Largescale growth of mostly monolayer molybdenum disulfide (MoS2) on quartz, sapphire, SiO2Si, and waveguide substrates is demonstrated by chemical vapor deposition with the same growth parameters. Centimeterscale areas with large flakes and films of MoS2 on all the growth substrates are observed. The atomic force microscopy and Raman measurements indicate the synthesized MoS2 is monolayer with high quality and uniformity. The MoS2 field effect transistors based on the asgrown MoS2 exhibit carrier mobility of 1–2 cm2V−1s−1 and OnOff ratio of ~104 while showing large photoresponse. Our results provide a simple approach to realize MoS2 on various substrates for electronics and optoelectronics applications
Trang 1Contents lists available atScienceDirect Current Applied Physics journal homepage:www.elsevier.com/locate/cap
thin films on various substrates and their optoelectronic properties
Van Tu Nguyena,b, Seongju Haa, Dong-Il Yeoma, Yeong Hwan Ahna, Soonil Leea, Ji-Yong Parka,*
aDepartment of Physics and Department of Energy Systems Research, Ajou University, Suwon, 16499, South Korea
bInstitute of Materials Science, Vietnam Academy of Science and Technology, Hanoi, 100000, Vietnam
A R T I C L E I N F O
Keywords:
CVD
MoS2
Monolayer
FET
Photoresponse
A B S T R A C T Large-scale growth of mostly monolayer molybdenum disulfide (MoS2) on quartz, sapphire, SiO2/Si, and wa-veguide substrates is demonstrated by chemical vapor deposition with the same growth parameters Centimeter-scale areas with large flakes and films of MoS2on all the growth substrates are observed The atomic force microscopy and Raman measurements indicate the synthesized MoS2is monolayer with high quality and uni-formity The MoS2field effect transistors based on the as-grown MoS2exhibit carrier mobility of 1–2 cm2V−1s−1 and On/Off ratio of ~104while showing large photoresponse Our results provide a simple approach to realize MoS2on various substrates for electronics and optoelectronics applications
1 Introduction
Two dimensional-transition metal dichalcogenides (2D-TMDCs), in
particular, MoS2has emerged as a potential material for various
ap-plications in electronics and optoelectronics due to its outstanding and
unique electrical and optical properties such as large and tunable band
gap, valley-dependent transport, and large exciton binding energies
[1,2] For practical applications, however, a low-cost synthesis method
for large area, high-quality MoS2film is highly desirable Up to now,
several methods have been developed for the growth of large-area MoS2
monolayers, such as physical vapor deposition [3], atomic layer
de-position [4,5], hydrothermal synthesis [6], and chemical vapor
de-position (CVD) [7–16] Among these, CVD is the most promising and
widely adopted method as large area, high-quality MoS2films can be
obtained and is suitable for scale-up of the production For the CVD
growth, many molybdenum (Mo)-containing source materials such as
Mo film [7], MoO3[8–12], MoCl5[13], MoS2powder [14], ammonium
heptamolybdate ((NH4)6Mo7O24) [15], and Mo(CO)6gas [16] can be
used Among these sources, MoO3 and S powder are widely used to
obtain large area, uniform, and highly crystalline MoS2 thin films
[8–12] On the other hand, the growth of monolayer MoS2is sensitive
to the growth parameters such as temperature, reaction time, pressure,
and gas flow rate Additionally, specific applications may require
pre-paration of MoS2films on different substrates by direct growth on them
rather than transfer from different growth substrates The optimization
and the reproducibility of thin MoS2growth on different substrates are
still a great challenge
In this work, we present a combination of suitable sets of growth parameters and a seeding promoter to demonstrate the direct growth of large-area, highly crystalline mostly mono-layer MoS2on various sub-strates such as quartz, sapphire, SiO2/Si, and waveguide The mor-phology, the number of layers and the quality of as-grown MoS2are investigated by optical microscopy, atomic force microscopy (AFM), and Raman spectroscopy We also investigate electrical and optoelec-tronic properties of devices based on as-grown monolayer MoS2 The On/Off ratio of ~104as well as the mobility of 1–2 cm2V−1s−1is ac-quired in back-gated MoS2 field effect transistors (FETs) while large photoresponses are observed under visible light illuminations
