Centimeter-Scale CVD Growth of Highly Crystalline Single-Layer

Mar 15, 2017 - Department of Electronic Engineering and Materials Science and Technology Research Center, The Chinese University of Hong Kong, Shatin,...
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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 S Supporting Information *

ABSTRACT: MoS2 monolayer attracts considerable attention due to its semiconducting nature with a direct bandgap which can be tuned by various approaches. Yet a controllable and low-cost method to produce large-scale, highquality, and uniform MoS2 monolayer continuous film, which is of crucial importance for practical applications and optical measurements, remains a great challenge. 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 toward 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 band structure ranging from metallic to semiconducting ones for band engineering.4−6 Single-layer (SL) MoS2 has a direct bandgap of 1.8−1.9 eV7 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 a uniform large-scale sample.14,15 Lots of substantial studies have been devoted to obtaining large-scale atomically thin MoS2.16−31 Thermal decomposition © 2017 American Chemical Society

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 MoCl5,28 and Mo(CO)630 have been demonstrated to produce SL MoS2 film with higher crystallinity. However, the detected crystalline sizes inside the CVDobtained SL MoS2 continuous film have rarely been reported, with typical values at submicrometer scales,18,28,30 although separate MoS2 triangular grains with sizes up to the 100 μm 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 Received: January 10, 2017 Accepted: March 15, 2017 Published: March 15, 2017 12073

DOI: 10.1021/acsami.7b00420 ACS Appl. Mater. Interfaces 2017, 9, 12073−12081

Research Article

ACS Applied Materials & Interfaces MoS2. Lower nuclei density results in larger 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. Meanwhile, 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 pretreatment to the material34,35 is involved in previous approaches, degrading their accessibility. A facile and nondestructive 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 evidence 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 has 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-length-insensitive electrical characteristics, unambiguously showing the spatial homogeneity of the film.

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Figure 1. Experimental setup for MoS2 monolayer fabrication. (a) Schematic diagram of the CVD setup for fabricating MoS2 monolayers. The distance between sulfur and ammonium molybdate d is 35 cm. The green dashed arrow indicates the low-rate diffusion process of Mo precursor to the substrate through the gap between boats. (b) Temperature evolutions of the reaction furnace and the heating belt for sulfur evaporation. related to the reaction temperature T, the distance between sulfur and ammonium 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 singlecrystalline grains A and B. (f−h) SAED patterns taken at grain A, grain B and across the GB, respectively.

of the large-area SL MoS2 film. Raman spectra, PL spectra, and optical absorption were measured at random points of the SL MoS2 film (Figure 4e−g), showing the monolayer nature of the MoS2 film with the Raman E12g and A1g peaks’ difference of ∼17.7 cm−1 and a direct energy bandgap of ∼1.81 eV.44 The PL A exciton peak shows a narrow fwhm of ∼70 meV, which is close to exfoliated samples.7 The AFM height image taken in a randomly selected area of the film, as is shown in Figure 4h, also verifies the MoS2 film is indeed a monolayer in thickness of ∼0.7 nm. The data are highly repeatable in whichever area of the film the measurement would be conducted (Figure S4h). PL mapping at 1.81 eV and TEM measurements were conducted on the MoS2 film to precisely verify its spatial homogeneity. PL mapping was carried out in a sizable area of the continuous MoS2 film with an occasional hole having sharp edges in order to provide a PL contrast against that in the area without MoS2, as shown in Figure 4a−d. The white dashed circle indicates a typical growth seed in the film, of which morphology is similar to that in MoS2 single crystal illustrated in Figure 3a. The mapping results present quite uniform PL intensity distribution in the MoS2 film area, manifesting the homogeneous feature of the exclusively SL MoS2 film on a large scale without any domain overlapping. It is worth noting that some dark sharp lines do exist on the mapping image portraying out the GBs of MoS2, which provides a strong evidence of the SL MoS2 film forming from the interconnection of MoS2 triangular single crystals. Several previous reports have also demonstrated that the GBs of TMD monolayer islands can be detected by PL intensity contrast.22,33−35,45 The GBs of MoS2 monolayers can be regarded as local defect lines, which results in the PL intensity decrease in the GB regions.33 The density of GB is estimated to be ∼0.045 μm−1 by dividing the total GB length over the totally counted area in Figure 4d.32 Because of the high uniformity of each MoS2 grain inside the continuous film and the large PL intensity contrast between

