Laterally Stitched Heterostructures of Transition Metal Dichalcogenide

Oct 31, 2016 - Laterally Stitched Heterostructures of Transition Metal Dichalcogenide: Chemical Vapor Deposition Growth on Lithographically Patterned ...
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Laterally Stitched Heterostructures of Transition Metal Dichalcogenide: Chemical Vapor Deposition Growth on Lithographically Patterned Area Henan Li,†,‡,§ Peng Li,‡,§ Jing-Kai Huang,‡ Ming-Yang Li,‡ Chih-Wen Yang,‡ Yumeng Shi,*,† Xi-Xiang Zhang,*,‡ and Lain-Jong Li*,‡ †

SZU-NUS Collaborative Innovation Center for Optoelectronic Science & Technology, Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China ‡ Physical Science and Engineering Division, King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia S Supporting Information *

ABSTRACT: Two-dimensional transition metal dichalcogenides (TMDCs) have shown great promise in electronics and optoelectronics due to their unique electrical and optical properties. Heterostructured TMDC layers such as the laterally stitched TMDCs offer the advantages of better electronic contact and easier band offset tuning. Here, we demonstrate a photoresist-free focused ion beam (FIB) method to pattern as-grown TMDC monolayers by chemical vapor deposition, where the exposed edges from FIB etching serve as the seeds for growing a second TMDC material to form desired lateral heterostructures with arbitrary layouts. The proposed lithographic and growth processes offer better controllability for fabrication of the TMDC heterostrucuture, which enables the construction of devices based on heterostructural monolayers. KEYWORDS: two-dimensional materials, heterostructure, transition metal dichalcogenides, chemical vapor deposition

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size, controllable thickness, and reproducible device performance.24 Recently, one-step or two-step CVD growth processes have been demonstrated to form lateral TMDC heterostructures, and these lateral junctions exhibit typical p−n junction properties which actually open a route for construction monolayer electronics.21,25−27 Since 2D materials are spatially separated, the lateral TMDC heterojunctions also offer the feature of easier band offset tuning.28,29 Yet, most of the TMDC synthesis can only produce randomly distributed TMDC heterostructures.30−32 The formation of the TMDC heterostructure is only limited at the edge boundaries of two adjacent TMDC monolayers without a directional control. Therefore, it is needed to explore the method to form heterojunctions with lithographic controllability; for example,

wo-dimensional (2D) atomically thin transition metal dichalcogenides (TMDCs) have been shown to exhibit unique electrical,1−5 optical,6−8 and mechanical proper9 ties. Driven by their extraordinary properties,10 TMDC monolayers have been widely studied as building blocks in compact and integrated electronic and optoelectronic systems.11,12 The semiconducting nature of TMDCs provides a high on/off current ratio in transistor switching for low-power consumption electronics.11,13 Their planar nature, good mechanical flexibility, and a reasonably large carrier mobility of TMDCs make them excellent candidates for lightweight wearable and flexible systems.14−16 The heterostructures consisting of both n- and p-type TMDC layers are the basic components required in functional devices and logic circuits.17−21 Compared with the manual transfer and stacking approach to form vertically stacked 2D layers,22,23 direct chemical vapor deposition (CVD) growth is an attractive alternative due to the possibility of large-scale mass production of various high-quality TMDC heterostructures with scalable © 2016 American Chemical Society

Received: September 26, 2016 Accepted: October 31, 2016 Published: October 31, 2016 10516

DOI: 10.1021/acsnano.6b06496 ACS Nano 2016, 10, 10516−10523

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Figure 1. (a) Schematic illustration of the formation of lateral 2D TMDC heterostructures. (b,c) OM images of CVD-grown monolayer WS2 before and after FIB patterning. (d−f) OM images of arbitrary patterns showing the FIB etching capabilities. “KAUST” logo in panel (d) is a registered trademark and service mark of King Abdullah University of Science and Technology, used with permission.

