Centimeter-Scale CVD Growth of Highly Crystalline Single-Layer

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Centimeter-Scale CVD Growth of Highly Crystalline Single-Layer MoS Film with Spatial Homogeneity and the Visualization of Grain Boundaries Li Tao, Kun Chen, Zefeng Chen, Wenjun Chen, Xuchun Gui, Huanjun Chen, Xinming Li, and Jianbin Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00420 • Publication Date (Web): 15 Mar 2017 Downloaded from http://pubs.acs.org on March 16, 2017

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ACS Applied Materials & Interfaces

Centimeter-Scale CVD Growth of Highly Crystalline Single-Layer MoS2 Film with Spatial Homogeneity and the Visualization of Grain Boundaries Li Tao†, Kun Chen†, Zefeng Chen†, Wenjun Chen‡, Xuchun Gui‡, Huanjun Chen‡, §, Xinming Li† and Jian-Bin Xu*† †

Department of Electronic Engineering and Materials Science and Technology Research Center,

The Chinese University of Hong Kong, Shatin, N.T., Hong Kong SAR, China ‡

State Key Lab of Optoelectronic Materials and Technologies, School of Electronics and

Information Technology and § Guangdong Province Key Laboratory of Display Material, Sun Yat-sen University, Guangzhou 510275, China

ABSTRACT MoS2 monolayer attracts considerable attention due to its semiconducting nature with a direct bandgap which can be tuned by various approaches. Yet controllable and low-cost method to produce large-scale, high-quality and uniform MoS2 monolayer continuous film, which is of crucial importance for practical applications and optical measurements, remains a great challenge.

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Most previously reported MoS2 monolayer films had limited crystalline sizes, and the high density of grain boundaries inside the films greatly affected the electrical properties. Herein, we demonstrate that highly crystalline MoS2 monolayer film with spatial size up to centimeters can be obtained via a facile chemical vapor deposition method with solid-phase precursors. This growth strategy contains selected precursor and controlled diffusion rate, giving rise to the high quality of the film. The well-defined grain boundaries inside the continuous film, which are invisible under an optical microscope, can be clearly detected in photoluminescence mapping and atomic force microscope phase images, with a low density of ~0.04 µm–1. Transmission electron microscopy combined with selected area electron diffraction measurements further confirm the high structural homogeneity of the MoS2 monolayer film with large crystalline sizes. Electrical measurements show uniform and promising performance of the transistors made from the MoS2 monolayer film. The carrier mobility remains high at large channel lengths. This work opens a new pathway towards electronic and optical applications, and fundamental growth mechanism as well, of the MoS2 monolayer. KEYWORDS: Molybdenum disulfide, continuous film, high crystallinity, grain size, grain boundary, chemical vapor deposition

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INTRODUCTION Though graphene opens the door of two-dimensional (2D) materials,1-3 its zero-bandgap electronic structure causes the poor on/off ratio in transistor devices, which severely limits its potential in semiconductor industry. Recently, 2D transition metal dichalcogenides (TMDs), among which MoS2 is representative, have attracted considerable attention because of their tunable bandstructure ranging from metallic to semiconducting ones for band engineering.4-6 Single-layer (SL) MoS2 has a direct bandgap of 1.8-1.9 eV,7 and is an ideal material for valleytronics, strain engineering and optoelectronics.8-13 Pursuing a simple and repeatable method to prepare large-area SL MoS2 film is of crucial importance for applications. Moreover, attainable characterization of optical properties such as the complex dielectric function of 2D MoS2, which provide valuable information on light-matter interaction inside the material, requires uniform large-scale sample.14-15 Lots of substantial studies have been devoted to obtaining large-scale atomically thin MoS2.16-31 Thermal decomposition of ammonium thiomolybdate has been proposed to gain scalable multilayer MoS2 film, but with nanoscale crystalline size due to the absence of grain growth process.16 Chemical vapor deposition (CVD) methods using Mo precursors such as MoO3,18-24 MoCl528 and Mo(CO)630 have been demonstrated to produce SL MoS2 film with higher crystallinity. However, the detected crystalline sizes inside the CVD-obtained SL MoS2 continuous film have rarely been reported, with typical values at sub-micron scales,18, 28, 30 although separate MoS2 triangular grains with sizes up to hundred-micron level can be achieved.22-24 Also, some of the approaches require pretreated substrate as growth template29 or highly toxic precursors,28,

30

which hinders their

practical potentials. There is a competition between the continuous film dimension and the crystalline size inside the film in most reported SL MoS2. Lower nuclei density results in larger

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single-crystalline MoS2 domain, but it gets hard to form continuous film; Higher nuclei density promotes the form of continuous film, but the MoS2 crystalline size degrades as the cost. To our knowledge, there still lacks growth process of uniform SL MoS2 film with both large-scale continuity and high crystallinity on bare SiO2 substrate, which can be much more low-cost and easier. On the other hand, intensive studies have focused on grain boundaries (GBs) of two adjacent MoS2 grains, which can generate degradation of electrical properties of the material.21-22, 32-33

