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Wafer-scale Highly Oriented Monolayer MoS2 with Large Domain Sizes
Qinqin Wang, Na Li, Jian Tang, Jianqi Zhu, Qinghua Zhang, Qi Jia, Ying Lu, Zheng Wei, Hua Yu,
Yanchong Zhao, Yutuo Guo, Lin Gu, Gang Sun, Wei Yang, Rong Yang, Dongxia Shi, and Guangyu Zhang
Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.0c02531 • Publication Date (Web): 24 Aug 2020
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Wafer-scale Highly Oriented Monolayer MoS2 with Large Domain
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Sizes
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9 Qinqin Wang1,2, Na Li1,2, Jian Tang1,2, Jianqi Zhu1,2, Qinghua Zhang1,2, Qi Jia1,2,
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11 Ying Lu 1,2, Zheng Wei1,2, Hua Yu1,2, Yanchong Zhao1,2, Yutuo Guo1,2, Lin Gu1,2,
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Gang Sun1,2, Wei Yang1,2,3, Rong Yang1,3,4, Dongxia Shi1,2,3, and Guangyu
14 Zhang1,2,3,4*
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18 1 Beijing National Laboratory for Condensed Matter Physics; Key Laboratory for
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20 Nanoscale Physics and Devices, Institute of Physics, Chinese Academy of Sciences,
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Beijing 100190, China
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23 2 School of Physical Sciences, University of Chinese Academy of Sciences, Beijing,
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25 100190, China
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27 Beijing Key Laboratory for Nanomaterials and Nanodevices, Beijing 100190,
28 China
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30 4 Songshan Lake Materials Laboratory, Dongguan, Guangdong, 523808, China
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*Corresponding author. Email: gyzhang@iphy.ac.cn
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37 Abstract
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40 Two-dimensional molybdenum disulfide (MoS2) is an emergent
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42 semiconductor with great potentials in next-generation scaled-up electronics, but
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44 the production of high-quality monolayer MoS2 wafers still remains a challenge.
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Here, we report an epitaxy route towards 4-inch monolayer MoS2 wafers with
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49 highly oriented and large domains on sapphire. Benefiting from a multi-source
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51 design for our chemical vapor deposition set up and the optimization of growth
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53 process, we successfully realized material uniformity across the entire 4-inch
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56 wafer and over hundred microns of domain size in average. These monolayers
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58 exhibit the best electronic quality ever reported, as evidenced from our
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60 spectroscopic and transport characterizations. Our work moves a step closer to

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3 the practical applications of monolayer MoS2.
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6 Keywords: wafer-scale; monolayer molybdenum disulfide; oriented; large domain
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8 sizes; high-quality
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10 2H-phase monolayer molybdenum disulfide (ML-MoS2) is a two-dimensional
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semiconductor with great potentials for electronics and optoelectronics.1-5 The
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15 controllable growth of wafer-scale uniform and high quality continuous monolayer
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17 MoS2 is highly demanded to realize its full potential for these applications.6-9 To date,
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to obtain wafer-scale high quality monolayer MoS2 (ML-MoS2) film, various
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22 approaches have been developed, including metal-organic chemical vapor deposition
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24 (MOCVD),6 atomic layer deposition (ALD),10 chemical vapor deposition (CVD),11 etc.
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26 Among these methods, CVD show great potential in growing MoS2 with high quality.
