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Synthesis of Ultrathin Composition Graded Doped Lateral WSe/WS Heterostructures 2

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Zhipeng Li, Jialu Zheng, Yupeng Zhang, Changxi Zheng, Wei-Yen Woon, Min-Chiang Chuang, HungChien Tsai, Chia-Hao Chen, Asher Davis, Zai-Quan Xu, Jiao Lin, Han Zhang, and Qiaoliang Bao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08668 • Publication Date (Web): 11 Sep 2017 Downloaded from http://pubs.acs.org on September 12, 2017

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ACS Applied Materials & Interfaces

Synthesis of Ultrathin Composition Graded Doped Lateral WSe2/WS2 Heterostructures ∥

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Zhipeng Li,†, Jialu Zheng,†, Yupeng Zhang, *,# Changxi Zheng, † Wei-Yen Woon, ‡ Min-Chiang Chuang, ‡ Hung-Chien Tsai,‡ Chia-Hao Chen, § Asher Davis,† Zaiquan Xu,† Jiao Lin,∆ Han Zhang, # and Qiaoliang Bao*, †,□ †

Department of Materials Science and Engineering, and ARC Centre of Excellence in Future Low-Energy Electronics Technologies (FLEET), Monash University, Clayton, Victoria 3800, Australia # College of Electronic Science and Technology, and College of Optoelectronics Engineering, Shenzhen University, China ‡ Department of Physics, National Central University, Jungli, 32054, Taiwan, ROC. § National Synchrotron Radiation Research Center, Hsinchu, 30076, Taiwan, ROC. ∆ School of Engineering, RMIT University, Melbourne, VIC 3001, Australia □ Institute of Functional Nano and Soft Material (FUNSOM), Jiangsu Key Laboratory for Carbon-Cased Functional Materials and Devices, and Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215123, China

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Abstract Lateral transition metal dichalcogenide and their heterostructures have attracted substantial attention, but there lacks a simple approach to produce large-scaled optoelectronic devices with graded composition. In particular, the incorporation of substitution and doping into heterostructure formation is rarely reported. Here, we demonstrate growth of a composition graded doped lateral WSe2/WS2 heterostructure by ambient pressure chemical vapor deposition in single heat-cycle. Through Raman and photoluminescence spectroscopy, we demonstrate that the monolayer heterostructure exhibits a clear interface between two domains and a graded composition distribution in each domain. The coexistence of two distinct doping modes, i.e., interstitial and substitutional doping, was verified experimentally. A distinct three-stage growth mechanism consisting of nucleation, epitaxial growth and substitution was proposed. Electrical transport measurements reveal that this lateral heterostructure has representative characteristics of a photodiodes. The optoelectronic device based on the lateral WSe2/WS2 heterostructure shows improved photodetection performance in terms of a reasonable responsivity and a large photoactive area Keywords: transition metal dichalcogenides, two-dimensional material, chemical vapor deposition, heterostructure, substitution, optoelectronic devices.

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1. INTRODUCTION Atomically thin transition metal dichalcogenides (TMDCs), such as monolayer tungsten disulfide (WS2) and tungsten diselenide (WSe2), have attracted substantial attention in recent years1-5. They are considered as counterparts of graphene with a high on/off ratio and a relatively high mobility6-8. Furthermore, monolayer TMDCs exhibit appealing electrical and optical properties, such as higher photoluminescence quantum efficiency, a large exciton binding energy, and an indirect to direct bandgap transition, which makes them notably different from the bulk counterparts.3,

9-10

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fabrication of heterostructures of two-dimensional TMDCs affords more tunability of the electrical and optical properties by restructuring the atomic layer materials. In particular, one may achieve a desired band alignment to create a built-in electric field that facilitates the separation of photocarriers, which is intriguing for light harvesting or detection. Recently, TMDC heterostructures have been incorporated into several devices, including light-emitting diodes (LEDs)4, 11, field effect transistors (FETs)9,

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and p-n diodes13. However, the controllable large-scale production of high quality