However, much less is known about the grain sizes and GBs inside the continuous SL MoS2

film at a large scale, and some advanced technique (e.g., high-resolution electron diffraction)18, 30 or irreversible pre-treatment to the material34-35 is involved in previous approaches, degrading their accessibility. A facile and non-destructive method to observe the GBs inside the film can bring benefits to GB engineering of MoS2 and is thus urgently needed. In this Article, we report on a facile, controllable, low-cost and eco-friendly CVD growth technique with selected precursor and controlled diffusion rate for producing highly crystalline centimeter-scale pure SL MoS2 film which exhibits promising electrical transport properties. Strong evidences including photoluminescence (PL)/Raman mapping, atomic force microscope (AFM), transmission electron microscopy (TEM) and direct optical contrast images of the MoS2 triangles and continuous film have been provided for demonstrating the spatial homogeneity of the film across a large scale. The SL MoS2 film is formed from the seamless interconnection of sizable MoS2 single crystals, and the sharp GBs with low density can be clearly identified in PL mapping and AFM phase images, resulting from their nature of local defects. Furthermore, transistors made from the SL MoS2 film exhibit promising, uniform and channel-lengthinsensitive electrical characteristics, unambiguously showing the spatial homogeneity of the film. 4


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EXPERIMENTAL SECTION Sample fabrication We have developed a home-built CVD system to fabricate high-quality SL MoS2 films. Figure 1a shows the schematic diagram of the experimental setup. A quartz boat holding high purity sulfur powder of 0.5 g were placed in the low-temperature upwind zone of a 2-inch CVD quartz tube wrapped with a heating belt. Another quartz boat holding ammonium molybdate ((NH4)6Mo7O24) solid were placed face-down in a large quartz boat in the high-temperature reaction zone of the quartz tube which was 35 cm away from the sulfur powder. The (NH4)6Mo7O24 solid was obtained from dehydrating 10 mg/ml (NH4)6Mo7O24 solution of 0.6 ml by heating at 250 °C for 30 min. An ultraclean silicon substrate covered atop a 300 nm-thick silicon dioxide (SiO2/Si substrate) with the size ~1.2 cm × 1.2 cm preprocessed by oxygen plasma treatment was placed in the large quartz boat leaning against to the boat with ammonium molybdate. Note that the cleanness of substrate gives rise to the ideal deposition environment, which is crucial for obtaining high-quality samples (Supporting Information S1). During the synthesis, the reaction furnace was heated to 850 °C at a rate of 20 °C/min in 100 sccm argon environment at the pressure of ~200 Pa. The sulfur powder began to be heated to 200 °C when the reaction furnace reached the temperature of 750 °C. After the growth of MoS2 for 20 min, the flow rate of argon was changed into 500 sccm for rapid cooling and precursor residue removal. Figure 1b gives the detailed furnace temperature evolutions of both the reaction zone and the upwind zone. The growth parameters should be carefully tuned to reach the optimized conditions. The quality of the as-grown MoS2 is significantly influenced by the precursor deposition rate which is related to the reaction temperature T, the distance between sulfur and ammonium 5



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molybdate d and the amounts of precursors (Supporting Information S1). In comparison to previous reports,18-19 we use ammonium molybdate powder as Mo precursor instead of MoO3 powder for the reason that ammonium molybdate can provide uniform MoO3 at relatively low temperature ( 65 µm) in the MoS2 monolayer film. (e) Dark-field TEM image of the MoS2 monolayer film. The contrast difference reveals two single-crystalline grains A and B. (f-h) SAED patterns taken at grain A, grain B and across the GB, respectively.

The characterization data in Figures 4, 5 and S7 have presented the perfect crystalline structure and high homogeneity of the as-grown SL MoS2 film, revealing that the continuous

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film is formed from large single-crystalline MoS2 triangular domains merging together with well-defined GBs. The grain sizes are much larger than those reported in Ref. 30, where complicated and high-cost metal-organic CVD method with gas-phase precursors was adopted.

