High-Mobility MoS2 Directly Grown on Polymer Substrate with Kinetics

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High-Mobility MoS Directly Grown on Polymer Substrate with Kinetics-Controlled Metalorganic Chemical Vapor Deposition Jihun Mun, Hyeji Park, Jaeseo Park, DaeHwa Joung, Seoung-Ki Lee, Juyoung Leem, Jae-Min Myoung, Jonghoo Park, Soo-Hwan Jeong, Won C. Chegal, SungWoo Nam, and Sang-Woo Kang ACS Appl. Electron. Mater., Just Accepted Manuscript • DOI: 10.1021/acsaelm.9b00078 • Publication Date (Web): 01 Apr 2019 Downloaded from http://pubs.acs.org on April 3, 2019

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ACS Applied Electronic Materials

High-Mobility MoS2 Directly Grown on Polymer Substrate with Kinetics-Controlled Metalorganic Chemical Vapor Deposition Jihun Mun†,∆, Hyeji Park†,‡,∆, Jaeseo Park†,§, DaeHwa Joung†,#, Seoung-Ki Lee○, Juyoung Leem¶, Jae-Min Myoung▲, Jonghoo Park#, Soo-Hwan Jeong‡, Won Chegal†, SungWoo Nam*,¶,▼, and Sang-Woo Kang*,†,§ †Advanced Instrumentation Institute, Korea Research Institute of Standards and Science, Daejeon 34113, Republic of Korea ‡Department of Chemical Engineering, Kyungpook National University, Daegu 41566, Republic of Korea §Science of Measurement, University of Science and Technology, Daejeon 34113, Republic of Korea #Department of Electrical Engineering, Kyungpook National University, Daegu 41566, Republic of Korea ○Functional Composite Materials Research Center, Korea Institute of Science and Technology, Jeollabuk-do 55324, Republic of Korea

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¶Department of Mechanical Science and Engineering, University of Illinois at UrbanaChampaign, Urbana, Illinois 61801, United States ▲Department of Materials Science and Engineering, Yonsei University, Seoul 03722, Republic of Korea ▼Department of Materials Science and Engineering, University of Illinois at UrbanaChampaign, Urbana, Illinois 61801, United States

KEYWORDS MoS2, kinetics-controlled MOCVD, low-temperature growth, direct growth, flexible FET

ABSTRACT

Batch growth of high-mobility (FE > 10 cm2V-1s-1) molybdenum disulfide (MoS2) films can be achieved by means of the chemical vapor deposition (CVD) method at high temperatures (> 500 ℃) on rigid substrates. Although high-temperature growth guarantees film quality, time- and cost-consuming transfer processes are required to fabricate flexible devices. In contrast, lowtemperature approaches (< 250 ℃) for direct growth on polymer substrates have thus far achieved film growth with limited spatial homogeneity and electrical performance (FE is unreported). The growth of a high-mobility MoS2 film directly on a polymer substrate remains challenging. In this study, a novel low-temperature (250 ℃) process to successfully overcome this challenge by kinetics-controlled metalorganic CVD (MOCVD) is proposed. Low-


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temperature MOCVD was achieved by maintaining the flux of an alkali-metal catalyst constant during the process; furthermore, MoS2 was directly synthesized on a polyimide (PI) substrate. The as-grown film exhibits a 4-inch wafer-scale uniformity, field effect mobility of 10 cm2V-1s-1, and on/off ratio of 105, which are comparable with high-temperature grown MoS2. The directly fabricated flexible MoS2 field effect transistors demonstrate excellent stability of electrical properties following a 1000 cycle bending test with a 1 mm radius.

INTRODUCTION

A large number of studies have demonstrated that an atomically thin two-dimensional (2D) molybdenum disulfide (MoS2) exhibits unique characteristics of electron mobility,1–4 flexibility,5–6 and tunable bandgap7 and is considered as a promising semiconducting material for next-generation electronic devices. Consequently, various synthetic methods such as thermolysis,8 intercalation,9,10 sulfurization,11,12 and chemical vapor deposition (CVD)13–18 have been developed. Among these, CVD is a versatile method with a wide range of process controllability functions, such as large-sized single crystal synthesis,19 wafer-scale synthesis,20 layer controlled growth,21 and in-situ doping.22 However, existing CVD methods for the growth of high-mobility MoS2 films have, thus far, been realized at high process temperatures (> 500 ℃) allowing for a large diffusion length at the surface. Although the high process temperature guarantees high-quality film growth, the melting temperatures of most polymer substrates are substantially lower than the MoS2 synthesis temperature. Therefore, an additional transfer process is necessary to use CVD-grown MoS2 for flexible device applications. Consequently, growing high-quality MoS2 films directly on polymer substrates using low-temperature growth