two-dimensional (2D) TMDC heterostructures remains challenging. More recently, increasing efforts have been made to realize numerous types of heterostructures with various geometries and band gap alignments. The direct transfer technique is considered to be a simple method for creating heterostructures with various combinations, and this method is widely used for proof-of-concept demonstrations of optoelectronic devices13-15. However, it lacks reasonable accuracy in the horizontal dimension based on the state-of-the-art technologies, and hence, only vertical heterostructures can be produced by this method. Recently, the two-step chemical vapor deposition (CVD) method has been successfully used to synthesize lateral WS2/MoS2 heterostructures. However, this method is considerably expensive and time-consuming because it requires two cycles of heating and cooling16-19. In a specially designed CVD system, a simplified method has been developed to produce lateral heterostructures of WS2/WSe2 and MoSe2/MoS2, which form lateral p-n diodes, with excellent current rectification behavior and photocurrent generation characteristics18, 20-21. Notably, the TMDC heterostructures produced by existing CVD methods exhibit oversized band bending, which introduces some disadvantages for carrier transport. Therefore, engineering the band alignment in TMDC heterostructures is a nontrivial task. Li et al. 22 reported the continuous lateral growth of composition graded bilayer MoS2(1–x)Se2x alloys, which exhibit a continuously modulated bandgap, along single triangular nanosheets using an improved chemical vapor deposition approach. More effort is required to fabricate graded composition TMDC heterostructures that exhibit graded band alignment for functional nanoelectronic and optoelectronic devices.



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In this study, we demonstrate that a monolayer composition graded doped lateral WSe2/WS2 heterostructure can be synthesized by growth method using a common CVD system with single heating cycle. Compared to previous works, a single-step technique rather than a two-step method is applied in an ambient pressure system, which results in a simpler and cheaper operation. Importantly, elemental substitution is introduced during the CVD growth, which enables a graded composition distribution and results in a tunable bandgap along single triangular nanosheets. This favors the separation and transport of photo-excited carriers along with an increase in the working area of the devices.

2. EXPERIMENT SECTION 2.1 Material growth. The growth of the WSe2/WS2 heterostructure was performed in a typical CVD quartz tube that is isolated from the external environment. Argon gas at atmospheric pressure was introduced as the carrier gas (flow rate: 250 sccm) accompanied with hydrogen gas (flow rate: 5 sccm) for the reaction. In a typical experiment, a mixture of sulfur powder (100 mg, Sigma-Aldrich, 213292) and selenium powder (500 mg, Sigma-Aldrich,) was loaded upstream and heated independently with a heating belt. WO3 (25 mg, Sigma-Aldrich 204781) was placed at the exact center of the furnace, and the substrates (sapphire or SiO2/Si) were placed downstream of the furnace. The heating belt and furnace are heated with a pre-set profile. The furnace was then cooled naturally with the cover open. 2.2 Device fabrication. The device fabrication includes plasma etching, ultraviolet (UV) lithography and electron-beam deposition. Plasma etching (SF6) was first used to remove a corner of TMDs in 20 second. Afterwards, the WS2/WSe2 heterostructure was coated with the photoresist (AZ 1512HS), and the pattern of the electrode was designed and placed directly on the heterostructure flake, which was grown on the SiO2/Si substrates using UV-light direct write lithography (Intelligent micropatterning, SF100 XPRESS). The highly doped n-type silicon of the SiO2/Si substrate serves as the back gate electrode. Subsequently, electron-beam evaporation was used to deposit the source and drain electrodes with Ti/Au (50 nm Au on top of 5 nm Ti) under vacuum (6×10−6 Torr). 2.3 Characterizations. Raman, photoluminescence (PL) and photocurrent spectra and image measurements were performed with a confocal microscope system (WITec, alpha 300R) with a 100× objective (NA = 0.9) at ambient pressure. Samples were placed on a crystal-controlled scanning stage and excited by a low-power visible light laser (532 nm, 50 µW). A grating of 600 line/mm was then used to collect the spectral data. The height and topology of the heterostructure were characterized by atomic force microscopy (AFM) (Bruker, Dimension Icon SPM) with tapping mode.


