Superlattices and Microstructures 147 (2020) 106698
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Superlattices and Microstructures
journal homepage: www.elsevier.com/locate/superlattices
A facile route to enhance the mobility of MoTe2 field effect
transistor via chemical doping
Muhammad Waqas Iqbal a, **, Ehsan Elahi a, Aliya Amin a, Sikandar Aftab b,
Imran Aslam c, Ghulam Hussain a, *, Muhammad Arslan Shehzad d
a
Department of Physics, Riphah International University, 14 Ali Road, Lahore, Pakistan
School of Electrical and Electronic Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul, 03722, Republic of Korea
c
Department of Basic Sciences & Humanities, University of Engineering and Technology Lahore, Narowal Campus, Pakistan
d
Department of Chemical and Biomedical Engineering, Marcus Nanotechnology Center, Georgia Institute of Technology, Atlanta, GA, USA
b
A B S T R A C T
Modulating electrical characteristics is essential to attain a progressive performance of electronic devices such as field effect transistors (FETs).
Chemical doping is very beneficial technique which is so simple and cost effective than other methods used in electronic devices for improvement of
electrical characteristics. Here we report the effect of chemical doping with dopant tetracyanoquinodimethane (TCNQ) for the multilayer (ML)MoTe2 field effect transistor (FET) to control its electrical properties. The threshold voltage (Vth) is shifted from negative to positive back gate
voltage which shows p-type doping, furthermore, it was confirmed by Raman spectroscopy, the peak A1g is shifted towards higher wave number
showing p-type doping effect in MoTe2 FET. Moreover, the full width at half maximum (FWHM) is reduced and the intensity ratio of the char­
acteristic peaks (A1g and E12g) is decreased with respect to reaction time. The electrical measurements revealed improved current on/off ratio from
105 to107, mobility from 26.2 cm2/V s to 178.73 cm2/V s and charge carrier density from 3.41 × 1012 cm− 2 to 7.9 × 1012 cm− 2. These results offer
the possibility of employing MoTe2 FETs in electronics.
1. Introduction
Transition metal dichalcogenide (TMDC) materials reveal remarkable properties for electronic applications due to their layered
configuration and semiconducting nature [1–8]. Basically these materials have band gap range of 1–2 eV having considerable on/off
ratio making them appropriate for logical operational devices [6,9,10]. The lack of interlayer covalent bonds and dangling bonds on
the surfaces give these materials significant chemical stability making them useful for CMOS and tunneling field effect transistors
(TFETs) [11,12].
MoS2 have gained attraction, however p-type transport exceeding phenomenon is difficult to attain in MoS2 due to strong pinning
effect at metal-MoS2 junction [13,14]. Furthermore, the large band gap in MoS2 (1.3–1.8 eV) [15] hinders its usage in TFET for
producing efficient tunneling in several cases. On the other side, WSe2 has extensive bandgap but smaller electron affinity which causes
enhancement in Schottky barriers for holes and electrons [16,17], however on making junction with metals, this effect causes a
reduction in the current density and mobility of device. Recently, Molybdenum ditelluride (MoTe2) played a vital role in electronic
industry due to its semi-metallic and metallic character [18,19]. MoTe2 have a direct band gap of 0.9–1.1 eV relying on its thickness
and lattice configuration [20–22]. This band gap is smaller than other several TMDC materials, which makes it substantial for TFETs
* Corresponding author.,
** Corresponding author.
E-mail addresses: waqas.iqbal@riphah.edu.pk (M.W. Iqbal), hussain.gik@gmail.com (G. Hussain).
https://doi.org/10.1016/j.spmi.2020.106698
Received 2 May 2020; Received in revised form 15 August 2020; Accepted 6 September 2020
Available online 14 September 2020
0749-6036/© 2020 Elsevier Ltd. All rights reserved.
Superlattices and Microstructures 147 (2020) 106698
M.W. Iqbal et al.
and optoelectronic devices with working range from visible to near infrared region. MoTe2 FETs show ambipolar transfer charac­
teristics with weak pinning effects in metal-MoTe2 junction as compared to other TMDCs [19,23]. The field effect mobility of p- and
n-type MoTe2 ranges from 0.3 to 20 cm2/V.s for holes and 0.03–30 cm2/V.s for electrons [19,24]. These values are less than Silicon (Si)
mobility (200 cm2/V.s) [25]; however the major drawback of Si is that it is a non-translucent material. Due to absence of dangling
bonds and effective mobility, MoTe2 has potential usage in various devices such chemical sensors [26], solar cells [27] and digital logic
circuits [19]. Ferroelectricity in 2D layered structures can be of importance in electronic devices [28]. The structure MoTe2/Al2O3 has
shown prominent IOFF/ION ratio about 106 with 1 μA currents in resistive random access memory (RRAM) design [29]. Recently, MoTe2
phase transition showed hysteresis loop in Raman spectra and this phenomenon can be controlled or reversed by gate voltage [30].