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methods would contribute considerable benefits to the practical and cost-effective fabrication of flexible electronic devices. Various approaches using plasma-enhanced CVD (PECVD)23, 24 and powder CVD25 have suggested potential means of direct growth of 2D materials. Although reported methods have demonstrated the possibilities, batch fabrication of field effect transistors (FETs) with highperformance is lacking owing to inferior material properties, often originating from the amorphous structure23 or inhomogeneity25 of directly-grown films. For example, as reported in our previous study,26 layer-by-layer growth is unfavorable at low process temperatures, owing to the short diffusion length on the surface; consequently, three-dimensional (3D) islands with a small grain size are formed. Furthermore, most direct growth methods are optimized for direct growth on polymeric substrates.23, 25 However, polymers are incompatible with the standard photolithography process, which is essential for the batch fabrication of transistors. Thus, overcoming these bottlenecks by developing (1) a novel low-temperature CVD method for the spatially homogeneous MoS2 film growth, and (2) a preparation strategy to prevent the deformation of polymer substrates would result in significant technological evolution for the practical application of 2D material-based flexible electronic devices. In this paper, we present the kinetics-controlled low-temperature metalorganic CVD (MOCVD) method for the direct growth of spatially homogeneous 2D MoS2 film on a polyimide (PI) substrate, and a systematic procedure for the fabrication of flexible FET devices via the standard photolithography process. The 4-inch scale monolayer MoS2 film is successfully grown at a record low process temperature of 250 ℃, by precise and continuous feeding of the alkalimetal catalyst during the MOCVD process. Furthermore, our novel polymer preparation method


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allows for direct growth of a continuous MoS2 film on a PI substrate, with superior electrical characteristics and mechanical durability.

EXPERIMENTAL SECTION

Film growth Growth of the wafer-scale MoS2 was conducted using in-house cold-wall-type MOCVD reactor. To grow the fully covered film, 250 ℃ and 30 Torr were maintained as process conditions during the growth time (8 h). A total of 200 sccm of hydrogen was used for enhanced dissociation of the Mo precursor. Mo(CO)6 (MHC, ≥ 99.9%, Sigma Aldrich, 577766) and high purity (99.9%) H2S (HS) were used as the Mo and S precursors, respectively. To enhance the lateral growth affinity, growth was conducted in the presence of excessive HS (PHS/PMHC = 400). The sodium chloride (NaCl) (Sigma Aldrich, 204439) was used as an alkali-metal catalyst and was loaded on to the MOCVD reactor along with the growth substrate. A certain amount of NaCl catalyst was added to the in-house enclosure to achieve a constant active area for sublimation and flux. To provide rich nucleation sites and hydrophilicity, both the PECVD and thermally-grown SiO2 substrates were pre-treated with a 0.5 wt.% KOH (Sigma Aldrich, 306568) solution for 5 min.

Film characterization The as-grown MoS2 films were characterized by Raman spectrometer (inVia, Renishaw), scanning electron microscope (SEM, S-4700, Hitachi), X-ray photoelectron spectrometer (XPS, K-alpha, Thermo Scientific), scanning transmission electron microscope (STEM, Titan cubed3 G2 60-300, FEI), spectroscopic ellipsometer (SE, M-2000, J. A. Woollam), optical microscope


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(BX51, Olympus), atomic force microscope (XE-150, Park Systems), UV/Vis spectrometer (Lambda 1050, PerkinElmer), and semiconductor parameter analyzer (4156A, Keysight).

Bilayer (BL) coverage analysis The BL coverage was obtained by calculating area ratio of bilayer and monolayer (ML). The ML and BL were identified according to contrast differences in SEM images.

TEM sample preparation To prepare the TEM samples, poly(methyl methacrylate) (950 PMMA A2, MicroChem) was spin-coated on the as-grown MoS2 films as a support layer. The PMMA/MoS2 layer was released from the substrate by deionized water and it was scooped with a TEM grid (HC300-Cu, Electron Microscopy Science). Subsequently, the sample was heat-annealed at 300 ℃ in a H2 and Ar (1:1) environment for 12 h to remove the support layer.

RESULTS AND DISCUSSION

Figure 1 presents the microscopic and spectroscopic analysis results of our lowtemperature grown film on a SiO2 substrate. The MoS2 film growth was carried out via lowtemperature MOCVD using MHC, which gradually sublimed at a constant temperature under the presence of excessive HS, maintaining a higher partial pressure ratio (PHS/PMHC) of 400 to enhance the lateral direction growth affinity. Such a method has been demonstrated as an effective procedure for the layer-by-layer growth of wafer-scale, uniform 2D transition metal dichalcogenides (TMDs).20 However, as presented in Figure 1a, at our process temperature


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(250 ℃), layer-plus-islands growth was dominated over lateral growth after exceeding critical coverage, and it causes poor coverage of the synthesized material. Homogeneous and continuous TMD films are desirable in various practical applications because achieving homogeneous electrical and optical properties over the entire film surface is critical for batch fabrication of devices. To improve homogeneity and continuity by increasing grain sizes of monolayer TMDs, we implemented alkali-metal-assisted MOCVD, which has been demonstrated by several researchers as an effective method for the growth of 2D TMDs with improved thickness homogeneity.27–29