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28 12,13
29 The uniformity of the wafer-scale films is important when use in large scale
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31 integrated circuit. However, the spatial inhomogeneity over a large-scale due to the
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33 nonuniform distribution of the source concentration in the vapor phase is the main
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reason for achieving large-scale uniform continuous film, which hinders their practical
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38 applications.14,15
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40 On the other hand, domain boundaries in MoS2 monolayers could degrade their
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electronic quality due to scattering effect.16-19 To achieve better materials, one should
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45 align and enlarge domains in ML-MoS2 films as most as possible. Recently, significant
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47 progresses have been made along this direction by choosing appropriate substrates and
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49 optimizing the growth process.11,16,20,21 By employing a high-temperature growth on
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52 sapphire substrates, we were able to achieve 2-inch wafer-scale bicrystalline monolayer
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54 ML-MoS2 films with ~1μm domain sizes.11 Centimeter-scale monolayer ML-MoS2
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56 films with 98% domains aligned in the same direction on gold substrates. 21 For the
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domain size control, the domain size can be also enlarged up to tens of even a hundred
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3 microns in ML-MoS2 films with random domain orientations.12,13,22 However, high
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6 quality ML-MoS2 films with domain sizes exceeding a hundred microns are still
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8 unavailable.
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10 In this work, we report the success on the epitaxy of highly oriented and large-
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domain ML-MoS2 films at 4-inch wafer-scale via a facile multi-source CVD growth
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15 method. Almost only 0° and 60° orientated domains are present in films and the domain
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17 size in average is over 180 μm at the largest. Although 4|4E type of domain boundaries
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exist but have no obvious impacts on their overall electronic properties at the room
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22 temperature. Our imaging, spectroscopy and transport measurements on these ML-
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24 MoS2 films suggest their best quality ever obtained, as evidenced from their wafer-scale
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26 homogeneity, nearly perfect lattice structure and intrinsically high electronic quality (in
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29 terms of their filed effect on/off ratios, mobilities, and threshold voltages).
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31 The epitaxial growth was performed in a specially designed 4-inch CVD system
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33 (Figure 1a & Fig. S1). In this setup, seven mini quartz tubes in the growth chamber
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serve as pockets for reaction sources and carrying gasses through them are
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38 independently delivered. As seen in inset Fig. 1a, MoO3 sources are evenly loaded
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40 within the outside six mini tubes and S source is loaded within the center mini tube.
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The carrying gasses for S and MoO3 sources are Ar and Ar/O2, respectively. This multi-
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45 sources design enables the homogenous cross-sectional source supply, which is a key
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47 to uniform growth at the 4-inch wafer scale. During the growth, temperatures of the S
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49 source, MoO3 sources and sapphire wafers are 120, 540 and 930 °C, respectively. Note
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52 that 4-inch sapphire wafers were vertically placed in the growth chamber to avoid the
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54 source concentration inhomogeneousity along the horizontal direction. Benefiting from
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56 this multi-sources design, we can reliably grow uniform MoS2 monolayer epitaxially
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over the entire 4-inch wafer.
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3 During the growth, Ar/O2 carrying gasses act as protecting the MoO3 sources
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6 from sulfurization thus ensures a steady evaporation. Besides, it can also balance the
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8 domain nucleation, growth and etching.23 Thus, domain sizes could be tuned by the O2
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10 flux; the more O2 flows, the less domain nucleates. Of course, too much O2 would stop
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the growth via etching and it is a matter of trade off. Fig. S2 show the optical
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15 microscope images of ML-MoS2 grown on sapphire for 20 min by employing different
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17 O2 flow rates. When varying the O2 flow rates from 0 to 10 sccm, the domain size in
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average increases from ~1 to ~180 µm. When the oxygen flow rate is over 10 sccm, the
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22 domain size drops gradually and it will be very difficult to form continuous films. We
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24 have further studied the temperature effect on the grain sizes. When keeping the oxygen
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26 flow rate at 10 sccm, we varied the growth temperature from 800 ℃ to 1000 ℃. At a
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29 low temperature of 800 ℃, we usually get bad growth, e.g. small domain sizes and
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31 presence of additional second layers. When the growth temperature increases from 870
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33 ℃ to 1000 ℃, the domain size in average decreases from ~400 to ~40 µm. Such domain
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36 size reduction comes from the oxygen etching effect. (Fig. S3). Note that when the
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38 temperature is below 930 °C, the domain orientation control is not good. Considering
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40 both the domain alignment and domain size, we choose the optimized growth
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43 temperature of 930 °C and oxygen flow rate of 10 sccm. Extending growth leads to
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45 merging of these domains into a continuous ML-MoS2 film. Note that these domains in
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47 our sample are highly oriented, we can clearly see almost only two domain orientations,
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i.e. 0° and 60° relative to the underneath sapphire substrate, being consistent with our
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52 previous observations on the 2-inch ML-MoS211. Note that the 4-inch sapphire wafer
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54 was vertically placed; while the substrate was horizontally placed in our previous 2-
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56 inch CVD growth. In this new set-up, substrates directly face to the sources and can
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59 have a much higher growth speed. As a result, a great improvement of domain size in
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3 the present ML-MoS2 can be achieved. Fig. 1b shows an as-grown 4-inch wafer of
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6 fully-covered ML-MoS2 on sapphire with the average domain size exceeding 100 µm.
