WS2 Heterojunctions on

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Lateral Epitaxy of Atomically Sharp WSe2/WS2 Heterojunctions on Silicon Dioxide Substrates Jianyi Chen,† Wu Zhou,‡,§ Wei Tang,† Bingbing Tian,† Xiaoxu Zhao,† Hai Xu,† Yanpeng Liu,† Dechao Geng,† Sherman Jun Rong Tan,† Wei Fu,† and Kian Ping Loh*,† †

Department of Chemistry and Centre for Advanced 2D Materials, National University of Singapore, 3 Science Drive 3, 117546 Singapore ‡ Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States § School of Physical Sciences, CAS Key Laboratory of Vacuum Physics, University of Chinese Academy of Sciences, 19A Yuquan Road, Beijing 100049, China S Supporting Information *

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confined growth of WS2 occur from the edges of WO3−xprotected WSe2 monolayer (see schematic in Figure 1a). We further report the scalable synthesis of regular WSe2/WS2 arrays from lithographically patterned edges. The length of the heterojunctions can reach several millimeters, which to the best of our knowledge, is the longest reported to date for

n recent years, 2-D transition-metal dichalcogenides (TMDCs) have received great interests because of the broader possibilities offered by their tunable band gaps, as opposed to gapless graphene which precludes application in digital electronics.1,2 TMDCs exhibit an indirect-to-direct band gap transition at the single atomic sheet state as well as optically accessible spin degree of freedom in valleytronics. Applications in transistors, sensors and piezoelectricity have been demonstrated.2−4 The similar lattice parameters make them the 2-D analogs of III-V semiconductors in terms of band structure engineering. 2-D layered materials of different compositions have been used to build vertical or lateral heterostructures. Atomically precise quantum well structures for highly efficient LED have been demonstrated using vertical heterostructures,5 whereas in-plane diode has been demonstrated from lateral p−n junctions.6 For these reasons, synthesis of heterostructures continues to attract research interests. Gong et al.6 reported a single-step growth process to synthesize WS2/MoS2 heterostructures, and subsequently a few modified routes have been developed to grow other heterojunctions,7,8 in which the epitaxial growth of lateral heterostructure has been achieved by the in situ switching of the vapor-phase reactants during CVD. However, it is difficult to avoid the formation of alloy arising from a reacted interface.9,10 Alternatively, a two-step CVD process was used to prepare one type of TMDC compound first, followed by the growth of a second compound which grows from the edges of the first, producing lateral heterojuctions. Because of a better lattice matching, atomically sharp, lateral p−n junctions were only grown on sapphire single crystals.11 However, it is nontrivial to transfer them onto SiO2/Si substrates for making back-gate FETs. Thus, it is highly desired to prepare highquality p−n junctions directly on silicon dioxide substrates in view of integration compatibility with current Si microelectronic processing techniques. We have reported the growth of p-typed WSe2 crystals on SiO2 substrates.12 However, the direct applications of these crystals as growth template are hampered by the presence of point defects which are easily attacked by reactive species during the second stage growth (Figure S1). Here we report a surface-protected CVD process to grow lateral p−n heterojunctions on SiO2/Si substrates. The edge© XXXX American Chemical Society

Figure 1. (a) Schematic illustration of the growth of WSe2/WS2 lateral heterojunctions. (b) AFM image of WO3−x-protected WSe2 crystals. (c) AFM image of WSe2 crystals after annealing. (d) Optical image of WSe2−WS2 heterojunctions. (e) AFM image of a WSe2−WS2 heterojunction. Received: August 30, 2016 Revised: September 30, 2016

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DOI: 10.1021/acs.chemmater.6b03639 Chem. Mater. XXXX, XXX, XXX−XXX