grain and GB, the GBs can be automatically recognized using edge detection algorithms in image processing (Figure S6a). A supplementary PL mapping image taken on a sizable area of the film without holes is shown in Figure S6b. The extracted GBs labeled in Figure S6c give a density value of ∼0.05 μm−1, which agrees with the calculation from Figure 4d. The GBs can also be identified from the AFM phase image shown in Figure 4i, while they are mostly invisible in the corresponding topographic image (Figure 4h). The atomic force interaction between the AFM tip and the MoS2 GBs differs from that between the AFM tip and the crystalline MoS2, thus revealing the trace of GBs in AFM phase image. The density of GB estimated from the AFM phase image is ∼0.031 μm−1, which is consistent with that estimated from PL mapping images. The crystalline structure characterization of the SL MoS2 film in detail was obtained by HRTEM measurements. Figure 5a presents the optical image of the SL MoS2 film transferred onto a SiN TEM holey grid. Figure 5b−d shows the HRTEM images taken at various locations of the film labeled in Figure 5a with zone [001] axis, and the insets are the corresponding fast Fourier transformation (FFT) patterns, revealing the honeycomb lattice with hexagonal symmetry of the SL MoS2 having the lattice constant d100 = d010 = 0.27 nm. Meanwhile, the same spatial orientation of the MoS2 lattice illustrated in the FFT patterns indicates a typical single-crystalline grain with large scale (spatial size of at least 65 μm) inside the SL MoS2 film. SAED measurements were also carried out to confirm the large single-crystalline grains in the SL MoS2 film (Supporting Information S7). Furthermore, we investigated the lattice structure and orientation across two adjacent grains in the continuous film with sharp GBs using electron diffraction techniques. As shown in Figure 5e, the dark-field TEM image presents contrast difference between two seamlessly interconnected MoS2 grains (A and B as labeled).22 The SAED patterns taken at each grain (Figure 5f,g) demonstrate clearly 12078

DOI: 10.1021/acsami.7b00420 ACS Appl. Mater. Interfaces 2017, 9, 12073−12081

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ACS Applied Materials & Interfaces

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 single-crystalline domain (green curve) and continuous MoS2 monolayer film in zone1 (red curve) and 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.

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 materials46,47 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 was further confirmed by examining the evolution of carrier mobility as the channel length gradually increases from 10 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.

two different single-crystalline lattice orientations, as labeled by the red and blue lines. Figure 5h presents the SAED pattern taken across the GB. Two sets of hexagonal spots with the colored lines indicating each orientation were observed, resulting from the contribution of both grains. The characterization data in Figures 4 and 5 as well as Figure S7 have presented the perfect crystalline structure and high homogeneity of the as-grown SL MoS2 film, revealing that the continuous 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 the complicated and high-cost metal−organic CVD method with gas-phase precursors was adopted. Electrical Properties of the Monolayer MoS2 SingleCrystalline 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 L of MoS2 single crystal device and SL MoS2 film device are 20 and 75 μm, respectively. It is found that the MoS2 single crystal device performs an n-type behavior with a threshold voltage of about −28 V. The field effect mobility μFE is estimated to be 9.6 cm2 V−1 s−1 from the equation μFE = (L/WCgVds)(dIds/dVg), where W is the channel width and Cg 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 and 1.7 cm2 V−1 s−1 for zone 1 film and zone 3 film, respectively, due to GB induced scattering of carriers.22,32,33 Figure 6b shows the drain-source 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 process introduces wrinkles in MoS2 film which impedes the electrical transport, the obtained electrical performances of devices on

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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 micrometers, 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 12079

DOI: 10.1021/acsami.7b00420 ACS Appl. Mater. Interfaces 2017, 9, 12073−12081

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the applications of integrated circuit industry and optical measurements.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b00420. 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 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +852 3943 8286; Fax +852 2603 5558 (J.-B.X.). ORCID

Li Tao: 0000-0002-7757-1149 Zefeng Chen: 0000-0002-0689-8443 Huanjun Chen: 0000-0003-4699-009X Xinming Li: 0000-0002-7844-8417 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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. 14204616, 14207515, and CUHK Group Research Scheme, as well as Innovation and Technology Commission ITS/096/14. J. B. Xu thanks the National Natural Science Foundation of China for the support, particularly via Grant No. 61229401.

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DOI: 10.1021/acsami.7b00420 ACS Appl. Mater. Interfaces 2017, 9, 12073−12081