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Ultrahigh sensitive MoTe2 phototransistors driven by carrier tunnelingLei Yin, Xueying Zhan, Kai Xu, Feng Wang, Zhenxing Wang, Yun Huang, Qisheng Wang, Chao Jiang, and JunHe Citation: Applied Physics Letters 108, 043503 (2016); doi: 10.1063/1.4941001 View online: http://dx.doi.org/10.1063/1.4941001 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/108/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Electrolytic phototransistor based on graphene-MoS2 van der Waals p-n heterojunction with tunablephotoresponse Appl. Phys. Lett. 109, 113103 (2016); 10.1063/1.4962551 Thickness-dependent electron mobility of single and few-layer MoS2 thin-film transistors AIP Advances 6, 065106 (2016); 10.1063/1.4953809 Superconductivity enhancement in the S-doped Weyl semimetal candidate MoTe2 Appl. Phys. Lett. 108, 162601 (2016); 10.1063/1.4947433 Enhancing photoresponsivity using MoTe2-graphene vertical heterostructures Appl. Phys. Lett. 108, 063506 (2016); 10.1063/1.4941996 Exfoliated multilayer MoTe2 field-effect transistors Appl. Phys. Lett. 105, 192101 (2014); 10.1063/1.4901527
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Ultrahigh sensitive MoTe2 phototransistors driven by carrier tunneling
Lei Yin,a) Xueying Zhan,a) Kai Xu, Feng Wang, Zhenxing Wang,b) Yun Huang,Qisheng Wang, Chao Jiang, and Jun Heb)
CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, National Center for Nanoscienceand Technology, Beijing 100190, People’s Republic of China
(Received 10 November 2015; accepted 18 January 2016; published online 26 January 2016)
Transition metal dichalcogenides (TMDs) demonstrate great potential in electronic and
optoelectronic applications. However, the device performance remains limited because of the poor
metal contact. Herein, we fabricate a high-performance ultrathin MoTe2 phototransistor. By intro-
ducing an electron tunneling mechanism, electron injection from electrode to channel is strikingly
enhanced. The electron mobility approaches 25.2 cm2 V�1 s�1, better than that of other back-gated
MoTe2 FETs. Through electrical measurements at various temperatures, the electron tunneling
mechanism is further confirmed. The MoTe2 phototransistor exhibits very high responsivity up to
2560 A/W which is higher than that of most other TMDs. This work may provide guidance to
reduce the contact resistance at metal-semiconductor junction and pave a pathway to develop high-
performance optoelectronic devices in the future. VC 2016 AIP Publishing LLC.
[http://dx.doi.org/10.1063/1.4941001]
Transition metal dichalcogenides (TMDs) are an attrac-
tive class of layered materials which have drawn much atten-
tion due to the unique electronic and optoelectronic
properties.1,2 Such properties have made TMDs intensively
studied for high performance two-dimensional (2D) elec-
tronic and optoelectronic devices over the past few years.3–7
Recently, many works on molybdenum ditelluride (MoTe2)
have been reported for electronic applications.8–12 The
reported mobility values of ambipolar back-gated MoTe2
FET are in the range of 0.03–3.7 cm2 V�1 s�1 for electrons
and 0.01–0.3 cm2 V�1 s�1 for holes,8,11 although the mobil-
ity can be improved via special processes, such as high-kdielectrics and ion liquid gating.12
However, metal-semiconductor interface is still one of
the main factors that limit the carrier conduction. So far,
there are few works on the metal contacts with MoTe2.
Recently, an ohmic homojunction contact for MoTe2 transis-
tors has been achieved via local phase transition.13
According to thermionic emission theory, electrons can eas-
ily cross over a low Schottky barrier. Thus, the choice of
metal contacts, in general, is based on the theoretical princi-
ple that low work function metals facilitate electron injec-
tion.14–20 However, based on the thermionic emission theory
of metal-semiconductor junctions, the existence of Schottky
barriers inevitably gives rise to a relatively large contact re-
sistance. Therefore, other conductive mechanisms need to be
explored. Additionally, though the phototransistors based on
TMDs have been largely reported, the responsivity of most
TMDs are still relatively low, which are in the range of 10�5
to 102 A/W.7,21–26 As a member of TMDs, MoTe2 possesses
a bandgap of about 1 eV in its bulk form.1,27 Meanwhile, due
to its strong light absorption ability, MoTe2 has been utilized
as electrodes in photoelectrochemical cells.28 Therefore, it
has great potential for high-performance optoelectronic
devices.
