Computational Investigation of Bilayer Alloy Contacts for p-Type 2D Semiconductor Transistors

We conducted an extensive set of density functional theory (DFT) simulations to examine how Ta incorporation alters the band structure, density of states (DOS), and Fermi level position in WSe₂. The findings show that Ta acts as an efficient substitutional p-type dopant, driving the Fermi level deep into the valence band without introducing mid-gap defects. This indicates that Ta doping enables high hole concentrations typical of degenerate doping. As the Ta content rises from x = 0.1 to x = 0.6, the bandgap gradually narrows and ultimately transitions into a pseudo-metallic state at high doping levels. The reduction in bandgap is accompanied by an upward shift of the valence band maximum and an increased DOS near the Fermi level, both of which facilitate improved carrier injection from metal contacts. This investigation guides the experimental realization of bilayer alloys that achieve record-low contact resistance (Rc ≈ 98 Ω.μm), independent of gate voltage, in p-type 2D field-effect transistors. [1]

Layer-resolved DFT results further emphasize the significance of the bilayer structure for minimizing contact resistance. In bilayer (2L) W_xTa_1-xSe₂, interlayer coupling broadens the valence band dispersion and boosts the DOS near the Fermi level by roughly 60% compared to the monolayer alloy (0.40 eV^-1 vs. 0.25 eV^-1 within ±0.1 eV). This elevated DOS enhances charge screening and tunneling at the metal–semiconductor interface, effectively simulating metallic contact behavior. Projected DOS and charge density difference analyses show that Ta substitution generates delocalized states overlapping strongly with Se p orbitals, enabling efficient hole conduction without localization or trap formation.

Overall, our DFT study offers a quantitative understanding of degenerate p-type doping in 2D TMD alloys. By connecting atomic-scale electronic effects to macroscopic contact properties, we demonstrate that controlled Ta substitution in bilayer WSe₂ can yield metallic-like hole transport while maintaining structural order. This first-principles framework outlines general design strategies for realizing low-resistance p-type contacts in 2D semiconductors via targeted alloy engineering.

[1] A. Azizi et al., 2024 IEEE International Electron Devices Meeting (IEDM), 1–4.