the first as-grown CVD TMDC monolayer can be patterned by lithography and followed by the growth of a second TMDC material to fill the etched area. However, it has been shown that the growth of TMDCs is initiated by the seeds,33,34 and thus the photoresist residues remaining on the etched areas act as effective nucleation sites which add more challenges for controlled synthesis of lateral heterojunctions. A patterning method free from interface contamination for TMDC nanofabrication is critical. Moreover, most reported electrical studies of TMDCs rely on conventional lithography methods, which unavoidably involve photoresist coating, exposure, and development.35 However, due to the weak van der Waals interaction between 2D TMDCs and underlying substrates, the conventional lithography normally imposes damage to the atomic structures or even delamination of the 2D TMDC layers, causing degradation of electrical properties in these ultrathin materials. Herein, we report a clean (photoresist-free), high-resolution, and location-specific lithographic method for 2D TMDC materials and the growth of inplane heterostructures of designed patterns including ultrafine features. The controlled patterning of the TMDC monolayer is realized by a photoresist-free focused ion beam (FIB) method where the FIB lithography method offers precise etching for electronic device fabrication. We show that the patterned TMDC edges serve as the front for lateral epitaxial growth of the second TMDC material. Electrical studies prove that

rectifying behaviors of the laterally stitched WSe2−WS2 heterostructures were from the WSe2−WS2 junctions. The advances in controllable patterning of ultrathin TMDC layers could offer opportunities to construct desired 2D structures for emerging concepts of devices and physics.36

RESULTS AND DISCUSSION Figure 1a schematically shows the formation of lateral 2D TMDC heterostructures, where a first-grown CVD TMDC monolayer can be patterned with FIB, followed by the CVD growth of a second TMDC material. Using a dual-beam FIB scanning electron microscope (SEM), the pristine monolayer WS2 triangles grown on sapphire substrates are imaged and located under the e-beam to avoid ion beam damage. The unwanted area can be etched precisely by the Ga+ ions to form a desired pattern. The CVD process for growing the second TMDC was then carried out in the FIB etched regions. Figure 1b,c display the optical microscopy (OM) images of the pristine WS2 and patterned WS2 (in the areas marked by the dash lines). The FIB is also capable of forming arbitrary patterns, as shown in Figure 1d−f. We further demonstrate that the edges of the FIB patterned WS2 serve as the growth seeds for the epitaxial edge of the WSe2 layers. The monolayer WS2 growth on sapphire substrates has been shown in our previous report.37 With a proper FIB etching condition, the lateral growth of WSe2 10517

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Figure 2. (a) Optical image of a WS2−WSe2 heterostructure formed by FIB patterning followed by epitaxial edge growth. (b) AFM phase image corresponding to the marked region in (a); WS2 and WSe2 regions are differentiated by the phase contrast. (c) Raman map displaying the distribution of WS2 (red) and WSe2 (blue). (d) PL measurements taken in the WS2 and WSe2 region (marked by the red and blue cross on the inset graphs) with two distinctive PL emissions; the inset shows the PL mapping of the region as shown by the optical image (a).

Figure 3. (a) SEM images of FIB patterned strips with a submicron width. The two enlarged micrographs correspond to the areas marked with red squares on the left image. (b) PL mappings for WS2 and WSe2 evidencing the presence of a thin strip (width ∼300 nm) of WSe2 across the WS2 triangle. The region is selected from the marked area on the OM image (left).

and thus form many smaller sized and merged WSe2 triangles surrounding the WS2. The WS2 and WSe2 domains can be distinguished by their optical contrast or, more obviously, in the phase image (Figure 2b) from atomic force microscope (AFM) and Raman mapping (Figure 2c), where the blue- and redcolored regions represent the WSe2 (∼250 cm−1) and WS2 (∼350 cm−1) vibration signatures.38−40 The AFM and Raman

initiated from WS2 edges can be promoted to complete the heterostructure formation.17 As shown in Figure 2a, a strip across the vertex and center of the large WS2 triangle was etched and filled with WSe2 during the second growth process. The OM image shows that the WSe2 fully covers the previously etched WS2 region. We note that the second TMDC material (WSe2) could also directly grow from the WS2 triangle edges 10518

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Figure 4. (a,b) Optical and SEM images showing partial filling of WSe2 on the FIB over-etched region after the second growth. (c) Optical images of the patterned TMDC heterostructures with increasing etching time (depth) under 8 kV. (d) PL spectra of the second TMDC material grown on the FIB etched region corresponding to the circle marks in (c).