1.1 Experimental details
The growth of MoS 2: MoS2flakes and films are grown on quartz (fused silica, diameter of 25 mm, thickness of 1 mm), sapphire (c-plane
⟨0001⟩), 300 nm-thick SiO2/Si, and waveguide (A fused silica substrate with Ge-doped waveguides for planar lightwave circuit cladding and core with the cross-sectional area of 6 × 6 μm2) substrates by atmo-spheric pressure CVD with molybdenum trioxide (MoO3) (99.95%, Sigma-Aldrich) and sulfur (S) (99.98%, Sigma-Aldrich) powders as precursors, and perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS) as a seeding promoter The growth setup is schematically shown in Fig 1(a), which is similar to the previously reported one [17,18] One substrate (Si substrate) with drop-casted PTAS promoter
https://doi.org/10.1016/j.cap.2019.07.007
Received 17 May 2019; Received in revised form 15 July 2019; Accepted 16 July 2019
*Corresponding author
E-mail address:jiyong@ajou.ac.kr(J.-Y Park)
Available online 16 July 2019
1567-1739/ © 2019 Korean Physical Society Published by Elsevier B.V All rights reserved
T
Trang 2(promoter substrate) and the target growth substrate are placed side by
side facing down toward MoO3power (5 mg) on a ceramic boat, which
is loaded into the middle of the quartz tube A second ceramic boat
containing 500 mg of sulfur powder is kept at the outside and the
up-stream part of the furnace, where temperature can be controlled
in-dependently by a heating tape Prior to the growth, all the substrates
are cleaned in a piranha solution for 24 h to get rid of the organic
contaminants and obtain a hydrophilic surface Afterward, the
sub-strates are washed by sonication in deionized (DI) water and dried with
N2gas before loading into the CVD setup The growth process is shown
inFig 1(b) Before heating, the whole CVD system is purged with 300
sccm of high purity Ar gas (99.999%) for 30 min to remove the
con-taminant Then, the furnace and the ceramic boat with sulfur powder
are heated to 650 °C and 200 °C with a ramping rate of 32 °C/min and
10 °C/min, respectively After holding the system for 10 min, the
fur-nace is turned off and cooled down to room temperature
Device fabrication: Photolithography is used to define electrode
patterns for FET devices on the substrates with as grown MoS2 Then,
titanium/gold (thickness: 3nm/47 nm) are deposited using e-beam
evaporation, followed by lift-off process in N-Methyl-2-pyrrolidone
(NMP) for one day to complete electrodes When necessary, another
photolithography and RIE dry etching process are employed to define
the MoS2channel
Characterizations: An optical microscope can be used for the quick
investigation of growth results such as shapes and coverages of MoS2
An AFM system (XE-100 from Park Systems) is used to characterize the
morphology and estimate the number of layers of MoS2 The
crystal-linity and the number of MoS2layers are investigated by Raman
spec-troscopy with an excitation wavelength of 532 nm A parameter
ana-lyzer (4200-SCS from Keithley) is employed to characterize the
electrical and optoelectronic properties of the MoS2FET devices
2 Results and discussions
Fig 2shows optical microscope images of MoS2grown on a quartz,
a sapphire, a SiO2/Si and a waveguide substrate, respectively The
tri-angular-shaped MoS2flakes with uniform color are observed on all the
substrates They tend to merge together to form a continuous film with high coverage The optical images of such regions of almost film-like MoS2on the same substrates are presented inFig S1 On the waveguide substrate, MoS2seems to grow over the core area of the waveguide, which is doped with Ge, the same way as on the rest of the substrate as shown inFig S2 The insets inFig 1show that MoS2on quartz and sapphire substrates grows almost in the centimeter scale AFM mea-surements inFig 3more clearly show the shapes and thickness of MoS2 flakes on each substrate The measured heights of MoS2flakes from these AFM images are in the range of 0.9–1.1 nm The variations can be
Fig 1 (a) A schematic of a CVD setup for MoS2synthesis (b) A CVD process for MoS2growth
Fig 2 Optical microscope images of CVD-grown MoS2on a (a) quartz, (b) sapphire, (c) SiO2/Si, and (d) waveguide substrates The insets in (a) and (b) show growth of cm-scale MoS2on a quartz and sapphire substrate, respectively
