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
Current Applied Physics 19 (2019) 1127–1131 Contents lists available at ScienceDirect Current Applied Physics journal homepage: www.elsevier.com/locate/cap Large-scale chemical vapor deposition growth of highly crystalline MoS2 thin films on various substrates and their optoelectronic properties T Van Tu Nguyena,b, Seongju Haa, Dong-Il Yeoma, Yeong Hwan Ahna, Soonil Leea, Ji-Yong Parka,* a b Department of Physics and Department of Energy Systems Research, Ajou University, Suwon, 16499, South Korea Institute of Materials Science, Vietnam Academy of Science and Technology, Hanoi, 100000, Vietnam ARTICLE INFO ABSTRACT Keywords: CVD MoS2 Monolayer FET Photoresponse Large-scale growth of mostly monolayer molybdenum disulfide (MoS2) on quartz, sapphire, SiO2/Si, 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 as-grown MoS2 exhibit carrier mobility of 1–2 cm2V−1s−1 and On/Off ratio of ~104 while showing large photoresponse Our results provide a simple approach to realize MoS2 on various substrates for electronics and optoelectronics applications Introduction Two dimensional-transition metal dichalcogenides (2D-TMDCs), in particular, MoS2 has emerged as a potential material for various applications 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 MoS2 film 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 deposition [4,5], hydrothermal synthesis [6], and chemical vapor deposition (CVD) [7–16] Among these, CVD is the most promising and widely adopted method as large area, high-quality MoS2 films 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], MoS2 powder [14], ammonium heptamolybdate ((NH4)6Mo7O24) [15], and Mo(CO)6 gas [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 MoS2 is sensitive to the growth parameters such as temperature, reaction time, pressure, and gas flow rate Additionally, specific applications may require preparation of MoS2 films on different substrates by direct growth on them rather than transfer from different growth substrates The optimization and the reproducibility of thin MoS2 growth 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 MoS2 on various substrates such as quartz, sapphire, SiO2/Si, and waveguide The morphology, the number of layers and the quality of as-grown MoS2 are investigated by optical microscopy, atomic force microscopy (AFM), and Raman spectroscopy We also investigate electrical and optoelectronic properties of devices based on as-grown monolayer MoS2 The On/Off ratio of ~104 as well as the mobility of 1–2 cm2V−1s−1 is acquired in back-gated MoS2 field effect transistors (FETs) while large photoresponses are observed under visible light illuminations 1.1 Experimental details The growth of MoS2: MoS2 flakes and films are grown on quartz (fused silica, diameter of 25 mm, thickness of 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 × μm2) substrates by atmospheric 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 Corresponding author E-mail address: jiyong@ajou.ac.kr (J.-Y Park) 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 Available online 16 July 2019 1567-1739/ © 2019 Korean Physical Society Published by Elsevier B.V All rights reserved Current Applied Physics 19 (2019) 1127–1131 V.T Nguyen, et al Fig (a) A schematic of a CVD setup for MoS2 synthesis (b) A CVD process for MoS2 growth (promoter substrate) and the target growth substrate are placed side by side facing down toward MoO3 power (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 upstream part of the furnace, where temperature can be controlled independently 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 substrates are washed by sonication in deionized (DI) water and dried with N2 gas before loading into the CVD setup The growth process is shown in Fig 1(b) Before heating, the whole CVD system is purged with 300 sccm of high purity Ar gas (99.999%) for 30 to remove the contaminant 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 furnace 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 MoS2 channel 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 crystallinity and the number of MoS2 layers are investigated by Raman spectroscopy with an excitation wavelength of 532 nm A parameter analyzer (4200-SCS from Keithley) is employed to characterize the electrical and optoelectronic properties of the MoS2 FET devices Fig Optical microscope images of CVD-grown MoS2 on a (a) quartz, (b) sapphire, (c) SiO2/Si, and (d) waveguide substrates The insets in (a) and (b) show growth of cm-scale MoS2 on a quartz and sapphire substrate, respectively substrates They tend to merge together to form a continuous film with high coverage The optical images of such regions of almost film-like MoS2 on the same substrates are presented