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Figure 1. Microscopic and spectroscopic analysis of low-temperature (250 ℃) grown MoS2 on a 100 nm thick thermal-SiO2 substrate. a) SEM images of growth process of MoS2 film prepared without NaCl catalyst (conventional MOCVD); scale bar is 100 nm. b) SEM images and grain size distribution histograms of three major types of growth modes; scale bar is 300 nm. c) SEM images of growth process of MoS2 film prepared with flux controlled NaCl catalyst (growth mode 3); scale bar is 100 nm. d,e) Coverage analysis and wavenumber differences between two major Raman modes (E12g and A1g) plots for MoS2 film grown without (blue) and with (red)


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NaCl for each growth process, respectively. f,g) Raman, PL, and XPS spectra of fully covered MoS2 film grown with (blue) and without (red) NaCl, respectively. The excitation wavelength of 488 nm was used for Raman and PL measurements. h,i) ADF-STEM and SAED pattern images of MoS2 monolayer grown with NaCl.

A major role of alkali-metal on the growth of TMD materials is suppressing nucleation sites.27 Although such a phenomenon has been verified experimentally, its detailed mechanism is still not fully understood, because various factors, such as the surface chemical state, residence time, diffusion length, and surface/adsorption/desorption energy may affect the nucleation. Among these, one hypothesis explains that the existence of alkali-metals modifies the density of hydroxyl groups at the SiO2 surface and consequently suppresses the number of nucleation sites at the SiO2 surface. To clarify the effect of surface hydroxyl groups on nucleation, we artificially modified the surface of the SiO2 substrate with KOH treatment to achieve high-density hydroxyl groups. After treatment, the contact angle of the SiO2 substrate was changed from 51.3˚ to 22.6˚ (Figure S1a) by hydroxylation on the surface.30 As shown in Figure S1b in Supporting Information, higher-density nuclei were created on the treated surface, and this result proves that the surface hydroxyl group plays major role in the nucleation of TMD materials on oxide substrates. Additionally, we conducted another control experiment to clarify the occupation effect of the alkali-metals by evaluating nuclei density change with respect to the amount of alkali-metal applied to the growth process. The results showed that further suppression of nucleation sites occurred with increased amount of alkali-metal (Figure S2).


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Another role of alkali-metal in the growth process is the catalytic reaction for enhanced growth affinity in the lateral direction. We hypothesize that addition of alkali-metal catalysts reduces the activation barrier for diffusion, and consequently, surface diffusion is enhanced owing to increased hopping rate. Recently, a similar research reported a similar idea; an alkalimetal catalyst reduces telluriding temperature by decreasing the Gibbs free energy.31 In this sense, control of catalytic reaction kinetics of alkali-metal can serve as a key factor during the growth of spatially homogenized MoS2 at a lower process temperature. Therefore, key findings in our work describe the role the amount and delivery flux (for the detailed method see Experimental Section in Supporting Information) of the NaCl catalyst plays during the growth process. We confirmed three major types of growth modes with respect to the combinations of the feeding flux and amount of NaCl. First, when excessive amount of NaCl was added, larger domains were grown (Figure 1b, growth mode 1). However, over-suppressed nucleation sites— because of an overdose of NaCl—led to an overpopulated hopping rate of adatom at the surface and eventually caused poor coverage of MoS2 over the substrate. As a result, a BL began to form on the surface of the ML islands. Mode 1 characteristics demonstrate that the catalyst overdose is eventually unfavorable for homogeneous film growth. Second, we injected NaCl with an uncontrolled delivery flux, and small-sized ML islands were observed at an intermediate growth step (Figure 1b, growth mode 2). Such island formations in the intermediate stage originated from the decrease of NaCl partial pressure over time. The active surface area of the NaCl catalyst decreased over time, which led to the formation of an inhomogeneous film (Figure S3). Mode 2 demonstrates that NaCl catalyst flux rate should be held constant for the growth of homogeneous film, particularly at a low temperature. Finally, we used a controlled NaCl flux and found an