Thin film of 1 T′ MoTe2 was fabricated by utilizing predeposited molybdenum and electrodeposited tellurium layers-ray diffraction,
AFM, Raman spectroscopy and XPS were performed, which resulted that, MoTe2 shows Tafel slope less than 70 mVdec− 1 in com­
parison with other reported MoTe2 catalysts [31]. In MoTe2 magneto resistance (MR) was suppressed and fragile anti-localization
(WAL) effects were observed. Also, DFT calculations indicated metallic or superconducting nature in intrinsic form while insulating
behavior in 1 T’ phase due to surface oxidation [32].
Therefore, a controllable and effective technique is necessary to modify the carrier concentration and mobility of MoTe2 FETs to
enable their usage in different operational devices.
In this regard, p-type chemical doping is an effective and simple method to modify the electrical characteristics of TMDC materials
[34]. This technique not only modifies the contact resistance but also modulates the channel resistance and enhances the stability of
Fig. 1. (a) Schematic diagram of FET based on MoTe2 Channel, Si/SiO2 was used as substrate and Cr/Au were used as metal contacts. (b) Optical
image of ML-MoTe2 based device. (c) AFM image of sample based on ML-MoTe2 flakes. (d) Thickness profile for MoTe2 sample showing a thickness
of about 3.3 nm.
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M.W. Iqbal et al.
the device [35–37]. We demonstrate the p-type doping in multilayer (ML) MoTe2 by using tetracyanoquinodimethane (TCNQ) as
chemical dopant. TCNQ has been used as a flexible building block for manufacturing organic conductors. Previously, F4-TCNQ was
used as chemical dopant in Black Phosphorus (BP) which played a significant role in enhancing the device mobility, extraordinary
drain current of 532 mA/mm and small value of on-state resistance of 3.2 Ω mm [38]. In this work, it is noticed that after doping TCNQ
in MoTe2, the threshold voltage is shifted from negative to positive gate voltage indicating p-type doping, which is also confirmed via
Fig. 2. Transfer characteristic curves (Ids -Vbg) of ML-MoTe2 FET (a) in pristine form, Ion/Ioff = 105. (b) After doping, Ion/Ioff = 107. (c) Ion/Ioff after
40 min of doping.
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M.W. Iqbal et al.
Raman spectroscopy.
2. Experimental section
We have purchased MoTe2 in bulk form HQ GRAPHENE Company through which MoTe2 flakes were formed by mechanical
exfoliation technique (basic scotch tape method) [39]. A thin layer of MoTe2 was made by adhesive tape from bulk materials and then
transferred to Si/SiO2 substrate. Then, the MoTe2 flakes were kept in acetone solution to remove residues of adhesive tape. The
structural morphology, thickness was investigated with optical microscope and atomic force microscopy (AFM) in tapping mode.
Photolithography and e-beam lithography were utilized to assemble source and drain metal contacts for device fabrication. Cr/Au (of
thicknesses 6 nm/80 nm) were deposited as metal contacts. The schematic illustration and the optical image of the device are pre­
sented in Fig. 1.
3. Results and discussion
Fig. 1(a) presents the schematic diagram and Fig. 1(b) represents real image of the ML- MoTe2 sample on SiO2 substrate with Cr/Au
metal contacts. Atomic force microscopy (AFM) was utilized to measure the thickness of the sample under the ambient conditions as
shown in Fig. 1(c) and (d) demonstrating the thickness of approximately 3.3 nm for the sample.
Electrical characteristics of the device were obtained via Picometer 6485 by two-probe measurements. The transfer curves (Ids-Vbg)
of the pristine ML-MoTe2 at constant source-drain voltage Vds = 0.5 V are illustrated in Fig. 2(a). The lowest value for the off-state
current was calculated to be about 10− 11. This low value could be due to the wide band gap and very thin nature of MoTe2, which
makes it very useful for electronic appliances. Also, the on/off current ratio of the device was calculated as 105 that makes it a potential
candidate for switching applications. The device revealed a threshold voltage of − 22 V attained by linear extrapolation of the curve.