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8 Fig. 1c-1e show the merging process of these highly oriented domains. When further
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10 increasing the growth time after the formation of a continuous monolayer film, second
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layer MoS2 will appear and prefer to seed above those domain boundaries (DBs) in the
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15 first layer (Fig. 1e and Fig. S4).17 In this way, we can visualize the domain size and
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17 DBs locations.
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Fig. 2a shows a typical atomic force microscope (AFM) image of the ML-MoS2
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22 film, the height profile across a scratched trench on the surface (Fig. S5) confirms the
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24 monolayer thickness of ~0.7 nm. Images of a ML-MoS2 wafer taken from different
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26 areas are show in Fig. S6. Ultra-clean and atomically flat surface with a 4-inch wafer-
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29 scale uniformity can be visualized. Contaminations or second layer are barely seen. We
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31 also imaged as-grown samples by fluorescence microscope (Fig. 2b). The uniform color
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33 contrast further confirms the optical uniformity of the ML-MoS2. An aberration-
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corrected scanning-transmission electron microscope (STEM), equipped with a high-
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38 angle annular dark-field detector (HAADF), was used to characterize the atomic
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40 structure of as-grown samples. Fig. 2c shows a atomic structure of 60° domain
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boundaries in the obtained ML-MoS2. This very straight boundary is an nearly perfect
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45 4|4E type without vacancies and distortions (Fig. 2d). According to previous studies,
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47 this type of boundary may show perfect metallicity. 24 A typical HAADF image and
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49 selected-area electron diffraction (SAED) pattern inside a domain are shown in Fig. 2e
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52 and Fig. 2f, respectively; we can see the nearly perfect honeycomb lattice of MoS2. We
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54 further evaluate the chemical composition of our achieved MoS2 films by X-ray
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56 photoelectron spectroscopy, a low oxygen concentration (~5%) is detected (Fig. S7).
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We also performed Raman and photoluminescence (PL) line-scan across the
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3 entire 4-inch wafer of ML-MoS2 on sapphire. Typical Raman/PL mapping results along
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6 the X- and Y- direction are shown in Fig. 3a/d and 3b/e, respectively. Spectra at
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8 different locations are nearly the same, revealing a wafer-scale uniformity. Four
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10 representative Raman/PL spectra are shown in Fig. 3c/f. Raman E2g and A1g peaks are
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located at 384 and 403 cm-1 with a full width at half-maximum of 4.8 and 6.8 cm-1,
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15 respectively. The peak distance between E2g and A1g is 19 cm-1, confirming the
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17 monolayer nature of the as-grown films. 25 We also characterized our as-grown sample
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by Raman and compared it with the exfoliated monolayer MoS2 on SiO2 and we did
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22 not observe a clear shift or split sub-peaks in the Raman spectrum (Fig. S8), it means
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24 the strain in our sample is small (less than 0.2%) possibly due to the relatively weak
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26 van der Waals interaction between MoS2 and the sapphire substrate. PL spectra of the
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29 MoS2 films show a single sharp excitonic A peak at ~1.88 eV. Its narrow full width at
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31 half-maximum, i.e. ~56 meV, also indicate very high crystalline quality. Also note that,
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33 the statistic Raman peak distance between E2g and A1g is 19.7±0.3 cm-1 from Fig.