Communication

Chemistry of Materials TMDC materials. Scanning transmission electron microscopy (STEM), Raman and photoluminescence (PL) studies evidence the formation of an atomically sharp interface made of spatially delineated regions with different optical signatures. Electrical measurements demonstrate that the carrier mobilities of WSe2 and WS2 crystals can reach about 30 and 40 cm2 V−1 s−1 respectively, and the p−n heterojunctions exhibit perfect current rectification behavior with a WSe2-control mode. Figure 1a shows the schematic of the surface-protected CVD method. The procedure consists of the growth of WO3−xprotected WSe2 crystals, followed by a lateral WS2 heteroepitaxy (see the SI). In WSe2 growth stage, excess H2 (15 sccm) with a constant flow of Ar (20 sccm) was introduced into the CVD system, which reacted with WO3 powders and selectively deposited a WO3−x film on WSe2 surface (Figure S2). In the subsequent WS2 growth stage, the as-made WO3−x layer, together with S powders, was used as precursors to grow WS2 crystals. Figure 1b show the AFM image of a WO3−x-protected WSe2 crystal. The height profile shows a step height of ∼0.8 nm, corresponding to monolayer WSe2. However, in contrast to previously reported results,12 the WSe2 surface becomes covered with a layer of WO3−x film whose thickness can be tuned by changing the H2 flow (Figure S3). The ultrathin WO3−x film plays a protective role for WSe2 defects during the second heating stage.13 The WO3−x film can be desorbed after heating, as verified by AFM measurements (Figure S4). The film is stable below 400 °C, and displays an apparent thermal volatility above 500 °C. At about 700 °C, most of the WO3−x film will evaporate, allowing a clean surface to be regenerated (Figure 1c). Therefore, the WO3−x film not only provides a protective coating to the WSe2 monolayer, but also it can be cleanly stripped off on demand. Figure 1d show an optical image of our heterojunctions. Discrete WSe2/WS2 crystals shows a triangular outline with clear heterostructures. The WSe2 crystals locate at the concentric regions, whereas WS2 are laterally fused to the edges of WSe2. The AFM image in Figure 1e reveals the microstructure of a typical WSe 2 /WS 2 heterojunction. The cross section contour displays a thickness of about 0.87 nm, which is evident of a monolayer heterostructure. To study the atomic structure of the as-grown monolayer film, we transferred samples onto TEM grids. Figure 2a shows a STEM annular dark field (ADF) image. The contrast difference between the left part and the right part is indicative of their different chemical composition. FFT patterns of the ADF image present a hexagonal structure (Figure 2b). However, a careful examination reveals that each diffraction spot consists of a pair of diffraction peaks (Figure 2c), corresponding to that of WSe2 and WS2 respectively. Monolayer WSe2 and WS2 share identical in-plane orientation, which favors the epitaxial growth of WS2 from WSe2 edges. Elemental mapping was recorded using electron energy-loss spectroscopy (EELS) across a WSe2/ WS2 heterojunction. The maps of Se (Figure 2d) and S (Figure 2e) show complementary spatial segregation across the interface, and their colorized overlapping image (Figure 2f) confirms the seamless connection between WSe2 and WS2. Aberration-corrected STEM-ADF imaging was used to further characterize the atomic arrangement. The image intensity increases with the atomic number of the elements, thus the atomic structure can be identified from the ADF image by quantifying the intensity of W, Se and S. As shown in Figure 2g, the bright spots corresponding to W atoms is nearly uniformly distributed, whereas the dimmer spots that correspond to Se

Figure 2. (a) Low magnification STEM ADF image of a WSe2/WS2 heterojunction. (b) FFT pattern of the ADF image. (c) Zoom-in view of the region outlined by the rectangle in panel b. (d−f) Elemental maps of Se (d) and S (e) and their colorized overlapping image (f). (g) High-resolution STEM ADF image. (h) Atomic model corresponding to the structure in panel g.

and S show obvious modulation due to the big difference between the atomic numbers of Se and S. Figure 2h shows the atomic model corresponding to the structure in Figure 2g. An atomically sharp interface between WSe2 and WS2 is formed. Almost all W atoms located at the interface were bridged to two pairs of Se atoms and one pair of S atoms, which suggest that the thermal stability of W-terminated edges is higher than that of Se-terminated edges. Duan et al.7 reported the growth of WSe2 from WS2 edges. Our results indicate that the reverse growth process can occur readily by controlling the flow rate of H2 to achieve selective selenide or sulphide growth. Due to the higher chemical reactivity of S than Se, a strong reducer like hydrogen is required in the selenization reaction of WO3, whereas WS2 deposition can occur under hydrogen-poor conditions at the edge of the pre-grown WSe2.14 Therefore, it can be appreciated that by controlling the flow of H2, the epitaxial growth of WS2 from the edges of WSe2 can avoid the formation of (WSSe) alloy at the interface. X-ray photoelectron spectroscopy (XPS) was performed to look at the binding state of W, Se and S (Figure S5). After correcting for atomic sensitivity, the W:Se:S ratio obtained from W 4f, Se 3d, S 2p XPS is about 1:0.45:1.53, suggesting that the heterojunctions are stoichiometric with some Se and S vacancies, which were reported as the dominant point defects in CVD-grown TMDs.15,16 To study their structural and optical properties, we conducted Raman and PL measurements. The spectra obtained from center and peripheral regions of a triangular heterojunction display the characteristic signals of WSe2 and WS2, respectively (Figure 3a). For WSe2, the prominent Raman peaks are located at ∼248 and ∼259 cm−1, which are assignable to the E12g and A1g modes.12,14 For WS2, the characteristic peaks corresponding to the A1g and E2g mode of WS2 monolayer are located at ∼419 and ∼353 cm−1.17 Figure 3b,c shows the mapping images centered at ∼248 and ∼353 cm−1, which demonstrate good spatial control of B