Herein, we enhance electron injection by introducing a
tunneling transport mechanism. Compared with the Cr-
contacted devices, the MoTe2 transistors using Au electrodes
display lower contact resistance and higher electron mobil-
ity. The electron mobility reaches up to 25.2 cm2 V�1 s�1,
which is superior to other back-gated MoTe2 FETs.8,11
Furthermore, through electrical measurements at various
temperatures, the tunneling mechanism in Au-MoTe2 junc-
tions is further confirmed. After optimization of the metal
contacts, MoTe2 phototransistor exhibits an ultrahigh respon-
sivity of 2560 A/W that is higher than most other TMDs
materials.7,21–26 This work may provide guidance to reduce
the contact resistance at metal-semiconductor junction and
pave a pathway to develop high-performance optoelectronic
devices in the future.
Layered MoTe2 possesses two stable phases: trigonal
phase (2H) and octahedral coordination phase (1T0) as shown
in Fig. 1(a).29–31 Naturally, MoTe2 shows trigonal (2H)
structure, where Mo atoms are between two atomic layers of
Te, and each Mo is coordinated to six Te atoms. The experi-
mental and theoretical investigations have confirmed that
2H-MoTe2 is a semiconductor.1,27 We first transferred the
layered 2H-MoTe2 crystals (99.995%, HQ Graphene) onto
Si substrates with 300 nm thick SiO2 by a mechanically exfo-
liation method. Electron beam lithography (EBL) was uti-
lized to pattern the source/drain electrodes and then Au
contact (Au/Cr/Au, 20 nm/8 nm/60 nm) and Cr contact (Cr/
Au, 8 nm/80 nm) are deposited by thermal evaporation. The
schematic diagram of MoTe2 FETs is shown in Fig. 1(b).
Figure 1(c) displays the atomic force microscopy (AFM)
image of the few layered MoTe2 with a thickness about
6.5 nm. The thicknesses of other devices were also character-
ized by AFM, as summarized in Table I. In Fig. 1(d), the
characteristic Raman-active modes of A1g (171 cm�1), E12g
(232 cm�1), and B12g (288 cm�1) are clearly observed using
a)L. Yin and X. Zhan contributed equally to this work.b)Electronic addresses: [email protected] and [email protected]
0003-6951/2016/108(4)/043503/5/$30.00 VC 2016 AIP Publishing LLC108, 043503-1
APPLIED PHYSICS LETTERS 108, 043503 (2016)
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a confocal Raman spectroscopy (Renishaw InVia, 532 nm
excitation laser).32–34 Besides, Raman mapping indicates the
MoTe2 flake with uniform thickness.
Figure 2(a) shows the band alignment of MoTe2, Cr, and
Au. Because MoTe2 owns a small bandgap and electron
affinity (v),35,36 the Fermi level of Au is located below the
valence band edge of MoTe2. Based on Schottky-Mott
model,17 there will be a high Schottky barrier for electrons at
the Au-MoTe2 junction. However, the Schottky barrier is
extremely thin because of significant band bending, so that
electrons may cross it by tunneling. To verify our conjecture,
electrical measurements were carried out on a probe station
(Lakeshore, TTPX) equipped with a semiconductor charac-
terization system (Keithley 4200). Both Au- and Cr-
conducted devices have the same thickness (7.2 nm) and ex-
hibit an ambipolar behavior in Fig. 2(b). The gate voltage
corresponding to minimum current (Vmin) for Au- and Cr-
contacted devices are found to be �20 and �12 V, respec-
tively. We estimated the mobility according to the generally
field-effect equation.21 The calculated electron mobility for
Au contacts is 15.1 cm2 V�1 s�1 which is better than that of
our Cr-conducted transistor and other back-gated MoTe2
FETs on SiO2 dielectrics.8,11 The electrical characteristics of
other devices are summarized in Table I. The influences of
thickness of MoTe2 on mobility and on/off are related to
coulomb scattering, screening effect, and quantum confine-
ment effect.37,38 Besides, we proved that carrier injection is
definitely related to work function, and Fermi level pinning
is negligible by comparing with the electrical characteristics
of Al-MoTe2 FET.
Figure 2(c) shows the output curves of MoTe2 FET for
Au and Cr contacts. It is worth noting that the Ids of Au-
MoTe2 FET is 100 times larger than that of the transistor
with Cr electrodes. According to the method presented by
Kim,39 from Fig. 2(c), the contact resistance (Rc) for Au- and
Cr-MoTe2 FETs can be evaluated to be 7.5� 104 and
7.3� 106 X�lm at Vgs¼ 80 V, respectively. The smaller Rc
indicates that Au contacts are easier to pass through for elec-
trons. What is more, the larger Rc of Cr-MoTe2 FET leads to
a lower effective Vgs, given by Vgs�eff ¼ Vgs � RcIds.15 It
explains the relative shift of Vmin for Au and Cr contacts
observed in Fig. 2(b). Moreover, the Ids�Vds curves of
MoTe2 FET with Au contacts in logarithmic scale suggest its
ambipolar nature and a good ohmic contact (Fig. 2(d)).