Trang 3attributed to the different roughness of the substrates and interaction
between MoS2flakes and the substrates Although these thickness
va-lues are larger than the expected thickness of monolayer MoS2
(0.65 nm), they seem to be all monolayer MoS2 as following Raman
measurements indicate [12]
Raman spectroscopy is a common tool to identify the number of
MoS2 layers and its crystallinity Raman spectra taken from MoS2
grown on these four kinds of substrates inFig 4clearly display two distinct peaks corresponding to E2g,and A1g, which are due to the in-plane vibration of Mo and S atoms and out of in-plane vibration of S atoms, respectively Especially, the spacings between these two peak
positions (Δk) can be used to roughly estimate the layer number of
MoS2 As depicted inFig 4(a)~(d), Δk of MoS2on the all substrates are smaller than 21 cm−1, which indicate they originate from monolayer
Fig 3 AFM topographical images of MoS2grown on a (a) quartz, (b) sapphire, (c) SiO2/Si, and (d) waveguide substrates Insets show cross-sectional height profiles along a white line in each figure
Fig 4 Raman spectra for MoS2grown on a (a) quartz, (b) sapphire, (c) SiO2/Si, and (d) waveguide substrates (e) A PL image of MoS2flakes on a SiO2/Si substrate
Trang 4MoS2[7,19] These values are consistent with their height profiles in
AFM images ofFig 3 Furthermore, the PL image taken from the SiO2/
Si substrate inFig 4(e) shows many triangular MoS2flakes with strong
PL signals All these results seem to confirm that these are monolayer
MoS2 Representative PL spectra taken from MoS2flakes on different
substrates inFig S3shows PL peaks in a range of 670–680 nm, which is
also consistent with the monolayer MoS2 Even though mostly
mono-layer MoS2flakes are obtained in this work, there are variations in the
density and size of the flakes even in the same substrate, which is
si-milar to the previous CVD growths of MoS2with MoO3and S powder as
precursors [8–11] Therefore, it is difficult to directly compare the
ef-fect of the different substrates on the nucleation density and the crystal
size of MoS2 In this work, rather, we focused on finding the optimized
growth parameters, which result in the synthesis of large-area, high
quality MoS2on various substrates
In order to investigate the electronic transport property of MoS2, we fabricated back-gated MoS2FET devices using MoS2grown on the SiO2/
Si substrate.Fig 5(a) shows the transfer characteristic (Ids vs Vg), in the
linear and logarithmic scale The device shows n-type behavior with
threshold voltage VT= −10 V and an On/Off ratio of 104 Output
characteristics (Ids vs Vds) measured at gate voltages Vgranging from
0 V to 50 V, exhibit strong gate-controlled features as shown in Fig 5(b) Additionally, the carrier mobility of the device is calculated
from the following formula = µ (dI dV ds/ bg)×[ /(L WC V ox ds)], where L is the channel length (15 μm), W is the channel width (10 μm), C ox
= 11.5 nFcm−2is the capacitance between the channel and the
back-gate per unit area, dI dV ds/ bgis the slope of transfer curve in the linear region The calculated carrier mobility is ~1.2 cm2V−1s−1, which is similar to the reported value from similar FET devices from CVD-grown MoS2measured at room temperature in the ambient conditions [20,21]
We also investigated the optoelectronic properties of MoS2grown
on various substrates We fabricated two terminal devices on each growth substrate as shown inFig 6(a) We found that all the devices display large photoresponses under white light illumination as shown in Fig 6(b)~(d) Any significant differences in the photoresponse from MoS2flakes grown on different substrates, larger than that among de-vices on the same substrates could not be found We also measured the
temporal response (Ids vs t) of the device on a quartz, a sapphire and a
SiO2/Si substrate as shown inFig 7 The temporal response of the devices typically shows rise times of few 10 s while decay times extend over few hundreds of seconds as shown inFig 7 These slow photo-responses were also reported previously on MoS2-based photodetectors [22,23] There are both fast and slow components in the time response The positive photoresponse can be explained as follows: In the dark, there are adsorbates (mostly, oxygen and water) on the surface of MoS2
