Electrical Transport Properties of Single-Layer WS2

Abstract: We report on the fabrication of field-effect transistors based on single 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 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/Vs at low temperatures (above 300 cm2/Vs in the case of bi-layers). 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.

• The paper demonstrates that annealing transitions WS2 FETs from p-type to n-type behavior, significantly enhancing mobility and contact transparency.
• Experimental results indicate an insulating-to-metallic transition with varying gate voltage, underlining WS2's potential in advanced nanoelectronics.
• The study provides a rigorous methodology for isolating intrinsic electrical properties, establishing WS2 as a competitive material for future electronic and valleytronic devices.

Electrical Transport Properties of Single-Layer WS$_2$

The paper conducts a comprehensive analysis of the electrical transport properties of field-effect transistors (FETs) using single-layer and bilayer tungsten disulfide (WS$_2$). Transition metal dichalcogenides (TMDs), notably single-layer MoS$_2$, have garnered substantial attention for their promising electronic properties, yet WS$_2$ has emerged as a formidable candidate in this category, primarily due to its direct band gap and strong spin-orbit coupling. This study elucidates the potential of WS$_2$ through an in-depth exploration of its electronic transport dynamics under various environment-induced conditions.

Experimental Setup and Methodology

The researchers fabricated FETs with distinct metal contacts on WS$_2$ flakes and studied their electronic performance under different environmental conditions, including in air and vacuum. The primary focus was the modulation of transport characteristics by annealing, which removes adsorbates and atmospheric contaminants. This process allows for a more accurate assessment of the intrinsic electrical properties of the WS$_2$ single layer.

Results and Analysis

Performance Enhancement through Annealing

One of the critical findings is the impact of prolonged annealing in vacuum on device performance. Before annealing, the devices exhibited p-type behavior with adversely affected conductance due to adsorbates. After annealing, not only was there a shift towards a more n-type character, but the contact transparency significantly improved, allowing precise four-terminal measurements. The sheet conductivity and field-effect mobility were notably enhanced, with room-temperature mobilities reaching approximately 50 cm$^2$/Vs and saturating at lower temperatures around 140 cm$^2$/Vs for single-layers, more pronounced in bilayers.

Insulating vs. Metallic Behavior

Temperature-dependent measurements highlighted a transition from insulating to metallic behavior as gate voltage increased, with a crossover region around certain voltage thresholds. This insulating-to-metallic transition aligns with trends observed in similar TMD materials, most notably MoS$_2$.

Implications of Doping and Annealing

The study reveals a significant dependency on in-situ annealing to counterbalance the doping effects instigated by environmental adsorbates. This procedure enhances carrier injection efficiency from metal contacts, substantiating the theories regarding WS$_2$’s intrinsic n-type character despite external influences.

Theoretical and Practical Implications

The paper asserts WS$_2$ as a viable candidate for future electronic, optical, and valleytronic applications, paralleling single-layer MoS$_2$ in performance metrics. The high on/off current ratio (~10$^6$) and robustness against short-channel effects position WS$_2$ as a potential material for next-generation electronic devices where reduced channel lengths and high mobility are crucial.

Speculation and Future Directions

Potential advancements in practical applications of WS$_2$ rest on improving control over dielectric environments and contact engineering to optimize device performance further. Future investigations could explore leveraging its unique spin-valley coupling for advanced applications in spintronics. Additionally, integrating high-$k$ dielectrics and exploring top-gated configurations could significantly enhance the performance metrics identified in the current back-gated setups.

In conclusion, the study corroborates the high-quality electronic properties of single-layer WS$_2$ FETs and spotlights its comparable performance with MoS$_2$-based devices, providing a foundational understanding for further exploration and application in nanotechnology and electronics fields.