Monolayer WSe₂ Nanoribbon Transistors with WO_x Passivated Edges

Two-dimensional (2D) semiconductors like transition metal dichalcogenides (TMDs) such as MoS₂ and WSe₂, have demonstrated record-high electron and hole mobility values with sub-nm body thicknesses,^1,2 showing great promise to sustain the transistor scaling trend beyond silicon complementary metal–oxide–semiconductor (CMOS) technologies. Since front-end silicon transistors are moving to a gate-all-around nanoribbon architecture, TMDs will adopt a similar stacked nanoribbon geometry to be competitive.³ However, as the channel width approaches sub-100 nm, the effects of edge states of the nanoribbon channel become pronounced, limiting the carrier mobility of TMD nanoribbons.⁴ The edge states must be passivated to fabricate high-performance, ultra-scaled TMD transistors.⁵

This work demonstrates a facile edge passivation method that significantly reduces edge disorders and enhances the electrical performance of p-type monolayer WSe₂ nanoribbon field-effect transistors (FETs). We achieved this by fabricating monolayer WSe₂ nanoribbon transistors with WO_x passivated edges. The process involved using nanolithography to deposit polymer masks on prefabricated microribbon transistors, followed by a controlled remote O₂ plasma treatment. To avoid device-to-device variation, we sequentially fabricated and measured two types of nanoribbons on the same devices – passivated-edge nanoribbons and open-edge nanoribbons, with a width ranging from 50 nm to 70 nm. Open-edge nanoribbons are the nanoribbons with dangling bonds at the edges, whereas passivated-edge nanoribbons are the nanoribbons with edge atoms covalently bonded to WO_x. Compared to the open-edge nanoribbon FETs, the passivated-edge nanoribbon FETs increased the maximum current by 7–120 times, improved field-effect mobility by 6–24 times and decreased subthreshold swing by an average of 38±9 %. Hole doping induced by edge-bound WO_x was ~1×10¹² cm⁻².The enhanced electrical performance in passivated-edge nanoribbon FETs primarily results from reduced disorders by eliminating dangling bonds, rather than the doping effect from WO_x at the edges. Here we report, for the first time, a working p-type transistor from TMD monolayers with a channel width smaller than 100 nm. Owing to its simplicity and robustness, this edge passivation method holds the potential to become a turnkey manufacturing solution for large-scale integration of high-performance, ultra-scaled WSe₂ p-FETs into commercial silicon foundries.

References:
(1) Liu, S.; Liu, Y.; Holtzman, L.; Li, B.; Holbrook, M.; Pack, J.; Taniguchi, T.; Watanabe, K.; Dean, C. R.; Pasupathy, A. N.; et al. Two-Step Flux Synthesis of Ultrapure Transition-Metal Dichalcogenides. ACS Nano 2023, 23, 59.
(2) Wang, Y.; Kim, J. C.; Wu, R. J.; Martinez, J.; Song, X.; Yang, J.; Zhao, F.; Mkhoyan, A.; Jeong, H. Y.; Chhowalla, M. Van Der Waals Contacts between Three-Dimensional Metals and Two-Dimensional Semiconductors. Nature 2019, 568 (7750), 70–74.
(3) O’Brien, K. P.; Naylor, C. H.; Dorow, C.; Maxey, K.; Penumatcha, A. V.; Vyatskikh, A.; Zhong, T.; Kitamura, A.; Lee, S.; Rogan, C.; et al. Process Integration and Future Outlook of 2D Transistors. Nat. Commun. 2023, 14 (1), 1–5.
(4) Chowdhury, T.; Sadler, E. C.; Kempa, T. J. Progress and Prospects in Transition-Metal Dichalcogenide Research beyond 2D. Chem. Rev. 2020, 120 (22), 12563–12591.
(5) Das, S.; Sebastian, A.; Pop, E.; McClellan, C. J.; Franklin, A. D.; Grasser, T.; Knobloch, T.; Illarionov, Y.; Penumatcha, A. V.; Appenzeller, J.; et al. Transistors Based on Two-Dimensional Materials for Future Integrated Circuits. Nat. Electron. 2021, 4 (11), 786–799.