in Fig S1 On the waveguide substrate, MoS2 seems 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 in Fig S2 The insets in Fig show that MoS2 on quartz and sapphire substrates grows almost in the centimeter scale AFM measurements in Fig more clearly show the shapes and thickness of MoS2 flakes on each substrate The measured heights of MoS2 flakes from these AFM images are in the range of 0.9–1.1 nm The variations can be Results and discussions Fig shows optical microscope images of MoS2 grown on a quartz, a sapphire, a SiO2/Si and a waveguide substrate, respectively The triangular-shaped MoS2 flakes with uniform color are observed on all the 1128 Current Applied Physics 19 (2019) 1127–1131 V.T Nguyen, et al Fig AFM topographical images of MoS2 grown 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 attributed to the different roughness of the substrates and interaction between MoS2 flakes and the substrates Although these thickness values 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 in Fig clearly display two distinct peaks corresponding to E2g, and A1g, which are due to the inplane vibration of Mo and S atoms and out of 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 in Fig 4(a)~(d), Δk of MoS2 on the all substrates are smaller than 21 cm−1, which indicate they originate from monolayer Fig Raman spectra for MoS2 grown on a (a) quartz, (b) sapphire, (c) SiO2/Si, and (d) waveguide substrates (e) A PL image of MoS2 flakes on a SiO2/Si substrate 1129 Current Applied Physics 19 (2019) 1127–1131 V.T Nguyen, et al In order to investigate the electronic transport property of MoS2, we fabricated back-gated MoS2 FET devices using MoS2 grown 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 Vg ranging from 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 = (dIds / dVbg ) ì [L/(WCox Vds )], where L is the channel length (15 μm), W is the channel width (10 μm), Cox = 11.5 nFcm−2 is the capacitance between the channel and the backgate per unit area, dIds / dVbg is 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 MoS2 measured at room temperature in the ambient conditions [20,21] We also investigated the optoelectronic properties of MoS2 grown on various substrates We fabricated two terminal devices on each growth substrate as shown in Fig 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 MoS2 flakes grown on different substrates, larger than that among devices 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 in Fig 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 in Fig These slow photoresponses 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 MoS2 channel [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 photoexcitedholes migrate to the surface of MoS2 and 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 SiO2 or 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 between desorption and re-adsorption of molecules is reached and the trap states are all filled with holes When the light is turned off, electron-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 (a) Transfer characteristics of a FET based on as-grown MoS2 on SiO2/Si substrate in linear and log scale (VDS = 0.25 V) (b) IDS-VDS output characteristics of the FET at various back-gate voltages Fig (a) Optical image of the device IDS-VDS curves and the photo response current of the device on (b) quartz, (c) sapphire, (d) SiO2/Si substrate MoS2 [7,19] These values are consistent with their height profiles in AFM images of Fig Furthermore, the PL image taken from the SiO2/ Si substrate in Fig 4(e) shows many triangular MoS2 flakes with strong PL signals All these results seem to confirm that these are monolayer MoS2 Representative PL spectra taken from MoS2 flakes on different substrates in Fig S3 shows PL peaks in a range of 670–680 nm, which is also consistent with the monolayer MoS2 Even though mostly monolayer MoS2 flakes are obtained in this work, there are variations in the 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MoS2, Nanoscale (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 MoS2, 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 In2Se3 phototransistors, Nano Lett 15 (2015) 7853–7858 We demonstrate the growth of monolayer MoS2 on 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 Asgrown 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 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(mostly, oxygen and water) on the surface of MoS2 as negative ions by capturing free electrons of MoS2 channel [24–26] Under light illumination with photon energy higher than the bandgap of MoS2, current... MoS2 measured at room temperature in the ambient conditions [20,21] We also investigated the optoelectronic properties of MoS2 grown on various substrates We fabricated two terminal devices on