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Annealing and transport studies of suspended molybdenum disulfide devices

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 105709 (http://iopscience.iop.org/0957-4484/26/10/105709) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 132.239.1.231 This content was downloaded on 15/06/2017 at 16:39 Please note that terms and conditions apply.

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Nanotechnology Nanotechnology 26 (2015) 105709 (5pp)

doi:10.1088/0957-4484/26/10/105709

Annealing and transport studies of suspended molybdenum disulfide devices Fenglin Wang, Petr Stepanov, Mason Gray and Chun Ning Lau Department of Physics and Astronomy, University of California, Riverside, CA 92521, USA E-mail: [email protected] Received 11 November 2014, revised 21 January 2015 Accepted for publication 26 January 2015 Published 20 February 2015 Abstract

We fabricate suspended molybdenum disulfide (MoS2) field effect transistor devices and develop an effective gas annealing technique that significantly improves device quality and increases conductance by 3–4 orders of magnitude. Mobility of the suspended devices ranges from 0.01 to 46 cm2 V−1 s−1 before annealing, and from 0.5 to 105 cm2 V−1 s−1 after annealing. Temperature dependence measurements reveal two transport mechanisms: electron–phonon scattering at high temperatures and thermal activation over a gate-tunable barrier height at low temperatures. Our results suggest that transport in these devices is not limited by the substrates, but likely by defects, charge impurities and/or Schottky barriers at the metal–MoS2 interfaces. Finally, this suspended MoS2 device structure provides a versatile platform for other research areas, such as thermal, optical and mechanical studies. S Online supplementary data available from stacks.iop.org/NANO/26/105709/mmedia Keywords: molybdenum disulfide, annealing, suspended device (Some figures may appear in colour only in the online journal) is a direct gap, ∼1.8 eV, and transitions to a 1.2 eV indirect gap in bulk material [11, 12, 15, 22]. Moreover, due to the large spin-orbit coupling endowed by the heavy Mo atoms and the presence to two valleys with spin-split bands [23], MoS2 has become a favorite platform for manipulating spin and valley degrees of freedom, and has demonstrated interesting optoelectronic properties such as valley selection, excitons and trions [24–27]. However, one significant drawback for MoS2-based electronics is its limited mobility [1]. Despite extensive research [1, 15, 28–31], mobility values of single layer and few-layer MoS2 field effect transistor (FET) devices have been mostly limited to 0.01–200 cm2 V−1 s−1, and the underlying mechanism of charge scattering and mobility bottleneck is not well understood. One common source of mobility impediment is the substrate, which can locally dope the few-layer atomic membranes, induce local corrugations and strains, and introduce scatterers such as charged impurities and surface phonons. In particular, Si/SiO2 substrates, on which the vast majority of MoS2 devices are fabricated, are notoriously known for their propensity to trap charges. Using alternative substrates or dielectrics such as high-κ dielectric [15] or

Two-dimensional (2D) materials have attracted a significant amount of attention over the last decade, as they provide an ideal platform for investigating behaviors of electrons and atoms in low-dimensional materials with properties ranging from semiconducting, metallic, ferromagnetic to superconducting [1]. Moreover, 2D materials often possess far superior materials properties than their bulk counterparts, thus are promising for a wide range of applications. Among the 2D materials, graphene is the most intensely studied, due to its Dirac charge carriers and extraordinary electrical, mechanical and thermal properties [2–4]. However, its gapless band structure renders graphene unsuitable for most digital electronics applications. Recently, 2D transition metal dichalcogenides have become very popular materials, such as WS2 [5–8], MoSe2 [9, 10], especially MoS2 [11–18]. Like graphene, bulk MoS2 is also a layered structure with inter-layer weak van der Waals forces [1, 19, 20]; when viewed from above, the atoms are arranged in a honeycomb lattice [21] with molybdenum and sulfur atoms occupying the sub-lattices, respectively. Unlike graphene, however, MoS2 is gapped, due to the breaking of the sub-lattice degeneracy. The band gap in monolayer MoS2 0957-4484/15/105709+05$33.00

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© 2015 IOP Publishing Ltd Printed in the UK

F Wang et al

Nanotechnology 26 (2015) 105709

Figure 1. (a), (b) Optical and AFM images of a typical few-layer

MoS2 sheet, scale bar: 10 μm. Inset: height profile indicates 8 nm thickness. (c) Schematics of the fabrication process. (d) SEM image of a suspended MoS2 device, scale bar: 1 μm.

Figure 2. (a) I–V characteristics of as-fabricated device at Vbg = 0 (blue), 10 V(green), 20 V(red) respectively. Inset: I–V at Vbg = 0 with reduced current range. (b) I–V characteristics after gas annealing at Vbg = 0(blue), 10 V(green), 20 V(red) respectively. Orange dashed line corresponds to the red curve in figure 1(a). (c) G(Vbg) of the device before (red curve) and after (green curve) gas annealing. (d) Comparison of conductance before (red) and after (green) gas annealing in samples with different thickness. (Conductance of this 4 nm thick device before annealing happens to be below the measurement’s noise floor (

Annealing and transport studies of suspended molybdenum disulfide devices.

We fabricate suspended molybdenum disulfide (MoS2) field effect transistor devices and develop an effective gas annealing technique that significantly...
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