ML-MoTe2 sample was doped with tetracyanoquinodimethane (TCNQ) for different periods of time. The chemical TCNQ in pure form
was dissolved in de-ionized water to form 1 M concentration of solution. The flakes of MoTe2 were dipped in solution for different
periods of time. The Solution played a role to decrease the energy barrier at electrode-MoTe2 interface and tunneling of holes become
dominant through Schottky barrier. The well-known attention in TCNQ as factor of charge-transfer (CT) systems is related to its
capability to occur in multiple, stable oxidation states. The unique quinonoid arrangement is a well π-acceptor that can undergo to one
Fig. 3. Transport measurements recorded at different reaction times of ML-MoTe2 FETs for 1 M concentration of TCNQ dopant solution. (b)
Transport measurements recorded at different reaction times of ML-MoTe2 FETs for 2 M Concentration (c) Ion/Ioff ratio of the device after 15 min of
doping for 2 M Concentration of TCNQ solution. (d) Transfer characteristics of the sample to examine the stability of the device.
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M.W. Iqbal et al.
electron reduction to create the stable radical monoanion, TCNQ˙- forms the doping of TCNQ, TCNQ2− which can act as a π-donor anion.
TCNQ creates energy state near the valence band, the charge transfer between dopant and host material reduces the Fermi level and
results in the modification of electrical and optical properties. TCNQ with lowest unoccupied molecular orbital (LUMO) at 4.5 eV is
prominent dopant for transferring charge by its radical TCNQ anion in FETs which leads to increase the hole mobility. The formation of
ion pair MoTe2-TCNQ improves the work function of electrodes and reduces the barrier. Furthermore, it improves electrode/organic
layer contact, hence reduces the contact resistance. After putting the device for 30 min in the solution, Ion/Ioff ratio was increased by
102 times and reached 107 because of enhancement in on-current as illustrated in Fig. 2(b). The value of Ion/Ioff was remained same as
107 but the source-drain was current enhanced to 115 μA after 40 min of doping as shown in Fig. 2 (c). Fig. 3(a) presents similar
transfer curves of the ML-MoTe2 device obtained at different reaction times. The drain current was enhanced abruptly after doping,
which is due to the decrease in impurities. The highest value of source-drain current of value 115 μA was attained at Vgs = − 100 V after
40 min of reaction time. This increase in Ids of the sample is due to the increase of hole concentration since TCNQ is a p-type dopant, as
verified by the shifting of threshold voltage and will be discussed in next section. These calculations were made at room temperature,
after this, we have made measurements with 2 M concentration of TCNQ. Fig. 3(b) shows the transfer characteristics of MoTe2 based
FET with 2 M concentration of TCNQ. We have attained the source-drain current Ids = 110 μA after 15 min of doping at same back gate
voltage Vgs = − 100 V. The rise in Ids is the result of reduction in Schottky barrier because of the doping concentration effect. The value
of Ion/I off was enhanced to 107 as compared to pristine sample after 15 min of doping when 2 M concentration was used as shown in
Fig. 3(c). After doping, the stability was examined after 7 days, 15 days and 30 days, respectively. Minor changes were observed in the
L
conductance and results are represented in Fig. 3(d). In addition, mobility of the sample was extracted with formula μ = Cg WV
(dIds ) and
ds dVg
dIds
is demonstrated in Fig. 4. Here L is the length, W is width of channel and (dV
) is the slope of transfer curve of FET whose value was
g
extracted through the fitting of linear regime of transfer curve. On the other hand, Cg is called gate capacitance with a value of 105 aF/
μm2. At Vbg = 0 V, the mobility of holes was determined to be 26.2 cm2/V s, while that of electrons is calculated as 10.55 cm2/V.s. It
was noticed that, the hole mobility was enhanced, and electrons mobility was reduced with reaction time at different negative gate
voltages. A maximum mobility of 178.73 cm2/V.s was obtained after dipping the device for 40 min in the dopant solution. To extract
charge carrier density, we used n = Cg (Vbg - Vth)/e, where ‘e’ is the charge on the hole with a value of +1.6 × 10− 19 C and Vth is
referred as threshold voltage of FET. The value of carrier density was observed to increase when the doping time was raised to 20 min.