Both the pristine and the FIB patterned WS2 edges can serve as the seeds for epitaxial edge growth of the WSe2 layers. On sapphire substrates, the second TMDC material (WSe2) can directly grow from the WS2 triangle edges, forming monolayer WSe2 triangles surrounding the WS2. However, for the epitaxial growth along the edges of FIB patterned WS2, the etching processing time needs to be controlled such that the etching stops at the stage when the TMDC layer is just completely removed. The over-etching of substrates would affect the second growth. The OM image in Figure 4a shows that the WSe2 cannot completely refill the gap when the substrate is over-etched by FIB. The SEM image in Figure 4b for the WSe2 inside the etched region indicates that both edge-promoted epitaxial growth17,42 and self-nucleation growth17,43 occur. As expected for the lateral heterostructure formation, WSe2 prefers to grow along WS2 edges.17,20,22 We also observed WSe2 growth in the center of the region. Different from the growth on a pristine c-plane sapphire, where the triangles are normally obtained, WSe2 with a round shape and irregular sawtooth edges is formed on the etched area, which is likely caused by the enhanced substrate roughness after FIB over-etching. The OM images in Figure 4c and the corresponding PL spectra in Figure 4d reveal the effect of different etching conditions on the growth of the heterostructure. Under the same operation voltage (8 kV) and current (0.11 nA), a shorter etching time (smaller etching depth) could result in under-etching (WS2 is not entirely removed) and thus the formation of WSxSey alloys during the second growth process44 as evidenced by the broadened PL peak positioned at ∼710 nm, which is between the characteristic peaks of pristine WS2 and WSe2. By contrast, a longer etching time (larger etching depth) would lead to the partial filling of the etched area. In addition to the etching time (or depth), the accelerating voltage and beam current also play important roles for FIB patterning. Especially in the case where a possible charging issue could arise from the nonconductive sapphire substrate, a proper selection of voltage and current would be essentially important. Generally, low voltages and

characterizations suggest the successful growth of heterostructures in designed patterns and the formation of a seamless WS2−WSe2 junction. The photoluminescence (PL) spectra in Figure 2d are taken from adjacent WS2 and WSe2 regions with characteristic peaks at 630 and 770 nm, respectively.40 The corresponding PL mappings of intensity distribution for the two materials are presented separately in the inset of Figure 2d. It is apparent that the PL intensity of the WSe2 formed on the FIB patterned region is relatively weaker than that epitaxially grown from the edge of the WS2. The epitaxial growth of WSe2 along the edges of WS2 results in a monolayer−monolayer lateral junction similar to the reported WS2−WSe2 heterojunction41 by one-batch CVD growth.27,29 Nevertheless, in the FIB etched areas, we find many additional small WSe2 triangles grown on top of the WSe2 monolayer across the etched area, which shall be discussed in detail later. Overall, the WSe2 grown on etched areas is not entirely monolayered, leading to a lower PL efficiency. Separate from the FIB etching technique, to form the WSe2−WS2 heterojunction using the proposed two-step growth process, it is easier to grow WS2 first followed by WSe2. We observe that WSe2 can be easily sulfurized under the growth condition of WS2 (Figure S1); hence, WS2 should be grown first to avoid the formation of WSxSey alloys. Different from the conventional lithography, the FIB etching was done directly on as-grown TMDC layers, and the submicron features can be achieved precisely. As shown in the SEM images and PL mapping in Figure 3, thin strips of WSe2 between WS2 can be observed with a minimum width of 340 nm. This one-step fast patterning method possesses significant advantages over the conventional photolithography and e-beam lithography, as the etching process does not involve any alignment marker, photoresist, mask and conductive coating, and those sophisticated and time-consuming processes. Note that the residual photoresists normally observed in conventional lithography behave as seeds for TMDC growth (Figure S2). 10519

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Figure 5. (a) AFM height image of WS2−WSe2 heterojunction (the cross-sectional height measurement is shown in Figure S3). (b) AFM phase contrast displaying the distribution of WS2 (dark brown) and WSe2 (bright yellow). (c−e) Bright-field STEM cross-sectional image of WSe2 on FIB patterned region with sapphire substrate, WS2 on the pristine sapphire substrate, and the joint edges of WSe2 multilayers and WS2 monolayer at the junction region, respectively. (f,g) High-resolution TEM images of WSe2−WS2 junction and the pristine monolayer WS2.