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optimized condition for the homogeneous film growth. The grown islands showed a narrower size distribution (full width half maximum of 24 nm versus 54 nm and 39 nm for modes 1 and 2, respectively), and demonstrated that the MoS2 synthesized with mode 3 had high uniformity. As shown in Figure 1c, with the implementation of an optimum amount (1 mg) and controlled flux of NaCl, the formation of additive islands on the ML islands (domain sizes: 80 - 100 nm) were notably minimized via the enhanced lateral growth affinity throughout the entire growth time; whereas, its nucleation density was not considerably changed. The material quality of our alkali-metal-assisted MOCVD-grown MoS2 is demonstrated in Figures 1c–g, by comparing the properties with the standard MOCVD-grown MoS2. The ML coverage (ML) of the MoS2 grown by a conventional MOCVD process began to increase considerably from a certain point in the process; however, ML stayed at the lowest range with the alkali-metal-assisted process, which implies that formation of BL was restricted until ML covered the entire substrate (Figure 1d). These coverage differences along the growth time were also clearly observable in the Raman spectra shown in Figure 1e. In case of the conventional MOCVD method, wavenumber difference between two major Raman modes (E12g and A1g) is much larger (22.2 cm-1) than that of the alkali-metal-assisted MOCVD method (19.8 cm-1). Because additional vertical layers in the MoS2 structure cause larger differences between E12g and A1g peaks,32 the spectral analysis agrees with observation of surface coverage (Figure 1d). Furthermore, the Raman and photoluminescence (PL) spectra from fully covered film (Figure 1f) also clearly show different levels of homogeneity. For films grown through the conventional method, higher intensity of E12g and A1g mode and shift of A1g mode were observed that are contributed by the interaction between additive islands and fully covered ML films. In addition, in photoluminescence measurement, the emission quantum yield of A exciton was decreased


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owing to partially covered BL-added islands for the films grown with the conventional MOCVD method.33 In the XPS measurement, the chemical energy states of the films were examined to confirm whether carbon or oxide impurities were incorporated during the process, owing to the low process temperature. As shown in Figure 1g, only Mo and S were found, and no other elements were bound with them (no binding energy for Mo6+ was observed at near 236 eV). Additionally, no shifts in each of the Mo and S peaks were observed, confirming no chemical doping with Na atoms, which only interact with the substrate.27 STEM images acquired by an annular dark-field STEM (ADF-STEM), and selected area electron diffraction (SAED) image are presented in Figures 1h,i to investigate the MoS2 structure grown by the alkali-metal-assisted MOCVD. The TEM images clearly show that the Mo and S atoms formed a hexagonal structure without structural defects. The image of a grain boundary shows that the connected ML domains with minimized dislocation or vacancy were observed that are similar to previously reported high-temperature grown films.14,15 The SAED pattern in Figure 1i also confirms the single crystallinity of the synthesized MoS2. In the following, we discuss the wafer-scale homogeneity of our film and its electrical characteristics.


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Figure 2. Homogeneity analysis of wafer-scale MoS2 film grown with NaCl catalyst. a) Digital photograph images of 4-inch scale bare thermal-SiO2/Si and MoS2 film grown on SiO2/Si substrate. b) Ellipsometry mapping plot of 4 inch scale MoS2 film grown on SiO2/Si substrate. The uniformity is calculated as 95.6%. c) Histogram of 100 points for two major Raman modes (E12g and A1g). The average wavenumber difference between the two modes is calculated as 19.9 cm-1, with a 0.2 cm-1 standard deviation. d) Optical microscope images of multiple two-terminal FET devices fabricated with standard photolithography (inset: structure of FET device). e) Transfer curve plots of two-terminal FET device showing FE = 10 cm2V-1s-1 and Ion/Ioff = 105 (Vtg = 2.0 V and Vds =0.1 V). f) Distribution plots of Ion/Ioff and FE of 100 randomly selected devices. For all devices, Vtg = 2.0 V and Vds = 0.1 V. g) Ids-Vds curve plots of two-contact device. The same device as in Figure 2e was used for characterization.

The wafer-scale homogeneity of our low-temperature grown film was confirmed via a synthesis on a 4-inch SiO2/Si wafer as shown in Figure 2a. The color of the wafer with MoS2 is notably distinguishable from its original SiO2/Si substrate. Furthermore, its uniform bluish color,


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which was maintained over the entire wafer confirms that our film is highly homogeneous. To quantitatively evaluate homogeneity of our film, the intrinsic optical properties of as-grown film were evaluated with SE thickness mapping (Figure 2b) and location-selected Raman spectroscopy (Figure 2c). For SE mapping, we established the atmosphere/MoS2/SiO2/Si optical model. The complex dielectric function and thickness were extracted from its multilayer optical calculation.34 The average thickness value measured from 300 points was 0.66 nm, with a standard deviation of 0.03 nm, and the uniformity was calculated as 95.6%. For the locationselected Raman spectroscopy, we obtained the spectral dataset from 100 points and calculated the average values of the individual peak locations of two major Raman modes (E12g and A1g). The average value of the wavenumber differences between two peaks was calculated to be 19.9 cm-1, with a 0.2 cm-1 standard deviation. Both uniform optical properties, which were measured over the entire area, prove the wafer-scale homogeneous characteristics of our grown film. The electrical characteristics of our low-temperature grown MoS2 monolayer were examined by fabricating two-contact FETs with the standard photolithography process. First, a fully covered MoS2 monolayer was etched to create an active channel by means of reactive ion etching, which was optimized with a CF4/O2 gas mixture; subsequently, Au/Ti source and drain electrodes were deposited by e-beam evaporation. After fabricating the MoS2 channels and electrodes, photo-patternable ion-gel was applied to serve as a top-gate dielectric. As presented in Figure 2d, we fabricated a large array of two-terminal contacted FETs on the as-grown MoS2, without any heat-annealing process. The electrical characteristics of the FETs were evaluated by measuring the source–drain current against applying top-gate and source–drain voltages at room temperature under atmospheric conditions. Although our film was grown at a very low process temperature, it shows high-mobility and on/off ratio that are comparable with those two-terminal