DOI: 10.1021/acs.chemmater.6b03639 Chem. Mater. XXXX, XXX, XXX−XXX

Communication

Chemistry of Materials

Figure 4. (a) IDS−VG transfer characteristics of a WSe2 FET. VDS = 0.5 V. (b) IDS−VG transfer characteristics of a WS2 FET. VDS = 1 V. The inset pictures show the IDS−VDS transfer characteristics of the devices. (c) Optical image of WSe2−WS2 devices. (d) Gate-tunable output characteristics of a WSe2/WS2 heterojunction.

Figure 3. (a) Raman spectra taken from WS2/WSe2 heterostructure. (b,c) Raman mapping images at ∼353 cm−1 (b) and at ∼248 cm−1 (c). (d) PL spectra taken from WS2/WSe2 heterostructure. (e,f) PL mapping images at ∼632 nm (e) and ∼760 nm (f). (g) Schematic illustration of the heterostructure arrays. (h,i) Corresponding optical images of heterostructure arrays grown on SiO2 (h) and Si3N4 (i).

stitching of WSe2 and WS2 forms lateral p−n diodes (Figure 4c), which display good rectification characteristics (Figure 4d). Contrary to that of previously reported WS2/WSe2 heterojunctions,8 our p-type WSe2 is in the normally “off” state at zero gate bias, with a relatively large negative turn-on voltage (Figure 4a), whereas the n-type WS2 is normally “on” due to a relatively high electron doping (Figure 4b). Therefore, current passes through the devices only when the n-type WS2 is negatively biased. The junction characteristics can be tuned by gate voltage. With increasing negative VG, the output current increases, suggesting that charge transport is partly limited by ptype WSe2. In summary, we have realized the growth of WSe2/WS2 lateral p−n heterojunctions via a surface-protected CVD method on the commonly used SiO2/Si platform. The in situ grown WO3−x capping layer protects the WSe2 monolayer from damage, and allows the edge-confined growth of lateral heterojunctions. These heterojunctions have atomically sharp interface, and display clear structural and optical characteristics. Using e-beam lithography, we have fabricated large scale WSe2/ WS2 arrays on dielectric substrate that show the promise for microelectronic processing. The ability to grow regular highdensity junctions on SiO2 will enable the integration of ultrathin 2-D electronics on the silicon platform.

chemical composition within a triangular crystal: a monolayer WSe2 domain exists in the core region, whereas WS2 locates in the peripheral region. There are clear boundaries between the two domains, but they are fused laterally by chemical bonds. Monolayer WSe2 and WS2 have a direct band gap of ∼1.65 and ∼2 eV, and their PL peaks are centered at ∼760 and ∼632 nm, respectively (Figure 3d).8,17 Similarly, the PL mapping studies (Figure 3e,f) also displayed the optical modulation of WSe2 and WS2. To be compatible with microelectronic processing, it is necessary to develops methods to fabricate a high-density of heterojunctions. Therefore, we patterned parallel channels in large-sized WSe2 crystals using EBL (see the SI and Figure 3g). We found that monolayer WS2 can nucleate and grow at these lithographically patterned edges (Figure 3h and S6), and the integrated length of heterojunctions in one WSe2 crystal can reach about 2.5 mm. The width of the WSe2 ribbons is about 5 μm, which could be further reduced by optimizing the lithography process (Figure S7). The process recipe was also tested on Si3N4 substrates (Figure 3i and S8). We have characterized the structure and optical properties of these arrays (Figure S9), in which the clear demarcations suggest the formation of parallel WSe2 and WS2 ribbons with good spatial control of chemical phase. To study their electronic quality, field-effect transistors (FETs) were fabricated on SiO2 dielectric layer using Ti/Au as the source/drain (S/D) electrodes for WS2 and Au as the electrodes for WSe2. For WSe2, IDS increases with increasing negative VG (Figure 4a), indicating p-type behavior; however, for WS2, IDS increases with increasing positive VG (Figure 4b), demonstrating n-type behavior. The inset pictures in Figure 4a,b describe the output characteristics. The linear change of the drain current (IDS) with drain voltage (VDS) indicates that contact barrier do not affect the device behavior. The on/off ratios are larger than 106, and the estimated mobilities are in the range of 5−40 cm2 V−1 s−1 for WS2, and 0.1−30 cm2 V−1 s−1 for WSe2. The mobilities of WS2 crystals are comparable with previous reports,18 and the values of WSe2 can be improved by high-k top gate dielectrics and interface engineering.12,19 The

