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Intrinsic transport in 2D heterostructures mediated through h-BN tunneling contacts Akshay A. Murthy, Teodor K. Stanev, Jeffrey D. Cain, Shiqiang Hao, Trevor LaMountain, Sung Kyu Kim, Nathaniel A. Speiser, Kenji Watanabe, Takashi Taniguchi, Chris Wolverton, Nathaniel P. Stern, and Vinayak P. Dravid Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b00444 • Publication Date (Web): 20 Apr 2018 Downloaded from http://pubs.acs.org on April 20, 2018
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Nano Letters
Intrinsic Transport in 2D Heterostructures Mediated through h-BN Tunneling Contacts Akshay A. Murthy,† Teodor K. Stanev,‡ Jeffrey D. Cain, †,§ Shiqiang Ho,† Trevor LaMountain,‡ Sungkyu Kim,◊ Nathaniel Speiser,‡ Kenji Watanabe,⊥ Takashi Taniguchi,⊥ Chris Wolverton,† Nathaniel P. Stern,‡ Vinayak P. Dravid *,†,◊,§
†
Department of Materials Science and Engineering, ‡ Department of Physics and Astronomy,
§
International Institute for Nanotechnology (IIN), and ◊Northwestern University Atomic and
Nanoscale Characterization Experimental (NUANCE) Center, Northwestern University, Evanston, Illinois 60208, USA ⊥
National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
*Corresponding author Vinayak P. Dravid:
[email protected] KEYWORDS: Transition metal dichalcogenides, heterostructures, tunneling contacts, scanning photocurrent microscopy, MoS2, WS2
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ABSTRACT: Understanding the electronic transport of monolayer transition metal dichalcogenides (TMDs) and their heterostructures is complicated by the difficulty in achieving electrical contacts that do not perturb the material. Typically, metal deposition on monolayer TMDs leads to hybridization between the TMD and the metal, which produces Schottky barriers at the metal semiconductor interface. In this work, we apply the recently reported hexagonal boron nitride (h-BN) tunnel contact scheme to probe the junction characteristics of a lateral TMD heterostructure grown via chemical vapor deposition. We first measure the electronic properties across the junction before elucidating optoelectronic generation mechanisms via scanning photocurrent microscopy. We find that the rectifying ratio of the encapsulated, tunnel contact scheme is almost two orders of magnitude smaller than that observed via conventional metal contact geometry, which implies that the metal/semiconductor Schottky barriers play large roles in this aspect. Furthermore, we find that both the photovoltaic as well as hot carrier generation effects are dominant mechanisms driving photoresponse, depending on the external biasing conditions. This work is the first time that this encapsulation scheme has been applied to lateral heterostructures and serves as a reference for future electronic measurements on this material. It also simultaneously serves as a framework to more accurately assess the electronic transport characteristics of 2D heterostructures and better inform future device architectures.
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Nano Letters
Transition metal dichalcogenides (TMDs) have entered the forefront of materials research due to their unique properties when spatially confined1-4. In particular, when these van der Waals materials are realized at the monolayer limit through chemical vapor deposition5,
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(CVD)
synthesis or mechanical exfoliation from bulk1, they demonstrate intriguing phenomena fundamentally distinct from their bulk counterparts. These include indirect to direct band gap transitions with decreasing layer number7-9 and coupled valley-spin physics10-12. Moreover, 2D materials can be stacked or stitched together to form van der Waal heterostructures, where each 2D layer acts in concert with the surrounding layers in order to achieve distinctive properties not exhibited by its individual constituents13-16. As a result, innovative devices have been constructed harnessing these components for the next generation of optoelectronics17-24. However, the same attributes of these atomically thin materials that have captivated the attention of scientists seeking new avenues for electronics has also proved challenging. The vertical carrier confinement that enables novel phenomena also makes the monolayer material particularly susceptible to its surrounding environment, such as chemical adsorbates or the metal contacts typically used to probe the transport properties of similar semiconducting materials. In particular, metal deposition on monolayer TMDs leads to hybridization between the neighboring TMD and metal atomic orbitals, as well as alloying of materials at the interface. This creates metal-induced gap states in the TMD layer along with charge redistribution in the hybridized material producing interfacial dipoles that modify the metal work function25-28. These features combine to pin the Fermi level within the bandgap (Fig. 1a) and lead to the formation of Schottky barriers. These barriers complicate analysis of transport through the monolayer or heterostructure while simultaneously leading to significant contact resistances and artificially diminished mobilities due to joule heating effects29. This issue is pervasive for monolayer
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TMDs, and a great deal of recent experimental work has explored methodologies to improve contact behavior30. These include phase engineering31, 2D metal contacts19, 32, one-dimensional edge contacts28, doped TMDs33, and ultra-thin tunnel barriers34-37. Of these, the last strategy has proven to be especially promising as it diminishes the metal-TMD interaction and unpins the Fermi level at the interface (Fig. 1b). As such, the Schottky barrier is replaced by a small tunneling barrier (30 meV) that allows for ohmic, field emission with applied bias36, 38.
Fig. 1. MoS2/WS2 lateral heterostructure. (a) Schematic of pinned Fermi level leading to Schottky barrier at interface. (b) Schematic of ohmic contact enabled via h-BN tunneling layer. (c) Atomic model of MoS2/WS2 lateral heterostructure, (d) Schematic of chemical vapor deposition setup. (e) Schematic of encapsulated heterojunction device.
In this study, we applied the recently developed hexagonal boron nitride (h-BN) tunnel contact scheme34, 36 to CVD-grown, MoS2-WS2 in-plane heterostructures (Fig. 1e). Through the use of these improved contacts, we were able to clarify unknown transport and optoelectronic properties at the interface of lateral heterojunctions. Samples were prepared by fully encapsulating CVD grown TMD heterostructures with a bottom layer (30 nm) and a top layer (
