Interfacial Properties of Monolayer and Bilayer MoS2 Contacts with Metals: Beyond the Energy Band Calculations

Abstract: Although many prototype devices based on two-dimensional (2D) MoS2 have been fabricated and wafer scale growth of 2D MoS2 has been realized, the fundamental nature of 2D MoS2-metal contacts has not been well understood yet. We provide a comprehensive ab initio study of the interfacial properties of a series of monolayer (ML) and bilayer (BL) MoS2-metal contacts (metal = Sc, Ti, Ag, Pt, Ni, and Au). A comparison between the calculated and observed Schottky barrier heights (SBHs) suggests that many-electron effects are strongly suppressed in channel 2D MoS2 due to a charge transfer. The extensively adopted energy band calculation scheme fails to reproduce the observed SBHs in 2D MoS2-Sc interface. By contrast, an ab initio quantum transport device simulation better reproduces the observed SBH in the two types of contacts and highlights the importance of a higher level theoretical approach beyond the energy band calculation in the interface study. BL MoS2-metal contacts have a reduced SBH than ML MoS2-metal contacts due to the interlayer coupling and thus have a higher electron injection efficiency.

• The paper demonstrates that ab initio quantum transport simulations yield more accurate Schottky barrier heights than traditional DFT for MoS2-metal interfaces.
• The study details strong hybridization effects at Sc and Ti contacts compared to weaker interactions with Ag and Au, impacting electron injection.
• It reveals that bilayer MoS2 generally exhibits lower barrier heights than monolayers, suggesting better performance for electronic device applications.

Analysis of Interfacial Properties of Monolayer and Bilayer MoS Contacts with Metals

The paper investigates the interfacial properties between monolayer (ML) and bilayer (BL) molybdenum disulfide (MoS$_2$) and a selection of metals—Sc, Ti, Ag, Pt, Ni, and Au—using sophisticated computational techniques encompassing ab initio and density functional theory (DFT) methodologies. This research moves beyond conventional energy band calculations to delve deeply into the Schottky barrier heights (SBHs) at these interfaces and examines the implications of these interfaces for electronic applications.

The study begins by setting the context for 2D MoS$_2$-metal contacts, emphasizing their crucial role in semiconductor devices like field-effect transistors (FETs). The characteristics of MoS$_2$, such as its band gap variations in ML and BL forms and the challenges posed by Schottky barriers at contacts, are extensively discussed.

Key Findings

• Schottky Barrier Height (SBH) Evaluations: The research highlights inadequacies in traditional DFT approaches for calculating the SBH of MoS$_2$-metal interfaces. It reveals that ab initio quantum transport calculations provide better agreement with experimental SBH values, particularly for interfaces involving Sc and Ti metals.
• Interfacial Chemistry and Structure: The paper reports strong adherence and substantial hybridization at interfaces involving metals like Sc and Ti, contrasted with weaker interactions at interfaces with Ag and Au. Geometrical configurations of these interfaces have been meticulously optimized, revealing equilibrium distances and binding energies that correlate with the electronic interaction strength between MoS$_2$ and metals.
• Charge Transfer Effects: The authors note a dramatic suppression of many-electron effects, likely due to charge transfer phenomena at these interfaces. Such suppression influences both the electron injection efficiency and the resultant SBH, necessitating corrections beyond standard DFT.
• Layer-Dependent Properties: A distinctive layer dependence of interfacial SBHs is observed, with BL MoS$_2$ generally exhibiting reduced barrier heights compared to ML MoS$_2$. This suggests better electron injection efficiency for BL interfaces, aligning with their higher practical potential for transistor applications.

Implications and Future Directions

The findings from this study underscore the need for employing higher-fidelity simulation techniques, such as coupled DFT-NEGF quantum transport approaches, when investigating semiconducting-metal interfaces, particularly for emerging 2D materials. The documented partial failure of traditional DFT methods in capturing realistic SBH values points to the necessity for theoretical advancements that incorporate many-body effects concessions better.

This work also provides a foundational understanding of 2D material-metal contacts that can facilitate the design of improved electronic devices, influencing future research into MoS$_2$ and similar materials. The insights into charge transport dynamics and energetics at these interfaces could lead to more efficient device architectures and inform wafer-scale fabrication processes of 2D electronic components.

In conclusion, this extensive evaluation of ML and BL MoS$_2$-metal interfaces offers a comprehensive view of the complex interactions at play, presenting a nuanced understanding that is critical for exploiting the full potential of 2D material-based electronics. Further exploration into other dimensional forms and metal combinations, as well as operational conditions, might provide additional avenues for lowering contact resistances and optimizing device performance.