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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/acs.chemmater.6b03639. Experimental details, Optical images, XPS, AFM, Raman and PL mapping (PDF)

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AUTHOR INFORMATION

Corresponding Author

*K. P. Loh, E-mail: [email protected]. Notes

The authors declare no competing financial interest. C

DOI: 10.1021/acs.chemmater.6b03639 Chem. Mater. XXXX, XXX, XXX−XXX

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(14) Huang, J.-K.; Pu, J.; Hsu, C.-L.; Chiu, M.-H.; Juang, Z.-Y.; Chang, Y.-H.; Chang, W.-H.; Iwasa, Y.; Takenobu, T.; Li, L.-J. Largearea synthesis of highly crystalline WSe2 monolayers and device applications. ACS Nano 2014, 8, 923−930. (15) Lin, Y.-C.; Bjorkman, T.; Komsa, H.-P.; Teng, P.-Y.; Yeh, C.-H.; Huang, F.-S.; Lin, K.-H.; Jadczak, J.; Huang, Y.-S.; Chiu, P.-W.; Krasheninnikov, A. V.; Suenaga, K. Three-fold rotational defects in two-dimensional transition metal dichalcogenides. Nat. Commun. 2015, 6, 6736. (16) Zhou, W.; Zou, X.; Najmaei, S.; Liu, Z.; Shi, Y.; Kong, J.; Lou, J.; Ajayan, P. M.; Yakobson, B. I.; Idrobo, J.-C. Intrinsic structural defects in monolayer molybdenum disulfide. Nano Lett. 2013, 13, 2615−2622. (17) Cong, C.; Shang, J.; Wu, X.; Cao, B.; Peimyoo, N.; Qiu, C.; Sun, L.; Yu, T. Synthesis and optical properties of large-area singlecrystalline 2D semiconductor WS2 monolayer from chemical vapor deposition. Adv. Opt. Mater. 2014, 2, 131−136. (18) Zhang, Y.; Zhang, Y.; Ji, Q.; Ju, J.; Yuan, H.; Shi, J.; Gao, T.; Ma, D.; Liu, M.; Chen, Y.; Song, X.; Hwang, H. Y.; Cui, Y.; Liu, Z. Controlled growth of high-quality monolayer WS2 layers on sapphire and imaging its grain boundary. ACS Nano 2013, 7, 8963−8971. (19) Chuang, H.-J.; Tan, X.; Ghimire, N. J.; Perera, M. M.; Chamlagain, B.; Cheng, M. M. -C.; Yan, J.; Mandrus, D.; Tománek, D.; Zhou, Z. High mobility WSe2 p- and n-type field-effect transistors contacted by highly doped graphene for low-resistance contacts. Nano Lett. 2014, 14, 3594−3601.

ACKNOWLEDGMENTS The authors acknowledge support from National Research Foundation, Prime Minister’s Office, under the Midsized Centre Grant (CA2DM)R-723-000-001-281. The electron microscopy work was supported in part by the U.S. Department of Energy, Office of Science, Basic Energy Science, Materials Sciences and Engineering Division (W.Z.), and through a user project at ORNL’s Center for Nanophase Materials Sciences (CNMS), which is a DOE Office of Science User Facility.

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DOI: 10.1021/acs.chemmater.6b03639 Chem. Mater. XXXX, XXX, XXX−XXX