as negative ions by capturing free electrons of MoS2channel [24–26] Under light illumination with photon energy higher than the bandgap
of MoS2, current increases immediately due to the photogeneration of electron-hole pairs (photoconductive effect) At this stage, the current dramatically increase (fast component) After that, some photoexcited-holes migrate to the surface of MoS2and recombine with negatively ionized adsorbates molecules, releasing them from the surface The unpaired photoexcited-electrons contribute to the current unless they are trapped again by re-adsorbed molecules on the surface after the light is turned off At the same time, some holes can also be trapped at the interface of MoS2 and SiO2or defects in MoS2 structure While trapped, these holes affect the channel conductance by effective gating (photogating effect) [27] Both processes result in the current increase
as more electrons will be available in the channel However, these are slow processes and the current will saturate when the equilibrium be-tween desorption and re-adsorption of molecules is reached and the trap states are all filled with holes When the light is turned off, elec-tron-hole recombination results in the fast decrease of the current, followed by a slow one due to the gradual adsorption of molecules and the discharging of trap states In the future work, we will try to improve photoresponse time by passivating defects on both MoS2 and SiO2 substrates, and controlling molecule adsorption on MoS2
Fig 5 (a) Transfer characteristics of a FET based on as-grown MoS2on SiO2/Si
substrate in linear and log scale (VDS= 0.25 V) (b) IDS-VDSoutput
character-istics of the FET at various back-gate voltages
Fig 6 (a) Optical image of the device IDS-VDScurves and the photo response
current of the device on (b) quartz, (c) sapphire, (d) SiO2/Si substrate
Fig 7 Time-dependent photocurrent response (I−t) of the device on (a) quartz, (b) sapphire and (c) SiO2/Si substrate, respectively VDS= 1 V for all measurements
Trang 53 Conclusions
We demonstrate the growth of monolayer MoS2on quartz, sapphire,
SiO2/Si, and waveguide substrates by CVD with the same growth
parameters The as-synthesized MoS2 samples, including monolayer
flakes and films, reveal high crystalline quality and uniformity as
confirmed by optical microscopy, AFM and Raman measurements
As-grown MoS2 on four substrates can be a good candidate for future
electronic and optoelectronic applications
Acknowledgement
This work was supported by “Human Resources Program in Energy
Technology” of the Korea Institute of Energy Technology Evaluation
and Planning (KETEP), granted financial resource from the Ministry of
Trade, Industry & Energy (No 20164030201380) and by the National
Research Foundation of Korea (NRF) grant funded by the Korea
gov-ernment (MSIT) (NRF-2018R1D1A1B07041804 &
NRF-2019R1A2C1007913)
Appendix A Supplementary data
Supplementary data to this article can be found online athttps://
doi.org/10.1016/j.cap.2019.07.007
References
[1] A Splendiani, L Sun, Y Zhang, T Li, J Kim, C.-Y Chim, G Galli, F Wang,
Emerging photoluminescence in monolayer MoS 2 , Nano Lett 10 (2010)
1271–1275
[2] Y Park, N Li, C.C.S Chan, B.P.L Reid, R.A Taylor, H Im, Optical polarization in
mono and bilayer MoS 2 , Curr Appl Phys 17 (2017) 1153–1157
[3] S Helveg, J.V Lauritsen, E Lægsgaard, I Stensgaard, J.K Nørskov, B Clausen,
H Topsøe, F Besenbacher, Atomic-scale structure of single-layer MoS 2
na-noclusters, Phys Rev Lett 84 (2000) 951–954
[4] A Valdivia, D.J Tweet, J.F Conley Jr., Atomic layer deposition of two dimensional
MoS 2 on 150 mm substrates, J Vac Sci Technol., A 34 (2016) 021515
[5] Y Huang, L Liu, W Zhao, Y Chen, Preparation and characterization of
mo-lybdenum disulfide films obtained by one-step atomic layer deposition method,
Thin Solid Films 624 (2017) 101–105
[6] S Muralikrishna, K Manjunath, D Samrat, V Reddy, T Ramakrishnappa,
D.H Nagaraju, Hydrothermal synthesis of 2D MoS 2 nanosheets for electrocatalytic
hydrogen evolution reaction, RSC Adv 5 (2015) 89389–89396
[7] Y Zhan, Z Liu, S Najmaei, P.M Ajayan, J Lou, Large‐area vapor‐phase growth and
characterization of MoS 2 atomic layers on a SiO 2 substrate, Small 8 (2012)
966–971
[8] S Wang, Y Rong, Y Fan, M Pacios, H Bhaskaran, K He, J.H Warner, Shape
evolution of monolayer MoS 2 crystals grown by chemical vapor deposition, Chem.