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The work function was measured by Kelvin probe force microscopy (KPFM), which is conducted on the same AFM instrument with a charge-contained tip. For X-ray photoelectron spectroscopy (XPS) and scanning photoelectron microscopy (SPEM) analysis, focused soft X-rays (photon energy hν = 380 eV, beam diameter ϕ = 100 nm) generated on the beam line (SPEM end station, 09A1) were used, and subsequently, the TMDC heterostructure films were transferred to an oxide-buffered SiO2/Si wafer and kept in ultrahigh vacuum (10−9 Torr). The electrical measurements were performed by a two-channel source meter unit (Agilent, B2902a) at room temperature in ambient conditions. 3. RESULTS AND DISCUSSION The typical experimental setup for synthesizing the monolayer WSe2/WS2 heterostructure is illustrated in Figure 1a. A mixture of sulfur powder and selenium powder was loaded upstream and heated independently with a heating belt; this functioned as a chalcogen source in the first heating region. Tungsten trioxide powder was then loaded in the center of the tubing furnace, which is denoted as the second heating region. Sapphire or SiO2/Si substrates were placed downstream of the furnace next to the tungsten source. The heating temperature profiles of the first and second heating regions are indicated in Figure 1b. During the growth, sulfur and selenium mixture was heated to 180°C for approximately 20 mins and then ramped to 280°C until the end of experiment. Meanwhile, WO3 (located in furnace) was heated to 900°C for approximately 30 mins, and the temperature was maintained for more than 2 hours until a natural cooling down occurred. The argon carrier gas was flown at a flow rate of 250 sccm with the addition of 5 sccm of H2, and the system was operated under ambient pressure. During the reaction, the S or Se vapors are designed to react with the vaporized WO3 to form WSe2 or WS2, respectively, and the products are deposited downstream on the substrate downstream. Because the melting points of S and Se are different, the ramping of the heating temperature from 180°C to 280°C in the first heating region is the key step that enables the varied composition distribution of S and Se in the vapor. Figure 1c shows a representative optical image of the monolayer WSe2/WS2 lateral heterostructure on a sapphire substrate synthesized by the aforementioned method. The contrast difference between the center and the periphery indicates the presence of different materials along the entire flake, and the clear interface suggests the successful synthesis of the heterostructure. The thickness and area distributions of the resulting heterostructures are illustrated in Figure 1d. It is observed that most of the resulting nanosheets are single layered heterostructures with an area of approximately 100-200 µm2. Raman and PL spectroscopy were performed to further probe the composition difference in the heterostructure. The Raman spectra at different positions (the edge and



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central region) are presented in Figure 1e from excitation by a 532 nm laser. The characteristic peaks of WS2 at 420 cm−1 and 350 cm−1, which correspond to the A1g and E12g resonance modes of WS223, and the characteristic peak of WSe2 at about 260 cm−1, which is attributed to the A1g resonance mode of WSe224, can be clearly identified over the entire flake. The variation of the peak intensities in the spectra obtained from the center and periphery indicates the composition difference. Figure 1f depicts the PL spectra of the WS2/WSe2 heterostructure excited by the same laser. The PL spectra from the center and periphery exhibit strong emission peaks centered at ∼655 nm and ~700 nm, respectively, which correspond to monolayer WS2 and WSe2. The redshift of the WS2 peak and the blueshift of the WSe2 peak presumably originate from the cation doping effect. These results confirm that two distinct materials, WSe2 and WS2, exist in the entire flake, but the composition of the different regions vary. The Raman images for the integrated intensity of the A1g resonance mode of WS2 and WSe2 are depicted in Figure 1g and 1h to further reveals the spatial distribution within the triangular domain. WSe2 comprises the majority of the central region, and WS2 is primarily in the peripheral region of the triangular domain. AFM topography image shows that this 2D domain has a uniform thickness of ~0.9 nm, as observed in Figure 1i, which indicates the monolayer thickness and in-plane nature of the heterostructure. The phase image (Figure 1i) is also recorded simultaneously, and a clear interface was observed between both materials because of their different adhesion of the surface. The distinct phase contrast indicates two different materials. Based on the characterization results above, we can conclude that a perfect lateral heterostructure of WSe2/WS2 has been produced. To investigate the morphology change and migration of the heterostructure, Raman imaging was performed on the samples’ typical flakes for different growth times, as shown in Figure 2. These three samples were prepared by adjusting the temperature holding time (30-120 mins) after the temperature in the first heating region was increased to 280°C. We observe that the ratio between the areas of WSe2 and WS2 increases when the holding time is increased. When the holding time is 30 mins, only a small area of WSe2 can be found in the central region (Figure 2a-c). When the holding time is 60 mins, large areas of WSe2 were observed in the central region (Figure 2d-f). When the holding time increases to 120 mins, a larger area of the WSe2 triangle with only a small region of WS2 at the edge can be observed (Figure 2g-i). The detailed modification of size, area ratio of location of the interface are depict in Table S1-S2 (see Support Information) by adjusting the growth time and temperature, which could also provide insight into the growth mechanism. It is proposed that the one-step growth of the lateral WSe2/WS2 heterostructure can be roughly divided into three stages, i.e., nucleation, epitaxial growth and substitution, as schematically shown in Figure 3a-c. First,


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when the chalcogen source was heated to 180°C, which is lower than the melting point of Se (217 °C), only sulfur can be vaporized from the chalcogen source, and only pure WS2 sheets are nucleated and formed (Figure 3a). Subsequently, when the temperature of the chalcogen source increases to 280 °C, Se begins to melt, and it is vaporized. Afterward, the steam of the S/Se mixture reduces WO3, and the Se-interstitially doped WS2 grows epitaxially in the peripheral region (Figure 3b). Lastly, upon the complete consumption of S in a short time (

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