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Ultrahigh sensitive MoTe2 phototransistors driven by carrier tunneling Lei Yin, Xueying Zhan, Kai Xu, Feng Wang, Zhenxing Wang, Yun Huang, Qisheng Wang, Chao Jiang, and Jun He 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 tunable photoresponse 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 Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 124.16.156.247 On: Thu, 29 Sep 2016 02:47:42
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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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Page 2: Ultrahigh sensitive MoTe2 phototransistors driven by ...

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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Page 5: Ultrahigh sensitive MoTe2 phototransistors driven by ...

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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