Figure 6. (a) Optical image of electronic devices, with Pd contacted to WSe2 and Ag contacted to WS2. (b) Electrical characteristics for the WSe2−WS2 lateral junction. Inset shows the I−V curves for WS2 (contacted by Ag S/D) and WSe2 (contacted by Pd S/D).

small currents are more likely to result in incomplete etching and unclear edges/boundaries. A high voltage or current would cause severe charging that broadens the incident focused beam or even deflects it out of the processing area. Regardless of voltages, the required etching time for a certain dose depends much on the current chosen. Although a low current can reduce the immediate charging effects, the long processing time needed could make the accumulated drift (caused by charging) more obvious, and thus the etched pattern would be broadened

or distorted and the junction will not be as sharp as those obtained by fast etching. We further exam the heterostructure formed in the FIB patterned region in greater detail. Figure 5a,b shows the AFM characterizations presenting the topography and phase contrast of the obtained WS2−WSe2 heterostructures. These AFM images show the successful growth of seamlessly stitched WS2− WSe2 heterostructure. Noticeably, the WeSe2 grown in the gap is composed of an underlying WSe2 monolayer, and some small triangle crystals (∼200 nm) also grow on the WSe2 monolayer 10520

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second step of WSe2 growth. The setup for WSe2 synthesis is similar to that for WS2, where the Ar/H2 gas flow was set at 70/5 sccm and the pressure was controlled at 10 Torr. The selenium source and center zone were heated to 230 and 925 °C, respectively, and held for 15 min for the synthesis and then naturally cooled to room temperature. FIB Patterning. A resistant-free and maskless patterning technique was adopted using an FEI Helios NanoLab 400S dual-beam FIB system. Electron and ion beams were aligned by beam shifts in order to view the same area of the specimen at the eucentric point. The pristine WS2 sample was imaged and located under an e-beam at 5 kV, 21 pA, and the patterning was done using the Ga+ ion beam dose of ∼1.9 pC/μm2 at 30 kV, 93 pA; a series of beam energies (8−30 kV) and probe currents (10 pA−1.5 nA) were attempted. TEM Characterization. The cross-sectional TEM sample was prepared by utilizing the in situ lift-out FIB technique on an FEI Helios NanoLab 400S with an Omniprobe. The TEM images were obtained using an FEI Titan ST instrument operated at 300 kV. Bright-field scanning transmission electron microscopy was performed at an acceleration voltage of 80 kV using a probe-corrected FEI Titan microscope. Spectroscopy and AFM Characterization. Photoluminescence and Raman spectra were collected using a Witec alpha 300 confocal Raman microscope with a RayShield coupler. The laser excitation wavelength was 532 nm, and the spot size was around 0.5 μm. The Si Raman peak at 520 cm−1 was used as the reference for wavenumber calibration. Objectives of 50× and 100× from Carl Zeiss Microscopy GmbH were used for PL measurements (300 lines/mm grating). The surface morphology and phase information were examined on a commercial multifunction AFM instrument (Cypher ES model from Asylum Research Oxford Instruments). Olympus (OMCL-AC240TS) Al-coated silicon cantilevers were used for AFM characterizations. The resonance frequency was ∼70 kHz; the spring constant was ∼2 N/m, and the tip curvature radius was ∼7 nm. Electronic Device Fabrication. The sample for transport property measurement was prepared by standard EBL processes using a PMMA 950 A4 (320 nm) photoresist. To avoid destroying the atomic sharp p−n interface between WSe2 and WS2, we directly nanofabricated the electrodes on the lateral heterostructure (WSe2− WS2) with a sapphire substrate, instead of transferring samples onto the most commonly used SiO2/Si substrate. Before EBL exposure, a 6 nm thick sputtered Au film was covered onto the photoresist to avoid severe charging effects of nonconductive substrates. First, the alignment markers were nanofabricated by EBL, followed by the sputtering of Ti(10 nm)/Au(40 nm) and a standard lift-off process with acetone. Next, the extra WSe2 far away from p−n junction was shaped by EBL and removed by a reactive ion etching process with oxygen, during which the p−n junction areas are protected by the PMMA photoresist. Finally, the small Pd (50 nm) electrodes for ptype WSe2 and Ag(10 nm)/Au(50 nm) for n-type WS2 were shaped by EBL separately, followed by e-beam evaporation.