Mater 26 (2014) 6371–6379
[9] S Najmaei, Z Liu, W Zhou, X Zou, G Shi, S Lei, B.I Yakobson, J.-C Idrobo, P.M Ajayan, J Lou, Vapour phase growth and grain boundary structure of mo-lybdenum disulphide atomic layers, Nat Mater 12 (2013) 754–759 [10] D Dumcenco, D Ovchinnikov, K Marinov, P Lazic, M Gibertini, N Marzari, O.L Sanchez, Y.-C Kung, D Krasnozhon, M.-W Chen, Large-area epitaxial mono-layer MoS 2 , ACS Nano 9 (2015) 4611–4620
[11] A.M Van Der Zande, P.Y Huang, D.A Chenet, T.C Berkelbach, Y You, G.-H Lee, T.F Heinz, D.R Reichman, D.A Muller, J.C Hone, Grains and grain boundaries in highly crystalline monolayer molybdenum disulphide, Nat Mater 12 (2013) 554–561
[12] J Chen, W Tang, B Tian, B Liu, X Zhao, Y Liu, T Ren, W Liu, D Geng, H.Y Jeong, Chemical vapor deposition of high‐quality large‐sized MoS 2 crystals on silicon dioxide substrates, Adv Sci 3 (2016) 1500033
[13] S Ganorkar, J Kim, Y.-H Kim, S.-I Kim, Effect of precursor on growth and mor-phology of MoS 2 monolayer and multilayer, J Phys Chem Solids 87 (2015) 32–37 [14] S Wu, C Huang, G Aivazian, J.S Ross, D.H Cobden, X Xu, Vapor–solid growth of high optical quality MoS 2 monolayers with near-unity valley polarization, ACS Nano 7 (2013) 2768–2772
[15] L Tao, K Chen, Z Chen, W Chen, X Gui, H Chen, X Li, J.-B Xu, Centimeter-scale CVD growth of highly crystalline single-layer MoS 2 film with spatial homogeneity and the visualization of grain boundaries, ACS Appl Mater Interfaces 9 (2017) 12073–12081
[16] K Kang, S Xie, L Huang, Y Han, P.Y Huang, K.F Mak, C.-J Kim, D Muller,
J Park, High-mobility three-atom-thick semiconducting films with wafer-scale homogeneity, Nature 520 (2015) 656–660
[17] Y.C Kim, V.T Nguyen, S Lee, J.-Y Park, Y.H Ahn, Evaluation of transport para-meters in MoS 2 /graphene junction devices fabricated by chemical vapor deposition, ACS Appl Mater Interfaces 10 (2018) 5771–5778
[18] V.T Nguyen, W Yim, S.J Park, B.H Son, Y.C Kim, T.T Cao, Y Sim, Y.-J Moon, V.C Nguyen, M.-J Seong, S.-K Kim, Y.H Ahn, S Lee, J.-Y Park, Phototransistors with negative or ambipolar photoresponse based on as-grown heterostructures of single-walled carbon nanotube and MoS 2 , Adv Funct Mater 28 (2018) 1802572 [19] X Ling, Y.-H Lee, Y Lin, W Fang, L Yu, M.S Dresselhaus, J Kong, Role of the seeding promoter in MoS 2 growth by chemical vapor deposition, Nano Lett 14 (2014) 464–472
[20] Y.-H Lee, L Yu, H Wang, W Fang, X Ling, Y Shi, C.-T Lin, J.-K Huang,
M.-T Chang, C.-S Chang, Synthesis and transfer of single-layer transition metal dis-ulfides on diverse surfaces, Nano Lett 13 (2013) 1852–1857
[21] Y.-C Lin, W Zhang, J.-K Huang, K.-K Liu, Y.-H Lee, C.-T Liang, C.-W Chu,
L.-J Li, Wafer-scale MoS 2 thin layers prepared by MoO 3 sulfurization, Nanoscale 4 (2012) 6637–6641
[22] P Han, E.R Adler, Y Liu, L.S Marie, A.E Fatimy, S Melis, E Van Keuren,
P Barbara, Ambient Effects on Photogating in MoS 2 Photodetectors, Nanotechnology 30 (2019) 284004
[23] O Lopez-Sanchez, D Lembke, M Kayci, A Radenovic, A Kis, Ultrasensitive pho-todetectors based on monolayer MoS 2 , Nat Nanotechnol 8 (2013) 497–501 [24] W Zhang, J.K Huang, C.H Chen, Y.H Chang, Y.J Cheng, L.J Li, High‐gain pho-totransistors based on a CVD MoS 2 monolayer, Adv Mater 25 (2013) 3456–3461 [25] Y Lee, J Yang, D Lee, Y.-H Kim, J.-H Park, H Kim, J.H Cho, Trap-induced photoresponse of solution-synthesized MoS 2 , Nanoscale 8 (2016) 9193–9200 [26] J Cha, K.-A Min, D Sung, S Hong, Ab initio study of adsorption behaviors of molecular adsorbates on the surface and at the edge of MoS 2 , Curr Appl Phys 18 (2018) 1013–1019
[27] J.O Island, S.I Blanter, M Buscema, H.S van der Zant, A Castellanos-Gomez, Gate controlled photocurrent generation mechanisms in high-gain In 2 Se 3 photo-transistors, Nano Lett 15 (2015) 7853–7858