Electrical properties of the monolayer MoS2 single-crystalline domain and continuous film Electrical transport properties are essential to semiconductor materials. We have transferred the as-grown MoS2 to another SiO2/Si substrate by polymer-protected method (Supporting Information S8) to prevent potential damage of substrate during MoS2 fabrication at high temperature. Bottom-gated MoS2 field-effect transistor (FET) devices were fabricated by a standard photolithography process. Figure 6a gives the transfer curves for FET devices made from MoS2 single-crystalline domain and continuous SL MoS2 film in both zone1 and zone 3 on the substrate. The channel lengths of MoS2 single crystal device and SL MoS2 film device are 20 µm and 75 µm, respectively. It is found that the MoS2 single crystal device performs an ntype behavior with a threshold voltage of about –28 V. The field effect mobility is estimated $

+,)*

' ()*

+('

to be 9.6 cm V " s " from the equation = %&

, where - is the channel width and ./

is the gate capacitance. The on/off ratio is up to ~105. For the SL MoS2 film devices, the estimated mobilities decrease to 3.7 cm V " s " and 1.7 cm V " s " for zone 1 film and zone 3 film, respectively, due to GB induced scattering of carriers.22, 32-33 Figure 6b shows the drainsource current Ids vs. the drain source voltage Vds at various gate voltages Vgs. The linear dependent Ids-Vds curves at low drain-source voltages indicate good ohmic contacts between MoS2 and gold electrodes. All the measurements of FET device properties were carried out at room temperature. By considering the large channel length chosen and the fact that transfer

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process introduces wrinkles in MoS2 film which impedes the electrical transport, the obtained electrical performances of devices on both MoS2 single crystal and SL MoS2 film are competitive in comparison to previous results taken from CVD MoS2,16, 19, 28 making our MoS2 promising for electronic and optoelectronic applications. The MoS2 device is expected to achieve higher mobility by dielectric engineering via utilizing high-k dielectric materials,46-47 and by interface engineering to suppress carrier scattering.48 Spatial uniformity of the FET device characteristics on SL MoS2 film has been demonstrated by probing additional FET devices at random locations over centimeter scale of the film (Supporting Information S9), which enables large-scale batch fabrication of numerous MoS2 FET arrays for integrated circuit applications. The homogeneity of the SL MoS2 film is further confirmed by examining the evolution of carrier mobility as the channel length gradually increases from 10 µm to 75 µm, as shown in Figure 6c. The mobility shows insensitivity to the channel length, resulting from the high uniformity of the density of GB in the SL MoS2 film. The promising and stable transport properties of the SL MoS2 film even at large channel lengths facilitate its practical applications for large-scale electronic and optoelectronic devices.

Figure 6 Electrical transport properties of the MoS2 monolayer devices. (a) Sheet conductance vs. gate voltage curves (transfer curves) for FET devices made from MoS2 singlecrystalline domain (green curve) and continuous MoS2 monolayer film in zone1 (red curve) and

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zone 3 (gold curve) on the substrate, respectively. Insets are optical images of the MoS2 single crystal device with the channel length of 20 µm (upper left) and the MoS2 monolayer film device with the channel length of 75 µm (middle left), respectively. Scale bars: 50 µm. (b) Drain-source current vs. drain-source voltage curves of the MoS2 single crystal device at different gating voltages. (c) Field-effect mobility of carriers in MoS2 monolayer film (zone 1) as a function of the channel length.

CONCLUSION In conclusion, we have developed a facile and low-cost CVD method with all solid-phase precursors to produce large-area, highly crystalline and uniform MoS2 monolayer film, and highquality sizable MoS2 single-crystalline domains as well. The dimensions of the continuous MoS2 film and discrete MoS2 domain are up to scales of centimeters and hundred microns, respectively. The growth process exclusively produces MoS2 monolayers, which is verified by various techniques including Raman spectroscopy, AFM, TEM and PL spectroscopy. Moreover, the highly crystalline nature of the MoS2 samples is confirmed by Raman mapping, AFM images, PL mapping and TEM/SAED images. The large-area MoS2 monolayer film is formed from triangular MoS2 single crystals in large sizes coalescing seamlessly, and the well-defined GBs with a low density of ~0.04 µm–1 are clearly detected. Electrical transport measurements conclude the high uniformity and promising electrical properties of the MoS2 FETs on the monolayer film. The carrier mobility remains high at large channel lengths. Uniform MoS2 monolayer film with such a large scale and low density of GBs is of essential importance in the applications of integrated circuit industry and optical measurements.

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ASSOCIATED CONTENT Supporting Information Predominant factors related to the quality of CVD MoS2; Mo precursor and quartz boat configuration optimization; MoS2 deposited on substrates beyond SiO2/Si; Large-scale identity of MoS2 monolayer film; Tip-notched triangular MoS2 domain; Identification of grain boundaries from PL mapping image; SAED measurements of the SL MoS2 film; MoS2 transfer process; Additional transfer curves of MoS2 FETs. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Tel: +852 3943 8286; Fax: +852 2603 5558.

ACKNOWLEDGMENT We are thankful to Dr. Ning Ke and Ms. Ya Deng for technical support. The work is in part supported by Research Grants Council of Hong Kong, particularly, via Grant Nos. N_CUHK405/12, 14207515, and CUHK Group Research Scheme, as well as Innovation and Technology Commission ITS/096/14. J. B. Xu would like to thank the National Natural Science Foundation of China for the support, particularly, via Grant No 61229401.

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Centimeter-Scale CVD Growth of Highly Crystalline Single-Layer

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