Electrical Transport Properties of Single-Layer WS₂

TLDR

WS₂’s promising electronic properties and strong spin–orbit coupling make it attractive for future electronic, optical, and valleytronic devices. The study reports the fabrication of field‑effect transistors from single‑layer and bilayer WS₂ and investigates their electronic transport properties. The authors fabricated WS₂ FETs, performed long in situ annealing to improve contact transparency, and carried out four‑terminal measurements to probe pristine electronic transport. They found that doping depends on the environment, long annealing restores pristine properties, the devices are n‑type with on/off ratios ~10⁶, show metallic behavior and mobilities up to 140 cm²/Vs (over 300 cm²/Vs in bilayers), and exhibit variable‑range hopping in the insulating regime with a ~2 nm localization length that grows as the Fermi level enters the conduction band.

Abstract

We report on the fabrication of field-effect transistors based on single layers and bilayers of the semiconductor WS2 and the investigation of their electronic transport properties. We find that the doping level strongly depends on the device environment and that long in situ annealing drastically improves the contact transparency, allowing four-terminal measurements to be performed and the pristine properties of the material to be recovered. Our devices show n-type behavior with a high room-temperature on/off current ratio of ∼106. They show clear metallic behavior at high charge carrier densities and mobilities as high as ∼140 cm2/(V s) at low temperatures (above 300 cm2/(V s) in the case of bilayers). In the insulating regime, the devices exhibit variable-range hopping, with a localization length of about 2 nm that starts to increase as the Fermi level enters the conduction band. The promising electronic properties of WS2, comparable to those of single-layer MoS2 and WSe2, together with its strong spin–orbit coupling, make it interesting for future applications in electronic, optical, and valleytronic devices.