Band alignment of two-dimensional transition metal dichalcogenides: application in tunnel field effect transistors (1308.0767v1)

Abstract: Tunnel field effect transistors (TFETs) based on vertical stacking of two dimensional materials are of interest for low-power logic devices. The monolayer transition metal dichalcogenides (TMDs) with sizable band gaps show promise in building p-n junctions (couples) for TFET applications. Band alignment information is essential for realizing broken gap junctions with excellent electron tunneling efficiencies. Promising couples composed of monolayer TMDs are suggested to be VIB-MeX2 (Me= W, Mo; X= Te, Se) as the n-type source and IVB-MeX2 (Me = Zr, Hf; X= S, Se) as the p-type drain by density functional theory calculations.

• The paper presents systematic DFT calculations to explore band alignments across 24 monolayer TMDs critical for efficient TFET tunneling.
• It identifies promising material pairs, using differences in work functions and band gaps, for n-type and p-type regions in TFET devices.
• The findings offer practical insights into strain engineering and vertical stacking strategies to optimize TFET performance while reducing interface defects.

Band Alignment of 2D Transition Metal Dichalcogenides and Implementation in TFETs

This paper presents a comprehensive analysis of band alignment in two-dimensional (2D) transition metal dichalcogenides (TMDs) and highlights the implications for their application in tunnel field-effect transistors (TFETs). Through systematic density functional theory (DFT) calculations, the authors explore the electronic structures of a range of TMDs, uncovering insights critical to the development of low-power electronic applications.

Key Findings and Methodology

The authors focus on the band alignment of monolayer TMDs, which is crucial for achieving efficient electron tunneling in TFETs with a "broken-gap" band alignment. The paper identifies promising material pairs: ⅥB-MeX₂ (Me = W, Mo; X = Te, Se) for n-type sources and ⅣB-Me₂ (Me = Zr, Hf; X = S, Se) for p-type drains. These combinations are selected based on their electronic properties calculated using DFT, showcasing potential high-efficiency electron tunneling.

The research involves evaluating electronic properties across 24 TMDs involving combinations of transition metals (ⅣB: Ti, Zr, Hf; ⅤB: V, Nb, Ta; ⅥB: Mo, W) and chalcogen anions (S, Se, Te). Notably, the paper distinguishes between trigonal prismatic coordination preferences for ⅥB-TMDs and octahedral coordination for ⅣB-TMDs, accounting for electronic configuration and crystal field splitting effects.

The paper deploys advanced DFT calculations using the Vienna Ab-initio Simulation Package (VASP), incorporating PAW pseudopotentials with both GGA and LDA to describe exchange-correlation. Spin polarization and spin-orbit coupling (SOC) are meticulously applied to enhance electronic property predictions.

Numerical Results

A prominent aspect of this work is the examination of work functions for ⅥB and ⅣB-TMDs, showing marked differences between these groups—ⅥB-TMDs exhibit work functions ranging between 4-5 eV, while ⅣB-TMDs display values around 6 eV. The calculated band gaps offer a nuanced understanding of the materials' electronic behavior, influencing TFET performance.

The findings also align with recent debates regarding monolayer MoS₂'s electronic band gap. The authors support a renewed understanding through "single-shot" calculations, verifying a larger quasiparticle energy gap indicative of more significant electronic interactions than previously measured excitonic gaps.

Theoretical and Practical Implications

This research serves as a pivotal reference for advancing TFET applications, where careful selection of TMDs for source-drain junctions can significantly enhance performance. It emphasizes the potential of strain engineering to mitigate intervalley scattering, thereby optimizing the efficiency of electron tunneling. The insights on band edge properties and universal band alignment foster further exploration in vertical stacking strategies for TMD-based devices.

The implications extend to device fabrication where interface state densities, often detrimental in traditional III-V materials, are less problematic in 2D TMDs due to inherent surface properties. The paper suggests that TFETs utilizing proposed TMD configurations could circumvent several limitations of conventional MOSFETs and III-V material-based TFETs by minimizing interface defects.

Future Directions

The research underscores the necessity of precise band alignment in practical TFET designs, urging further exploration into multilayer and heterogeneous junctions. Future studies should investigate interface-related effects, such as charge transfer and dipole formation, to understand better the interaction dynamics in stacked 2D-TMD systems.

This work provides a fundamental understanding critical for the development of future low-energy electronics, establishing a theoretical foundation for ongoing and future studies in the field. As the researchers suggest, subsequent efforts will need to explore examining real-world TFET implementations and the material growth techniques necessary to realize these semiconductor devices effectively.