Through above analysis, we find that Au-contacts can
reduce Rc more effectively. However, to elucidate how con-
tacts influence the carrier transport in MoTe2 FETs, we per-
formed Ids�Vgs measurements at various temperatures
(Figs. 3(a) and 3(b)). As well known, carriers can be injected
into channel by thermionic emission or tunneling across over
barriers. Their contributions to carrier transport depend on
temperature and gate bias. Interestingly, we observe that the
n-type region of Cr-MoTe2 FET vanish below 120 K. This
phenomenon reveals that the thermionic emission dominates
the electron transport in n-type region of Cr-MoTe2. When
temperature is lower than a critical value, electrons do not
have enough energy to cross over a high barrier at the Cr-
MoTe2 junctions (Fig. 3(e-II)). However, Ids of Au-MoTe2
do not have pronounced reduction with the temperature
FIG. 1. (a) Scheme of the crystal struc-
tures: trigonal phase (2H) and octahe-
dral coordination phase (1T0) of MoTe2.
(b) Schematic diagram. (c) AFM image
and optical image (inset) of a typical
MoTe2 device. (d) Raman spectra and
mapping (inset) of MoTe2 nanosheet.
TABLE I. Summary of the thickness and electrical characteristics MoTe2
FETs.
Device Thickness (nm)
On/off ratio Mobility (cm2 V�1 s�1)
Electron Hole Electron Hole
Au-1# 11.4 1.8� 105 94 25.2 6.3� 10�2
Au-2# 9.6 4� 104 370 15 0.4
Au-3# 6.5 2.1� 105 5.7� 104 2.79 1.2
Au-4# 7.2 1.7� 105 3.1� 103 15.1 0.7
Cr-1# 11.3 1.5� 103 2� 104 7.1� 10�2 1.5
Cr-2# 7.2 2� 104 2.5� 104 0.4 0.5
043503-2 Yin et al. Appl. Phys. Lett. 108, 043503 (2016)
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FIG. 2. (a) Band alignments of Au, Cr,
and MoTe2. (b) Ids�Vgs of 7.2-nm-
thick MoTe2 FET with Au and Cr con-
tacts. Vds¼ 500 mV and T¼ 300 K. (c)
Ids�Vds of the corresponding devices
at various gate voltages. (d) Ids�Vds
of Au-contacted MoTe2 FET in loga-
rithmic scale.
FIG. 3. (a) and (b) Temperature de-
pendence of Ids as the function of Vgs
for Cr- and Au-contacted MoTe2 FET
at Vds¼ 0.5 V. (c) and (d) Ids of the
corresponding devices normalized by
the square of the temperature as a func-
tion e/kBT. Dashed lines stand for the
linear fit curves. (e) and (f) Extracted
effective UB at various Vgs for Cr and
Au contacts. The insets are band dia-
gram at Vgs� 0 and Vgs � 0 for Cr-
MoTe2 and Au-MoTe2 FETs. Blue
arrows represent primary transport
mechanism. Red and blue balls are
hole and electron, respectively.
043503-3 Yin et al. Appl. Phys. Lett. 108, 043503 (2016)
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decreasing at Vgs � 0. Because the Fermi level of Au is
located far below the valence band edge of MoTe2, the bar-
rier of Au-MoTe2 is too high (>1 eV) to transport electrons
by conventional thermionic emission. Considering the signif-
icant band bending between Au and MoTe2, the barrier is
high but very thin (Fig. 3(f-II)). Electrons have the ability to
pass the barrier by tunneling. Besides, there is a limited band
bending at Vgs¼ 0 V so that the dominant conductive mecha-
nism is thermally assisted tunneling. In contrast to n-region,
holes in p-region can be injected into channel material from
electrode by thermionic emission (Fig. 3(f-I)). It should be
noted that because MoTe2 is a semiconductor, its conductiv-
ity must be increased with temperature.
