Improved Carrier Mobility in Few-Layer MoS2 Field-Effect Transistors with Ionic-Liquid Gating

Abstract: We report the fabrication of ionic liquid (IL) gated field-effect transistors (FETs) consisting of bilayer and few-layer MoS2. Our transport measurements indicate that the electron mobility about 60 cm2V-1s-1 at 250 K in ionic liquid gated devices exceeds significantly that of comparable back-gated devices. IL-FETs display a mobility increase from about 100 cm2V-1s-1 at 180 K to about 220 cm2V-1s-1 at 77 K in good agreement with the true channel mobility determined from four-terminal measurements, ambipolar behavior with a high ON/OFF ratio >107 (104) for electrons (holes), and a near ideal sub-threshold swing of about 50 mV/dec at 250 K. We attribute the observed performance enhancement, specifically the increased carrier mobility that is limited by phonons, to the reduction of the Schottky barrier at the source and drain electrode by band bending caused by the ultrathin ionic-liquid dielectric layer.

• The paper demonstrates that IL gating in few-layer MoS2 FETs enhances electron mobility to 220 cm²/V·s at 77 K, far exceeding traditional methods.
• The paper reports ambipolar behavior with a high ON/OFF current ratio and a near-ideal subthreshold swing, evidencing superior charge control.
• The paper reveals that IL-induced band bending reduces the Schottky barrier at MoS2/metal contacts, leading to more efficient electron transport.

Overview of Improved Carrier Mobility in Few-Layer MoS<sub\>2</sub> Field-Effect Transistors with Ionic-Liquid Gating

The paper explores the enhancement of carrier mobility in few-layer molybdenum disulfide (MoS<sub\>2</sub>) field-effect transistors (FETs) through the implementation of ionic-liquid (IL) gating. It addresses the limitations of traditional silicon/silicon-dioxide back-gated MoS<sub\>2</sub> FETs and presents the IL-FETs as a viable alternative with remarkable improvements in device performance. The central finding is the substantial reduction of the Schottky barrier at MoS<sub\>2</sub>/metal contacts achieved through band bending facilitated by IL gating, leading to enhanced electron transport properties.

Key Results and Observations

1. Enhanced Mobility: The paper reports that the IL-gated MoS<sub\>2</sub> FETs exhibit a significant increase in electron mobility, reaching values of up to 220 cm<sup\>2</sup>/V·s at 77 K. This is in stark contrast to the electron mobility under traditional back-gated configurations, which often range between 0.1 and 10 cm<sup\>2</sup>/V·s.
2. Ambipolar Behavior: The IL-FET devices consistently showed ambipolar conduction with a high ON/OFF current ratio (>10<sup\>7</sup> for electrons and 10<sup\>6</sup> for holes) and a subthreshold swing near 50 mV/decade at 250 K, indicating efficient control over charge carriers in the channel.
3. Temperature Dependence: The temperature dependence of mobility follows a µ ~ T^-γ pattern with γ ≈ 1, suggesting that phonon scattering primarily limits mobility in these devices rather than extrinsic factors such as impurity scattering or contact resistance.
4. Schottky Barrier Reduction: The use of IL gating enables a thin electrical double layer at the MoS<sub\>2</sub>/metal interface, leading to reduced contact resistance and enhanced tunneling efficiency. This results from pronounced band bending induced by the high capacitance of the IL gate.

Implications and Future Directions

The implications of this research are significant for the development of high-performance electronic devices based on two-dimensional materials. By demonstrating the role of IL gating in improving carrier mobility and lowering contact resistance, the study opens up potential advancements in the design of low-power, high-efficiency electronic and optoelectronic systems.

1. Applications in Nanoelectronics: The IL-gated MoS<sub\>2</sub> FETs hold promise for use in energy-efficient, flexible nanoelectronics, surpassing conventional silicon technology in specific applications due to their mechanical flexibility, chemical stability, and scalability. These findings could catalyze further research into similar ionic-liquid gated systems to enhance other transition-metal dichalcogenides.
2. Material Engineering: Future research could explore the optimization of ILs to fine-tune their electronic properties and investigate their interactions with different TMDCs, tailoring the gating mechanism to a wider range of semiconductor materials.
3. Device Architecture: Continued exploration of IL gating may lead to novel device architectures that leverage the unique properties of MoS<sub\>2</sub> and similar materials, potentially leading to breakthroughs in transistor scaling and quantum computing components.

In conclusion, the presented work on IL-gated few-layer MoS<sub\>2</sub> FETs provides a valuable contribution to the field of nanoelectronics, displaying significant mobility enhancement and reduced Schottky barrier limitations. This approach offers a promising route to overcome critical performance bottlenecks in TMDC-based devices, aligning with the continual pursuit of more energy-efficient and scalable electronic technologies.