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1a&b, indicating the superior monolayer thickness uniformity of the 4-inch wafer-scale
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38 ML-MoS2 on sapphire.
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40 In contrast to the previously reported wafer-scale ML-MoS2 continuous films,
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these highly oriented and large domain samples should have improved electronic
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45 quality. To make a simple comparison, we thus transferred our films on 300-nm-thick
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47 SiO2/Si substrates and fabricated back-gated filed effect transistors (FETs). Fig. S9
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49 shows a batch production of 22500 devices. Output and transfer curves of a typical
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52 device with channel length/width (L/W) of 10/25 μm are shown in Fig. 4a and 4b,
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54 respectively. This device presents a high carrier mobility of 82 cm2V-1s-1, low off
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56 current of ~15 fA, high on/off ratio of ~2×1010, and the threshold voltage is around 0
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V, suggesting an excellent device performance. The velocity saturation occurs under a
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3 relative high bias voltage (Vds) since our FETs are in the long-channel regime. 8 The
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6 saturated current is about 1.22 mA and the saturated current density (Ids/W) can reach
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8 ~49 μA/μm for a channel length of 10 μm (Fig. S10). We measured 100 randomly
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10 picked devices in the batch and the statistical data are shown in Fig. 4c&d and Fig S9d.
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We can see that the device mobility averages at 70 cm2V-1s-1 and the highest mobility
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15 can reach 93 cm2V-1s-1. The average mobility of 70 cm2V-1s-1 is just slightly smaller
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17 than the number achieved for single crystal samples (averages at 78 cm2 V-1 s-1, Fig.
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S11), suggesting that the domain boundary densities in present films are really low and
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22 these twin domain boundaries have weak effects on electrical properties14. Note that
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24 the achieved devices’ Ion/Ioff ratio is much higher than those achieved in previous CVD
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26 MoS2; 12,21,22,26-28 and to our best knowledge, the present ML-MoS2 films have the
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29 highest electrical quality (Fig. 4e and Table S1). And the threshold
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31 voltage value is mainly located at 0-5V (Fig. S9d). We also fabricated inverter, NAND,
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33 NOR and AND gate devices based on such films. The voltage transfer curves of inverter
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is plotted in Fig. 4f. In this inverter, a sharp voltage switching is realized, and a high
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38 voltage gain of 90 is achieved at the drive voltages of 5 V. Furthermore, the logic
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40 function can successfully achieved in our devices (fig 4g-i), which means the obtained
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films have great potential for integrated circuits. We also performed the growth of MoS2
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45 directly on SiO2/Si which is a universal substrate for semiconductor processes (Fig.
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S13). The domain size (~5 μm) is smaller than those on sapphire due to the relatively
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high surface roughness of SiO2. The amorphous nature of SiO2 also leads to a random
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53 orientation of MoS2 domains. We characterized the electrical properties of achieved
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55 films on SiO2/Si substrate. And the FET devices presents a carrier mobility of ~35
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cm2V-1s-1, and on/off ratio of ~108. This performance is worse than our highly-oriented
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3 In summary, we report the realization of 4-inch wafer-scale expitaxy of highly-
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6 oriented MoS2 monolayers with large domain sizes. Benefiting from a multi-source
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8 design of CVD process, as-grown MoS2 monolayers on sapphire show remarkable
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10 uniformity across the entire wafers. Domain sizes, orientations and boundaries in these
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monolayer films were visualized by optical, scanning probe and transmission electron
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15 microscopes. Nearly perfect 4|4E type of domain boundaries were observed but
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17 contribute not significantly to their field effect properties. We fabricated field effect
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transistors (FETs) based on these produced monolayers and achieved an average room-
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22 temperature device mobility of ∼70 cm2V-1s-1 and on/off ratio of ~109 on SiO2. High
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24 performance logic inverters, NOR, NAND and AND gates were also demonstrated. Our
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26 electronic and spectroscopic characterizations suggest that the produced ML-MoS2 4-
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29 inch wafers are of the highest electronic quality so far and ready for use for many
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31 fascinating applications.