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Table 1. Summary data of electrical characteristics of the FET from previous results. Reference

Field effect

On/off

Mobility (cm2V-1s-1)

ratio

Growth Method

Substrate

number

temperature (℃)

14

850

CVD

SiO2/Si

4.3

106

15

700

CVD

SiO2/Si

3-4

105-107

16

800

CVD

SiO2/Si

7

106

20

550

MOCVD

SiO2/Si

29

106

35

850

CVD

SiO2/Si

13

-

SiO2/Si

10

105

SiO2/PI

1

103

This 250 work

MOCVD

FET characteristics of high-temperature grown CVD MoS2 (Table 1).14-16, 20, 35 The transfer curves measured from randomly selected data within such a device array are presented in Figure 2e, and the devices exhibited conventional n-type transistor characteristics with an on-state current of 10-5A and off-state current of 10-10A. The calculated field-effect mobility was 10 cm2V-1s-1 and the on/off ratio was 105. Such high field-effect mobility and on/off ratio possibly originated from the structural continuity of the MoS2 with minimized defects and a well-merged boundary interface.20 The measured mobility and on/off ratio of 100 multiple devices exhibited distributions of 7 to 12 cm2V-1s-1 and 104 to 107, respectively (Figure 2f), proving the spatial homogeneity of our low-temperature grown film. The output characteristics of the two-terminal device with ion-gel capping exhibited slightly nonlinear characteristics as presented in Figure 2g. This non-linear Ids–Vds possibly originated from the local biasing effect of the ion-gel capping layer, induced by the accumulation of ions with the opposite polarity in the vicinity of the source


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and drain electrodes.36 From both Figures 1 and 2, we have demonstrated the feasibility of the low-temperature MOCVD method to grow spatially homogenized monolayer MoS2. Furthermore, the quantitative uniformity analysis and electrical performance evaluation proved that our film quality is comparable with samples grown with the high-temperature CVD method. Although we demonstrated the feasibility of the low-temperature method, direct growth of homogeneous films on a polymer substrate and ease of handling remained to be demonstrated. To overcome such challenges, we developed a novel substrate preparation process (Figure 3a), most importantly, preparation of a polymer substrate. Scratched and locally deformed areas are often observed at the surface of commercially available PI films (Figure S4). Such defects cause inhomogeneous film growth owing to surface energy variations (Figure 3a, marked by number 1).25 Furthermore, the PI substrate itself is easily bent and deformed because of thermal strain during the growth process at elevated temperatures, which consequently increases handling difficulties. To overcome this challenge, we manufactured an in-house PI film, which was gently laminated onto a glass substrate. The PI film using this method provides a very smooth surface (Figure S4) for the growth of homogeneous MoS2, and the underlying glass substrate provides rigid support during the standard photolithography processes and prevents deformation of the PI film during the MoS2 synthesis process. Another process is the ultra-high vacuum heat-annealing of the PI/glass substrate. Although PI film can endure our process temperature (250 ℃), under high temperatures and vacuum conditions, PI may release water and gas phase substances absorbed in the ambient environment (Figure 3a, marked by number 2). Because these water molecules and gas phase substances are undesirable in the MoS2 growth process (Figure S5), the PI/glass substrate was pre-annealed at a slightly higher temperature (300 ℃) than the MoS2 growth temperature under the ultra-high vacuum condition ( 400 ℃) process using commercial PI, sparse MoS2 islands are grown, owing to the roughened surface of the commercial PI. Moreover, the transparency of the PI is changed to opaque owing to its higher process temperature (marked by number 1). Although in-house PI lamination on glass substrates provides a smooth surface and rigid form, the growth of MoS2 is disturbed by outgassing from the PI/glass substrate (marked by number 2). The heat-annealing and capping (with PECVD SiO2) procedures prevent the outgassing phenomenon, and further absorption of water molecules and gas phase substances in the ambient environment. Consequently, a highly homogeneous film is grown (marked by number 3). b) Digital photograph images of PECVD SiO2/PI/glass substrate and fully covered MoS2 film on that substrate. The transparency is maintained owing to our low process temperature (250 ℃). c) Transmittance spectrum of directly grown MoS2 film. The PI substrate spectrum is compensated. d) PL spectra of directly grown MoS2 film. Owing to the strong luminescence from the PI substrate, measurement is conducted by transferring to a rigid SiO2/Si substrate. The excitation wavelength of 488 nm was used for PL measurement. e,f) Cross-sectional ADF-STEM images of directly grown MoS2 film and corresponding EELS spectra of C, S and O, respectively.