To take a deep insight into the carrier transport mecha-
nism, we extract the effective Schottky barriers height (UB)
of the region (Figs. 3(e-II) and 3(f-I)), in which the current is
dominated by thermionic emission theory. According to
thermionic emission equation, Ids is approximately described
as follow: Ids ¼ AA�T2 exp ð�eUB=kBTÞ,9 where A*, A, kB, e,
and T are the Richardson constant, the area of contact, the
Boltzmann constant, the electron charge, and the tempera-
ture, respectively. In Figs. 3(c) and 3(d), we plot Ids normal-
ized by T2 as a function of e/kBT for various Vgs. The slop of
the linear fit curves in Figs. 3(c) and 3(d) in the high temper-
ature region is related to the effective UB. Figures 3(e) and
3(f) show the extracted effective UB for Cr- and Au-
contacted devices. In the high positive gate voltage region
(Vgs � 0), UB for Cr-MoTe2 FET linearly depends on the
Vgs, manifesting only thermionic emission current contrib-
utes to the current flow through the device (Fig. 3(e-II)). In
the high negative gate voltage region (Vgs� 0), UB for holes
deviates gradually from linear (Fig. 3(e-I)) and the thermally
assisted tunneling become possible. In Fig. 3(f), we note
effective UB in n-region is in the range from �0.012 eV to
0.28 eV that are smaller than above estimated barrier �1 eV.
This is because electrons pass through the barrier in n-region
of Au-MoTe2 FET by tunneling instead of thermionic emis-
sion. Thermionic emission theory is not suitable to extract
UB in this region. This further proves that thermionic emis-
sion theory is invalid and electron tunneling is dominant in
the case. The detailed band diagram is shown in the inset of
Fig. 3(f-II). According to direct tunneling equation,40 the
estimated width of Schottky barrier due to the significant
band bending is about 1.1 nm.
After optimization of the devices by contact-metal engi-
neering, we investigated the performance of Au-MoTe2 photo-
transistors under illumination of a 473 nm laser. From Fig.
4(a), we observe that the Ids�Vgs curves shift toward negative
Vgs with increasing laser power. Additionally, the remarkable
photocurrents (Iph ¼ Ilight � Idark) are attained at different Vgs
in Fig. 4(b). These results can be explained by the photogating
effect.41,42 The photoexcited holes are captured by the trap
states; meanwhile, dramatically photoexcited electron injection
is equivalent to n-doping the channel, thus generating a nega-
tive shift of Ids�Vgs curves and prominent Iph. Besides, with
the increase in Vgs from 0 V to 80 V, the exponent a calculated
by Iph Pa,43 where P is the laser power density, decreases
from 0.81 to 0.48. The sublinear photoreponse with laser
power density suggests that the recombination of photoexcited
carriers becomes prominent. As discussed above, the trap
states could be recombination centers for holes. Furthermore,
as critical parameters for phototransistors, responsivity (Rk)
and photogain (G) of the devices were estimated, respectively,
as shown in Figs. 4(c) and 4(d). Responsivity can be calculated
by the formula: Rk ¼ Iph=PS, S is the effective illumination
area. The best responsivity is measured up to 2560 A/W at
P¼ 2.6 mW/cm2 and Vgs¼ 80 V. The responsivity is higher
than that of most other 2D materials used in the back-gated
phototransistors.7,21–26 Photogain is related to Rk by the equa-
tion Rk ¼ Iph=PS ¼ gGe=ht,44 where g is the external quan-
tum efficiency, h is Planck’s constant, and t is the frequency
of the incident laser. Assuming g¼ 100%, the maximal G over
FIG. 4. (a) Transfer curves of Au-
MoTe2 phototransistor under different
light illumination. The photocurrent
(b), responsivity (c), and photogain (d)
of Au-MoTe2 phototransistor change
with the laser power at different gate
voltages.
043503-4 Yin et al. Appl. Phys. Lett. 108, 043503 (2016)
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6700 is obtained at P¼ 2.6 mW/cm2 and Vgs¼ 80 V. The
ultrahigh responsivity and photogain are also largely attributed
to the tunneling mechanism. Because of the extremely thin
Schottky barrier deriving from significant band bending at the
contacts, photoexcited electrons are prone to participate in
conducting, which reduces the recombination of photocarriers.
In summary, we fabricate the ambipolar MoTe2 photo-
transistors and study the electrical and photoelectrical proper-
ties. Compared with Cr-contacted devices, Au-contacted
MoTe2 display significantly lower contact resistance and
higher electron mobility, which reaches 25.2 cm2 V�1 s�1.
We find that a thin Schottky barrier in Au-contacted devices
promotes the tunneling of electrons and results in an ideal
ohmic contact at Au-MoTe2 junctions. Moreover, the MoTe2
phototransistors exhibit an ultrahigh responsivity of 2560 A/W
and photogain over 6700. Its low contact resistance and high
responsivity make MoTe2 be a promising candidate for next-
generation electronic and optoelectronic devices in the future.
This work was supported by the National Natural
Science Foundation of China (Nos. 21373065 and
61474033), 973 Program of the Ministry of Science and
Technology of China (No. 2012CB934103), Beijing Natural
Science Foundation (No. 2144059), and CAS Key
Laboratory of Nanosystem and Hierarchical Fabrication. The
authors gratefully acknowledge the support of K. C. Wong
Education Foundation.
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