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35 Methods
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37 CVD growth of MoS2 on sapphire. The multi-source CVD system was manufactured
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40 by Dongguan Join Technology co., ltd. For a typical growth, the center mini quartz tube
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42 was loaded by S (Alfa Aesar, 99.9%, 8 g) and flowed by Ar (40 sccm); and the outside
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44 six mini quartz tubes were individually loaded by MoO3 (Alfa Aesar, 99.999%, 30 mg)
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and flowed by Ar/O2 (240/10 sccm). Sapphire wafers were annealed in oxygen
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49 atmosphere at 1000 °C for 4 hours prior to the growth to form atomically flat surface
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51 for subsequent MoS2 growth. A typical growth lasts for ∼40 min and the pressure in
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the growth chamber is ∼1 Torr.
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56 Structural and spectroscopic characterizations. AFM imaging was performed by
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58 Asylum Research Cypher S. PL and Raman characterizations were performed in a
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3 Horiba Jobin Yvon LabRAM HR-Evolution Raman system with the excitation laser
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6 wavelength of 532 nm and laser power of 1 mW. SAED was performed in a TEM
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8 (Philips CM200) operating at 200 kV; while HRTEM was performed in an aberration-
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10 corrected scanning transmission electron microscope JEM ARM200F (JEOL)
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operating at 200 kV.
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15 Device Fabrication and Measurements. ML-MoS2 films were etched off substrates
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17 in KOH solution and transferred onto SiO2/Si substrates. The transferred 4-inch wafer-
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scale ML-MoS2 was then patterned into FETs by UV-lithography (MA6, Karl Suss),
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22 oxygen plasma etching (RIE, Plasma Lab 80 Plus, Oxford Instruments Company),
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24 electron beam evaporation and lifting-off processes. The contact electrodes are
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26 Au/Ti/Au (2/2/30 nm). Electrical measurements were carried out by semiconductor
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29 parameter analyzer (Agilent 4156C) in a four-probe vacuum station with a bass pressure
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31 of ~10−6 mbar.
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35 Supporting Information
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37 The Supporting Information is available free of charge via the internet at
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39 http://pubs.acs.org. Photo of the 4-inch multisource CVD system, more growth results
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41 under different oxygen flux and temperature, AFM and STEM images on additional
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44 layer growth on grain boundaries, AFM images on different locations of a wafer, XPS
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46 and Raman characterizations to confirm the oxygen concentration and strain in MoS2
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48 films, more electrical measurements for MoS2 films, detailed structure of logic circuits
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and the MoS2 growth results on SiO2/Si substrates.
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55 Author Contributions
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G.Z. designed the research; Q.W. performed the epitaxial growth, device fabrications
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60 and measurements. N.L., J.T. helped on fabrication of the logic circuits. J.T., Q.Z. and

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3 L.G. performed TEM and structural analysis. Q.J. and Y.L. performed the Fluorescence
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6 microscope measurements. Q.W. and G.Z. wrote and all authors commented on the
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8 manuscript.
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12 Acknowledgements
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14 This work was supported by the National Science Foundation of China (NSFC) under
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17 Grant Nos. 11834017, 11574361 and 61888102, the Strategic Priority Research
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19 Program of Chinese Academy of Sciences (CAS) under Grant No. XDB30000000, the
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21 Key Research Program of Frontier Sciences of the CAS under Grant No. QYZDB-
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SSW-SLH004, the Youth Innovation Promotion Association CAS (No. 2018013), the
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26 National Key R&D program under Grant No. 2016YFA0300904, and the Research
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28 Program of Beijing Academy of Quantum Information Sciences under Grant No.
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30 Y18G11.
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3 monolayer MoS2 films (the step structure is from the underlying sapphire surface). b)
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39 c/f) Corresponding Raman/PL spectra at four randomly picked locations on the wafer.
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