This trend of carrier density against reaction time is demonstrated in Fig. 5(a) with a maximum value of 7.9 × 1012 cm− 2 at T = 300 K.
The rise in the carrier concentration is due to the p-type doping effect of TCNQ, as evidenced by the shift of threshold voltage in Fig. 5
(b). After 40 min of doping, Vth is shifted from − 22 V to 80 V confirming its p-type behavior.
Raman spectra of pristine and doped samples of ML-MoTe2 were obtained with Renishaw micro-spectrometer with laser wave­
length of 514 nm. Our results are consistent with the previous reports [30,31,40–43]. The laser beam was focused to a region of 1 μm
and the power of laser was maintained at 1 mW to overcome local heating effect and defects by the laser. These Raman curves are
illustrated in Fig. 6(a) showing major peaks at 173.96, 232.06 and 288.03 cm− 1, which corresponds to A1g, E12g and B12g modes,
respectively. After doping, Raman peaks A1g, E12g and B12g were shifted towards higher wave numbers (176.08 cm− 1, 233.13 cm− 1 and
289.43 cm− 1) and this phenomenon leads to blue shift which indicates p-type doping as presented in Fig. 6(b). Fig. 6(c) illustrates the
full width at half maximum (FWHM) of Raman peaks e.g. A1g and E12g with respect to reaction time, which shows that FWHMs of A1g
and E12g get reduced with the doping time. The crystalline quality of MoTe2 doped film was observed by examining the peak line
widths. FWHM of Raman peak is generally used to estimate the crystal quality of material, with narrower peaks analogous to higher
Fig. 4. The trend of hole and electron mobility as a function of doping time. The mobility of holes shows a monotonic increase while that of
electrons it decreases.
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M.W. Iqbal et al.
Fig. 5. (a) Carrier density vs. reaction time after doping of MoTe2 based FET. (b) The shift of threshold voltage as a function of treatment time.
Fig. 6. (a) Raman spectra of pristine and doped ML-MoTe2 samples as a function of reaction time. (b) The trend of peak shift, which moves towards
higher wavenumber as the reaction time is increased. (c) Full width at half maximum as function of reaction time, the value of FWHM reduces with
the passage of reaction time. (d) Intensity ratio of peaks as function of reaction time showing a monotonic decrease.
crystallinity and vice-versa. Any alteration to MoTe2 lattice causes an impact to the Raman peak width. After doping with TCNQ, there
was major effect on A1g peak and peak E12g not affected too much, width of A1g peaks were reduced with doping time, the peaks
became sharp, that’s why FWHM was deceased. The FWHM of thinned layered showed slight change (i.e broadening) as compared to
their pristine form, representing that the structural integrity of the film is fully conserved. Furthermore, the intensity ratio of peaks
reveals a monotonic decrease with respect to reaction time as presented in Fig. 6(d).
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4. Conclusions
Here, we have exfoliated multilayered MoTe2 sample by standard mechanical exfoliation technique and stated p-type chemical
doping via tetracyanoquinodimethane (TCNQ). The thickness of the multilayered sample was confirmed by utilizing atomic force
microscopy (AFM). The sample was doped with TCNQ dopant for different values of reaction time and the time dependent electrical
transport measurements were carried out by using two probe technique. P-type doping was confirmed by the shift of threshold voltage
from negative to positive gate voltage. Also, the sample showed enhanced current on/off ratio from 105 to 107, mobility from 26.2
cm2/V s to 178.73 cm2/V s and charge carrier density from 3.41 × 1012 cm− 2 to 7.9 × 1012 cm− 2 after using TCNQ as a dopant. Raman
spectra confirmed p-type doping, as A1g and E12g peaks were shifted towards higher wave numbers. Moreover, the FWHMs of A1g and
E12g were reduced with respect to the doping time. This technique is more stable to utilize for enhancing the performance of MoTe2
FETs.
Authors contribution
M.W. Iqbal designed the experiment. E. Elahi, A. Amin and M.W. Iqbal performed experiments. S. Aftab, M. A. Shehzad fabricated
the devices. I. Aslam did AFM characterization and analysis. E. Elahi, G. Hussain and M.W. Iqbal analyzed the data and lead the
manuscript write up.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to
influence the work reported in this paper.
Acknowledgements
This work is funded by Higher Education Commission (HEC) of Pakistan under the National Research Program for Universities
(NRPU) with project no. HEC/R&D/NRPU/2017/7876.
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