Abstract
Atomically thin two-dimensional semiconductors such as MoS2 hold great promise for electrical, optical and mechanical devices and display novel physical phenomena. However, the electron mobility of mono- and few-layer MoS2 has so far been substantially below theoretically predicted limits, which has hampered efforts to observe its intrinsic quantum transport behaviours. Potential sources of disorder and scattering include defects such as sulphur vacancies in the MoS2 itself as well as extrinsic sources such as charged impurities and remote optical phonons from oxide dielectrics. To reduce extrinsic scattering, we have developed here a van der Waals heterostructure device platform where MoS2 layers are fully encapsulated within hexagonal boron nitride and electrically contacted in a multi-terminal geometry using gate-tunable graphene electrodes. Magneto-transport measurements show dramatic improvements in performance, including a record-high Hall mobility reaching 34,000 cm(2) V(-1) s(-1) for six-layer MoS2 at low temperature, confirming that low-temperature performance in previous studies was limited by extrinsic interfacial impurities rather than bulk defects in the MoS2. We also observed Shubnikov-de Haas oscillations in high-mobility monolayer and few-layer MoS2. Modelling of potential scattering sources and quantum lifetime analysis indicate that a combination of short-range and long-range interfacial scattering limits the low-temperature mobility of MoS2.
Highlights
Here we developed a van der Waals heterostructure device platform where MoS2 layers are fully encapsulated within hexagonal boron nitride, and electrically contacted in a multi-terminal geometry using gate-tunable graphene electrodes
The theoretical upper bound of the electron mobility of monolayer (1L) MoS2 is predicted to be from several tens to a few thousands at room temperature (T) and exceed 105 cm2/Vs at low T depending on the dielectric environment, impurity density and charge carrier density 23–25
We have previously demonstrated that encapsulation of graphene within hexagonal boron nitride (hBN) reduces scattering from substrate phonons and charged impurities, resulting in band transport behaviour that is near the ideal acoustic phonon limit at room T, and ballistic over more than 15 μm at low T 34
Summary
We demonstrate a vdW heterostructure device platform in which an atomically thin MoS2 layer is encapsulated by hBN and contacted by graphene. The vdW heterostructure provides a standard device platform that enables us to measure intrinsic electrical transport of 2D materials and achieve high mobility 2D devices for studying the unique transport properties and novel quantum physics. By forming robust and tunable electrical contacts and dramatically reducing interfacial impurities, intrinsic electron-phonon scattering can be observed at high T, and substantially improved mobility can be achieved at low T. This enables the first observation of Shubnikov-de Haas oscillations in MoS2. Modeling and quantum lifetime analysis suggest that a combination of short-ranged and long-ranged interfacial scattering limits the low-T mobility, indicating that further improvements should be possible
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