Hysteresis-Free High Mobility Graphene Encapsulated in Tungsten Disulfide

Abstract

High mobility is a crucial requirement for a large variety of electronic device applications. The state-of-the-art for high quality graphene devices is based on heterostructures made with graphene encapsulated in > 80 nm-thick flakes of hexagonal boron nitride (hBN). Unfortunately, scaling up multilayer hBN while precisely controlling the number of layers remains an elusive challenge, resulting in a rough material unable to enhance the mobility of graphene. This leads to the pursuit of alternative, scalable materials, which can be simultaneously used as substrate and encapsulant for graphene. Tungsten disulfide (WS2) is a transition metal dichalcogenide, which was successfully grown in large (∼mm-size) multi-layers by chemical vapour deposition. However, the resistance vs gate voltage characteristics when gating graphene through WS2 exhibit largely hysteretic shifts of the charge neutrality point (CNP) in the order of ∆n ∼2.6•10 11 cm −2 , hindering the use of WS2 as a reliable encapsulant. The hysteresis originates due to the charge traps from sulfur vacancies present in WS2. In this work, we report for the first time the use of WS2 as a substrate and the overcoming of hysteresis issues by chemically treating WS2 with a super-acid, which passivates these vacancies and strips the surface from contaminants. The hysteresis is significantly reduced below the noise level by at least a factor five (to ∆n <5•10 10 cm −2 ) and, simultaneously, the roomtemperature mobility of WS2-encapsulated graphene is as high as ∼6.2•10 4 cm −2 V −1 s −1 at a carrier density n ∼1•10 12 cm −2 . Our results promote WS2 to a valid alternative to hBN as encapsulant for high-performance graphene devices.

Two-Step Flux Synthesis of Ultrapure Transition-Metal Dichalcogenides

“…However, defects in TMDs can induce charge disorder, diminish graphene mobility, and introduce hysteresis. Recent work has shown that devices with graphene encapsulated in superacid-treated WS 2 show minimal hysteresis, reduced charge disorder ∼10 11 cm –2 , and room-temperature mobility at or near the acoustic phonon limit. Here, we use the same methodology to establish that the WSe 2 crystals described above can serve as an ultralow-disorder substrate.…”

“…On this plot, the red lines show the intersection point between the constant conductivity at low density and the linear behavior at high density, which provides a good measure of the charge inhomogeneity ( n *) in the graphene . The derived value of n * = 1.4 × 10 10 cm –2 is much smaller than that achieved in the CVT device and an order of magnitude lower than that reported for superacid-passivated WS 2 . This value is well-below the concentration of defects identified as isovalent by STM (Figure above), confirming that these defects have negligible contribution to the charge inhomogeneity in the graphene.…”