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The digital photograph image in Figure 3b shows the SiO2/PI/glass substrate and MoS2 film grown on that substrate. With the MoS2 film, its color changed from yellowish to brownish with a uniform color contrast over the entire substrate. To prove homogeneity property of our directly grown film, we performed Raman and PL mapping analysis and results showed that the directly grown film is spatially homogenous (Figure S6). The transmission spectra of the directly grown film are presented in Figure 3c, showing a transparency of more than 90% in the visible range. Furthermore, excitonic peaks generated from A, B, and C excitons were observed near 660 nm, 610 nm, and 450 nm, which is consistent with previously reported results,37 and the spectroscopic data supported that our directly grown film contains ML to BL MoS2. The Raman and PL measurements were conducted with the as-grown film transferred onto a rigid SiO2/Si substrate to avoid strong luminescence peaks from the PI film itself (Figure 3d). Further, a structural analysis was performed with a cross sectional ADF-STEM image (Figure 3e) and electron energy loss spectroscopy (EELS). First, the ADF-STEM image clearly indicates two slightly bent layers and blurred overlapped multiple layers, which originated from the simultaneous measurement of the front and back side layers. Such overlapped layers are normally unobservable in case of MoS2 grown on a rigid substrate. However, our SiO2 was deposited by PECVD onto the PI film, with a higher surface roughness than thermally grown SiO2 (Figure S4), exhibiting a bent structure and overlapped film in the cross-sectional imaging. Second, the EELS results show the chemical composition of the interfacial layers and prove that our film was directly grown on the PI substrate. The peak position of each element is well matched with its corresponding layers.


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Figure 4. a,b) Schematic illustration of flexible MoS2 FET fabricated using directly grown MoS2. c) Digital photograph of flexible MoS2 FET. The image shows its original color and flexibility (inset: structure of flexible FET device, where dark brown color indicates continuous MoS2 active channel). d,e) Electrical performance of flexible MoS2 FET showing FE = 1 cm2V1s-1

and Ion/Ioff = 103. f–h) Durability test results of flexible MoS2 FET. The cyclic tests were

conducted at three different bending radii (1, 3, and 5 mm)


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A schematic illustration of the fabricated flexible MoS2 FET device is presented in Figure 4a. The flexible device was fabricated using exactly the same methods as those used for the device fabrication on SiO2/Si, and the peeling-off process at the final stage was the only additional step (Figure 4b). The flexible FET devices fabricated by this procedure are presented in Figure 4c, showing its bendable and transparent characteristics. Compared with other directly grown films,25 our film and device maintained its original color and transparency because of our lower process temperature. In the optical and microscopic images in the inset of Figure 4c, the patterned MoS2 active channel with electrodes and top-gate dielectric are shown. To evaluate the electrical properties, source–drain current was measured by applying topgate and source–drain voltages of a randomly selected device among the multiple two-terminal FET devices, and the results are shown in Figures 4d and 4e, respectively. As same as the rigid FET, all tests were performed at room temperature and under atmospheric conditions. The transfer curve plots (Figure 4d) indicate an on-state current of 10-6A and off-state current of 109A,

and the calculated field effect mobility is 1 cm2V-1s-1, which is one order of magnitude lower

than that of the rigid FET device. Such degradation of the device performance can be explained by the surface roughness of the underlying SiO2 film, as well as leakage currents between source and drain electrodes. In particular, a SiO2 film was deposited by the PECVD process onto PI film to achieve homogeneous nucleation sites. However, as we showed in Figure S4 in Supporting Information, the surface roughness of the PECVD SiO2 was 2 times higher than the roughness of the thermally grown SiO2. In atomically thin material, roughness scattering is a major phenomenon that affects carrier transportation, as carriers interact strongly with dangling bonds on the roughened SiO2 surface.38, 39 Furthermore, roughened surface of PECVD SiO2 provide much


