High-Performance Monolayer WS2 Field-effect Transistors on High-k Dielectrics

Abstract: The combination of high-quality Al2O3 dielectric and thiol chemistry passivation can effectively reduce the density of interface traps and Coulomb impurities of WS2, leading to a significant improvement of the mobility and a transition of the charge transport from the insulating to the metallic regime. A record high mobility of 83 cm2/Vs (337 cm2/Vs) is reached at room temperature (low temperature) for monolayer WS2. A theoretical model for electron transport is also developed.

• The paper demonstrates that interface engineering with an ultrathin Al2O3 layer and thiol functionalization significantly enhances electron mobility in monolayer WS2 FETs.
• The authors employ a four-probe measurement method to accurately assess mobility, reporting 83 cm²/Vs at room temperature and 337 cm²/Vs at low temperatures.
• Methodological improvements yield a 49% reduction in charge trap density and a 40% performance enhancement, suggesting a scalable approach for advanced 2D electronics.

High-Performance Monolayer Tungsten Disulfide Transistors on High-κ Dielectrics

This paper presents a comprehensive investigation into enhancing the electron mobility of monolayer tungsten disulfide (WS$_2$) field-effect transistors (FETs) through meticulous interface engineering. Building on the notion that semiconducting transition metal dichalcogenides (TMDs) are promising materials for advanced electronic applications, this study underscores the potential of WS$_2$ to bridge the performance gap identified in MoS$_2$-based devices due to its theoretically superior phonon-limited mobility.

Methodological Innovations and Results

The authors employed a systematic modification of the transistor's dielectric interface to mitigate extrinsic factors—such as Coulomb impurities and charge traps—that traditionally impede high electron mobility. The strategic introduction of an ultrathin Al$_2$O$_3$ dielectric layer beneath the WS$_2$ channels and subsequent thiol chemical functionalization formed the core of the experimental approach. The combination of these modifications resulted in a remarkable mobility increase, achieving highest reported values of 83 cm$^2$/Vs at room temperature and 337 cm$^2$/Vs at low temperatures.

The study utilized a four-probe measurement approach to ensure the precise determination of mobility, while controlling for contact resistance effects. Initial experiments demonstrated a 49% reduction in charge trap density when comparing WS$_2$ FETs fabricated on Al$_2$O$_3$-coated substrates to those on bare SiO$_2$. Further thiol functionalization amplified this improvement, showing a 40% increase in device performance metrics over as-exfoliated samples.

Theoretical Analysis and Implications

The authors developed an empirical model that integrates charge trap and Coulomb impurity scattering effects to align quantitatively with their experimental data. Interestingly, although the model congruently explains low-temperature mobility behaviors, it falls short at high temperatures, implying the presence of additional scattering mechanisms. The study tentatively attributes this discrepancy to surface optical phonons from the high-κ Al$_2$O$_3$ dielectric, a hypothesis warranting future in-depth analysis.

This work emphasizes the utility of high-κ dielectrics in reducing Coulomb scattering, thus elevating intrinsic mobility levels that are critical for the next generation of electronic devices. The demonstrated methodology provides a scalable pathway to exploit TMDs' potential in various technological arenas, particularly where low power and high-speed operations are paramount.

Future Directions

Given the insights from this research, future studies may enhance the understanding of WS$_2$ FETs by decoupling and quantifying distinct scattering processes at elevated temperatures. The refinement of the model to account for such processes could provide further clarity and control, advancing the application of TMDs in commercial and industrial solutions. The exploration of other TMD-dielectric interfaces and the continuous development of chemical functionalization techniques also present promising avenues to harness the full potential of these materials.

In sum, this research contributes significantly to the body of knowledge regarding 2D material engineering and demonstrates the impactful role of interface control in optimizing electronic transport properties.