(see the AFM cross-sectional height in Figure S3), forming small-domain double/multilayers. The lateral heterojunction was further investigated by the cross-sectional transmission electron microscopy (TEM), as shown in Figure 5c−g. The comparison between the FIB patterned (Figure 5c) and pristine substrates (Figure 5d) indicates that the FIB induces the damage to ∼4−15 nm deep sapphire substrates for the FIB etched area, and the ion irradiated substrate is slightly higher than the pristine substrate, due to the change of sapphire substrate (caused by ion injection, crystal structure damage, and amorphization of the sapphire surface). The junction area can thus be recognized by the change of sapphire underneath the TMDC layers. Figure 5e shows the observed continuous TMDC layer across the junction, with double or triple layers present at the FIB etched sections. Figure 5f,g displays the highresolution TEM images of the junction and pristine WS2 grown on sapphire substrates. For the device fabrication, electron beam lithography (EBL) was directly employed to make the contact electrodes on the asgrown WS2−WSe2 heterostructure on sapphire substrates (Figure 6a). Ascribed to the well-defined lateral heterostructures, the junction shows obvious rectification behaviors in the Ids−Vds curves (Figure 6b),17,45 in which current is amplified significantly with p-type WSe213,46 positively biased (across s−t). From the plot in Figure 6b, a threshold voltage can be determined about 1.2 V under forward bias. To ensure the origin of rectifying characteristics in our lateral heterostructures, the I−V curves of WSe2 contacted by Pd source/ drain (S/D) (across l−s) and WS2 contacted by Au S/D (across t−m) were checked carefully.13,17 The nearly symmetric and weak nonlinear Ids−Vds curves in the inset of Figure 6b demonstrate the negligibly small Schottky barriers between metals and TMDCs. Our electrical properties clearly illustrate the successful formation of epitaxial lateral p−n or rectifying junctions with the proposed WSe2−WS2 lateral heterostructures.

CONCLUSIONS In summary, we have developed a photoresist-free method of monolayer TMDC patterning. The patterned monolayer TMDCs can be used to form lateral TMDC heterostructures directly on the sapphire substrate. High-resolution TEM images confirm the lateral van der Waals epitaxy of TMDC layers from the patterned edges seamlessly. Devices fabricated on the synthesized TMDC heterostructure show excellent stability and obvious rectification behaviors in the I−V measurements. The photoresist-free lithography and epitaxial edge growth strategy developed here offer opportunities to construct various twodimensional lateral devices.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b06496. Figures S1−S3 (PDF)

METHODS Chemical Vapor Deposition of TMDCs. First, a WS2 monolayer was grown by the chemical vapor deposition method. The WO3 powder (0.3 g) was placed in a quartz boat located in the heating zone center of the furnace. The sapphire growth substrate was put at the downstream side, next to the quartz boat. The sulfur powder was put at the upper stream side of the furnace, and the temperature was maintained at 120 °C during the reaction. The gas flow was from Ar/ H2 (Ar = 90 sccm, H2 = 5 sccm), and the chamber pressure was controlled at 10 Torr. The center heating zone was heated to 925 °C. After the desired growth temperature was reached, the heating zone was kept for 15 min and the furnace was then naturally cooled to room temperature. After FIB patterning of the WS2 monolayer with the desired layout, the sample was then put into a separate furnace for the

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Author Contributions §

H.L. and P.L. contributed equally to this work.

Notes

The authors declare no competing financial interest. 10521

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