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abundant nucleation sites (Figure S7) than thermally grown SiO2 consequently MoS2 film with smaller domain (20 - 30 nm) were grown on PECVD SiO2/PI substrate which may affect to degradation of device performance.16 Moreover, ion-gel is a well-known electrical double-layer (EDL), which has a high capacitance value. Owing to its EDL, ion-gel inherently causes higher leakage currents,40 which may degrade the device performance and exhibit non-linear Ids–Vds characteristics (Figure 4e). Although we applied ion-gel as a top-gate dielectric owing to its handiness, considering that the performance degradation originated from a roughened SiO2 surface and natural limitations of the ion–gel, this could be solved by other means, such as enhancing the deposition process for a smoother SiO2 film and introducing a buffer layer (such as Al2O3) in the middle.41 The stability of the electrical properties of our one set of MoS2 flexible FET devices was tested by a mechanical bending test, using a bending machine, with a controlled bending radius ranging from 1 to 5 mm. We performed cyclic tests by measuring its mobility every 200th cycle. No other heat treatment was carried out between measurements, and the same cyclic tests were conducted at three different bending radii (1, 3, and 5 mm). The cyclic durability test results demonstrated excellent stability of the electrical performance of our device by maintaining its mobility between 0.6 and 0.9 cm2V-1s-1 (Figure 4f– h). From Figures 3 and 4, we have demonstrated our unique procedure for the direct fabrication of flexible MoS2 FET, by proving that the preparation of a suitable polymer substrate is essential for the direct growth of homogeneous MoS2 film on a flexible substrate. The direct growth of MoS2 film on polymer substrates shows advantages in term of expanding its application of 2D materials, without significant performance degradation.


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CONCLUSIONS

In this study, we demonstrate the growth of a spatially homogenized MoS2 monolayer film with an alkali-metal-assisted MOCVD method at a record low process temperature of 250 ℃. Our main finding in this work is that introducing a precisely controlled NaCl flux is essential for our low-temperature growth process for a large area (wafer-scale), uniform MoS2 with high crystallinity. Furthermore, the direct growth of a homogeneous MoS2 film on a flexible substrate was demonstrated, and fabricated into large-area FET arrays. By proving the high electrical performance of the low-temperature grown film, which is comparable with the other high-temperature grown films, we introduce the opportunity for a versatile cost-effective way to fabricate 2D material-based flexible electronics including FETs, photodetectors, and sensors.

ASSOCIATED CONTENT Supporting Information Contact angle and SEM images of MoS2 grown on pristine and KOH-treated thermal SiO2 substrate (Figure S1); SEM images of MoS2 domain sizes and nucleation sites according to NaCl amount (Figure S2); SEM images of coverage change at different growth times for growth mode 2 (Figure S3); AFM images of commercial PI film, PECVD SiO2/in-house PI/glass and pristine thermal-SiO2 substrate (Figure S4); Thermal desorption spectra of PI substrate and annealing effect (Figure S5); PL and Raman mapping results of MoS2 film transferred from PECVD SiO2/PI substrate (Figure S6); Contact angle of KOH-treated PECVD SiO2 and grown MoS2 on that substrate (Figure S7)


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Author Contributions ∆These

authors contributed equally. The manuscript was written through contributions of all

authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Technology Innovation Program (project no. 10062161, Direct low-temperature synthesis of two-dimensional materials and heterostructure on flexible substrate for next-generation high-mobility electronic devices) funded by the Ministry of Trade, Industry, and Energy (MOTIE, Korea) and development of advanced ultra-thin measurement standard technology, funded by Korea Research Institute of Standards and Science (KRISS-2018GP2018-0012) ABBREVIATIONS CVD, chemical vapor deposition; MOCVD, metalorganic chemical vapor deposition; PI, polyimide; 2D, two-dimensional; PECVD, plasma-enhanced chemical vapor deposition; 3D,


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three-dimensional; FET, field effect transistor; TMD, transition metal dichalcogenide; BL, bilayer; ML, monolayer; PL, photoluminescence; XPS, X-ray photoelectron spectrometer; STEM, scanning transmission electron microscope; ADF-STEM, annular dark-field scanning transmission electron microscope; SAED, selected area electron diffraction; SE, spectroscopic ellipsometry; EELS, electron energy loss spectroscopy; EDL, electrical double layer. REFERENCES (1) Radisavljevic, B.; Radenovic, A.; Brivio1, J.; Giacometti, V.; Kis, A. Single-Layer MoS2 Transistor Nat. Nanotech. 2011, 6, 147-150. (2) Kaasbjerg, K.; Thygesen, K. S.; Jacobsen, K. W. Phonon-Limited Mobility n-Type SingleLayer MoS2 from First Principles Phys. Rev. B 2012, 85, 115317. (3) Wang, H.; Yu, L.; Lee, Y.-H.; Shi, Y.; Hsu, A.; Chin, M. L.; Li, L.-J.; Dubey, M.; Kong, J.; Palacios, T. Integrated Circuits Based on Bilayer MoS2 Transistors Nano Lett. 2012, 12, 46744680. (4) Radisavljevic, B.; Kis, A. Mobility Engineering and a Metal–Insulator Transition in Monolayer MoS2 Nat. Mater. 2013, 12, 815-820. (5) Castellanos-Gomez, A.; Poot, M.; Steele, G. A.; van der Zant, H. S.; Agraït, N.; RubioBollinger, G. Mechanical Properties of Freely Suspended Semiconducting Graphene-Like Layers Based on MoS2 Nanoscale Res. Lett. 2012, 7, 233. (6) Peng, Q.; Deab, S. Outstanding Mechanical Properties of Monolayer MoS2 and Its Application in Elastic Energy Storage Phys. Chem. Chem. Phys. 2013, 15, 19427-19437. (7) Lee, H. S.; Min, S.-W.; Chang, Y.-G.; Park, M. K.; Nam, T.; Kim, H.; Kim, J. H.; Ryu, S.; Im, S. MoS2 Nanosheet Phototransistors with Thickness-Modulated Optical Energy Gap Nano Lett. 2012, 12, 3695-3700.


