Hopping Transport through Defect-induced Localized States in Molybdenum Disulfide

Abstract: Molybdenum disulfide is a novel two-dimensional semiconductor with potential applications in electronic and optoelectronic devices. However, the nature of charge transport in back-gated devices still remains elusive as they show much lower mobility than theoretical calculations and native n-type doping. Here we report transport study in few-layer molybdenum disulfide, together with transmission electron microscopy and density functional theory. We provide direct evidence that sulfur vacancies exist in molybdenum disulfide, introducing localized donor states inside the bandgap. Under low carrier densities, the transport exhibits nearest-neighbor hopping at high temperatures and variable-range hopping at low temperatures, which can be well explained under Mott formalism. We suggest that the low-carrier-density transport is dominated by hopping via these localized gap states. Our study reveals the important role of short-range surface defects in tailoring the properties and device applications of molybdenum disulfide.

• The paper identifies sulfur vacancies as key contributors to hopping transport in few-layer MoS2 using detailed experimental setups and DFT analysis.
• It demonstrates that charge conduction transitions from nearest-neighbor hopping at high temperatures to variable-range hopping at lower temperatures, as modeled by Mott's formalism.
• The findings highlight practical pathways for improving MoS2-based devices through defect engineering and optimized material synthesis.

Hopping Transport through Defect-induced Localized States in Molybdenum Disulfide

This paper presents an intricate study on charge transport mechanisms within few-layer molybdenum disulfide (MoS$_2$) by examining defect-induced localized states using a combination of experimental techniques and computational modeling. In particular, the research provides insights into the effects of sulfur vacancies, acting as electron donors, which introduce localized states within the bandgap of MoS$_2$.

Experimental and Computational Investigations

The authors employ a comprehensive experimental approach that includes aberration-corrected transmission electron microscopy (TEM), variable-temperature transport measurements, and high-level quantum mechanical calculations based on density functional theory (DFT). TEM characterizations reveal a notable density of sulfur vacancies (~10$^13$ cm$^-2$) on the surface of MoS$_2$, with a vacancy distance of approximately 1.7 nm. These vacancies are confirmed to be intrinsic rather than induced by electron beam irradiation.

Transport measurements were conducted on devices fabricated using a few layers of MoS$_2$, demonstrating insulating behavior across the entire temperature range and carrier density spectrum investigated. Under low carrier density, electrical conduction exhibits nearest-neighbor hopping (NNH) at elevated temperatures and variable-range hopping (VRH) at reduced temperatures. Mott's formalism successfully models these observations, providing a coherent theoretical framework that aligns with the measured conductivity profiles.

The authors extend their analysis through DFT calculations that elucidate the impact of sulfur vacancies on the electronic structure of MoS$_2$. These calculations show that sulfur vacancies give rise to defect states within the bandgap, converting the direct bandgap to an indirect one, and resulting in strong electron localization around the vacancies. A critical carrier density where transition from localized to band-like transport occurs was also identified.

Implications and Future Directions

The study underscores the substantial influence of short-range structural disorders on the low carrier density transport properties of MoS$_2$. It offers a microscopic explanation for the metal-insulator transition and observed n-type doping behavior in MoS$_2$ devices, elucidating the role of sulfur vacancies in these phenomena.

On a theoretical level, the paper enhances the understanding of 2D semiconductor materials by providing a model of transport dominated by hopping within defect-induced localized states. These findings could guide future theoretical and experimental research targeting defect engineering for optimizing electronic properties in two-dimensional materials.

From a practical perspective, the results suggest potential pathways for improving the performance of MoS$_2$ based devices through defect management. Specifically, advancing sample quality by reducing defect density or employing chemical vapor deposition techniques might enhance device capabilities.

Conclusion

By marrying advanced experimental techniques with robust theoretical modeling, this research offers critical insights into the charge transport mechanisms in MoS$_2$. It highlights the importance of understanding and controlling defect distributions in 2D materials to harness their full potential in electronic applications. This comprehensive approach to exploring transport phenomena could serve as a template for analogous studies in other transition metal dichalcogenides and related materials, further contributing to the evolution of semiconductor technology.