes.
Characterizations: The AFM images were obtained using a
VeecoDimension-Icon system. Raman and PL spectra were collected in
aconfocal Raman/PL system (NT-MDT). The wavelength of the laser
was473 nm (2.63 eV), and the spot size of the laser beam was 0.5 m.
Thestep size of the Raman spatial mapping was 0.5 m, and the
spectralresolution was 3 cm1 (obtained with a 600 grooves mm1
grating). Ahigh grating (1800 grooves mm1) was also used to obtain
more detailsof the line shapes of the Raman band, and the spectral
resolution was1 cm1. The Si peak at 520 cm1was used as a reference
for wavenumbercalibration, and the peak frequency was extracted by
fitting a Raman peakwith a Lorentz function. The electrical
measurements were performedunder ambient or vacuum conditions using
a Keithley semiconductorparameter analyzer, model 4200-SCS. A 532
nm laser was used tomeasure the photoresponse of the devices, and
the spot size was 1 mm.
Supporting Information
Supporting Information is available from the Wiley Online
Library orfrom the author.
AcknowledgementsThis research was supported by Academia Sinica
(IAMS and Nanoprogram) and National Science Council Taiwan
(NSC-99-2112-M-001-021-MY3).
Received: March 19, 2013Revised: April 13, 2013
Published online: May 24, 2013
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