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Multilayer Graphene−WSe2 Heterostructures for WSe2 Transistors Hao-Ling Tang,†,‡,# Ming-Hui Chiu,†,# Chien-Chih Tseng,† Shih-Hsien Yang,‡,§ Kuan-Jhih Hou,‡ Sung-Yen Wei,⊥ Jing-Kai Huang,† Yen-Fu Lin,§ Chen-Hsin Lien,*,‡ and Lain-Jong Li*,† †
Physical Science and Engineering Division, King Abdullah University of Science & Technology (KAUST), Thuwal 23955-6900, Saudi Arabia ‡ Institute of Electronics Engineering, National Tsing Hua University, Hsinchu 300, Taiwan § Department of Physics, National Chung Hsing University, Taichung 402, Taiwan ⊥ SulfurScience Technology Co. Ltd, Taipei 106, Taiwan S Supporting Information *
ABSTRACT: Two-dimensional (2D) materials are drawing growing attention for nextgeneration electronics and optoelectronics owing to its atomic thickness and unique physical properties. One of the challenges posed by 2D materials is the large source/ drain (S/D) series resistance due to their thinness, which may be resolved by thickening the source and drain regions. Recently explored lateral graphene−MoS21−3 and graphene−WS21,4 heterostructures shed light on resolving the mentioned issues owing to their superior ohmic contact behaviors. However, recently reported fieldeffect transistors (FETs) based on graphene−TMD heterostructures have only shown n-type characteristics. The lack of p-type transistor limits their applications in complementary metal-oxide semiconductor electronics. In this work, we demonstrate p-type FETs based on graphene−WSe2 lateral heterojunctions grown with the scalable CVD technique. Few-layer WSe2 is overlapped with the multilayer graphene (MLG) at MLG−WSe2 junctions such that the contact resistance is reduced. Importantly, the fewlayer WSe2 only forms at the junction region while the channel is still maintained as a WSe2 monolayer for transistor operation. Furthermore, by imposing doping to graphene S/D, 2 orders of magnitude enhancement in Ion/Ioff ratio to ∼108 and the unipolar p-type characteristics are obtained regardless of the work function of the metal in ambient air condition. The MLG is proposed to serve as a 2D version of emerging raised source/drain approach in electronics. KEYWORDS: graphene, WSe2, heterostructure, transition metal dichalcogenides, transistor, contact
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vapor deposition (CVD). Recent progresses in CVD have enabled the growth of large-area monolayer TMDs23−25 and various heterostructures, such as graphene−hBN,18 TMD− TMD,2,22 graphene−TMD,1−4 and hBN−TMD.2 In addition to the challenges in growth, the conventional bulk (3D) metal contact to 2D materials will dominate the overall transistor size and total resistance of the devices, limiting the scaling roadmap of 2D nanotechnology.1,26−29 In addition, the channel type of the 2D transistors is highly sensitive to the metal used in metal−TMD contacts. This severely hinders its application in the circuit. Very recently, several groups have explored lateral graphene−MoS21−3 and graphene−WS21,4 heterostructures using CVD synthesis, where the graphene edge contacts provide a low intrinsic device size along with a low contact
he research on two-dimensional (2D) materials has been stimulated since single-layer graphene was first isolated from a graphite crystal by mechanical exfoliation.5,6 Semimetal graphene is an ideal material for contacts and interconnections owing to its excellent conductivity.7,8 Other layered materials have also been revisited including insulators such as hexagonal boron nitride (hBN),9 metals such as NbSe2,10 and semiconductors such as black phosphorus 11,12 and transition metal dichalcogenides (TMDs).13−15 The heterostructures based on these 2D materials have become important building blocks for future nanoscale modern optoelectronics and electronics with multifunctionality.16−22 The stacking of van der Waals heterostructures in the vertical direction can be accomplished by mechanical transfer.16,17,19−21 Nevertheless, mechanical exfoliation and transfer are not realistic for large-scale production. The heteroepitaxy to fully cover one material with another or the atomic stitching of 2D materials in the lateral direction can only be achieved by epitaxial growth or spatially controlled chemical © XXXX American Chemical Society
Received: November 1, 2017 Accepted: November 28, 2017 Published: November 28, 2017 A
DOI: 10.1021/acsnano.7b07755 ACS Nano XXXX, XXX, XXX−XXX
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Cite This: ACS Nano XXXX, XXX, XXX−XXX
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ACS Nano
Figure 1. (a) Schematic illustration for the process flow to form MLG−WSe2 lateral heterostructures. (b, c) Top-view OM images presenting patterned MLG before and after WSe2 CVD growth, respectively (MLG labeled as G and WSe2 labeled as W). (d, e) Raman spatial mappings of (d) graphene G band and (e) WSe2 E′ band, respectively. (f) Raman spectra of graphene before and after WSe2 CVD growth. (g, h) Point PL and Raman spectra of WSe2, respectively. Scale bars are 5 μm.
(schematic illustration in Figure S1a)32,33 and then transferred onto c-plane sapphire substrates.34 Optical lithography and dry etching are used to form the desired graphene patterns. MLG− WSe 2 heterostructures are formed after growing WSe 2 monolayers on the regions without graphene, where we schematically illustrate the process in Figure 1a. The patterned graphene films occupy the left- and right-hand sides, and the sapphire surface is exposed in the center region as shown in the optical micrograph in Figure 1b. With our reported CVD growth (tungsten trioxide and selenium precursors are used for the CVD growth under Ar/H2),23 the WSe2 monolayer selectively fills the center region, and there is no visible gap as shown in Figure 1c. The CVD setup for the growth of WSe2 is schematically illustrated in Figure S1b, and the detailed description is given in the Methods. The photoluminescence and Raman spectroscopies are adopted to characterize the MLG−WSe2 heterostructures. Figure 1d and e are the Raman intensity mappings for the G band (1577 cm−1) of graphene and the degenerate E′ and A′1 bands for WSe2, respectively. These mappings prove that WSe2 exists only in the center region but not on the top of the graphene layers. Figure 1f compares the single-point Raman spectra of MLG before (curve i, as-patterned graphene) and after (curve ii, graphene with WSe2 stitched laterally) the area-selective WSe2 growth. The Raman peak height ratio of 2D/G (