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TABLE OF CONTENTS GRAPHIC


31


Figure 1. Microscopic and spectroscopic analysis of low-temperature (250 ℃) grown MoS2 on a 100 nm thick thermal-SiO2 substrate. a) SEM images of growth process of MoS2 film prepared without NaCl catalyst (conventional MOCVD); scale bar is 100 nm. b) SEM images and grain size distribution histograms of three major types of growth modes; scale bar is 300 nm. c) SEM images of growth process of MoS2 film prepared with flux controlled NaCl catalyst (growth mode 3); scale bar is 100 nm. d,e) Coverage analysis and wavenumber differences between two major Raman modes (E12g and A1g) plots for MoS2 film grown without (blue) and with (red) NaCl for each growth process, respectively. f,g) Raman, PL, and XPS spectra of fully covered MoS2 film grown with (blue) and without (red) NaCl, respectively. The excitation wavelength of 488 nm was used for Raman and PL measurements. h,i) ADF-STEM and SAED pattern images of MoS2 monolayer grown with NaCl. 86x155mm (300 x 300 DPI)


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Figure 2. Homogeneity analysis of wafer-scale MoS2 film grown with NaCl catalyst. a) Digital photograph images of 4-inch scale bare thermal-SiO2/Si and MoS2 film grown on SiO2/Si substrate. b) Ellipsometry mapping plot of 4 inch scale MoS2 film grown on SiO2/Si substrate. The uniformity is calculated as 95.6%. c) Histogram of 100 points for two major Raman modes (E12g and A1g). The average wavenumber difference between the two modes is calculated as 19.9 cm-1, with a 0.2 cm-1 standard deviation. d) Optical microscope images of multiple two-terminal FET devices fabricated with standard photolithography (inset: structure of FET device). e) Transfer curve plots of two-terminal FET device showing μFE = 10 cm2V-1s-1 and Ion/Ioff = 105 (Vtg = 2.0 V and Vds =0.1 V). f) Distribution plots of Ion/Ioff and μFE of 100 randomly selected

devices. For all devices, Vtg = 2.0 V and Vds = 0.1 V. g) Ids-Vds curve plots of two-contact device. The same device as in Figure 2e was used for characterization. 128x57mm (300 x 300 DPI)


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Figure 3. a) Schematic of direct growth process of MoS2 on PI substrate. At a high-temperature (> 400 ℃) process using commercial PI, sparse MoS2 islands are grown, owing to the roughened surface of the commercial PI. Moreover, the transparency of the PI is changed to opaque owing to its higher process temperature (marked by number 1).25 Although in-house PI lamination on glass substrates provides a smooth surface and rigid form, the growth of MoS2 is disturbed by outgassing from the PI/glass substrate (marked by number 2). The heat-annealing and capping (with PECVD SiO2) procedures prevent the outgassing phenomenon, and further absorption of water molecules and gas phase substances in the ambient environment. Consequently, a highly homogeneous film is grown (marked by number 3). b) Digital photograph images of PECVD SiO2/PI/glass substrate and fully covered MoS2 film on that substrate. The transparency is maintained owing to our low process temperature (250 ℃). c) Transmittance spectrum of directly grown MoS2 film. The PI substrate spectrum is compensated. d) PL spectra of directly grown MoS2 film. Owing to the strong luminescence from the PI substrate, measurement is conducted by transferring to a rigid SiO2/Si substrate. e,f) Cross-sectional ADF-STEM images of directly grown MoS2 film and corresponding EELS spectra of C, S and O, respectively. 163x70mm (300 x 300 DPI)



Figure 4. a,b) Schematic illustration of flexible MoS2 FET fabricated using directly grown MoS2. c) Digital photograph of flexible MoS2 FET. The image shows its original color and flexibility (inset: structure of flexible FET device, where dark brown color indicates continuous MoS2 active channel). d,e) Electrical performance of flexible MoS2 FET showing μFE = 1 cm2V-1s-1 and Ion/Ioff = 103. f–g) Durability test results of flexible MoS2 FET. The cyclic tests were conducted at three different bending radii (1, 3, and 5 mm) 113x103mm (300 x 300 DPI)


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High-Mobility MoS2 Directly Grown on Polymer Substrate with Kinetics

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