Insertion of a Layered Electride for Low-Resistivity 2D Semiconductor Contacts

A major challenge for 2D semiconductor-based devices is achieving low-resistance, barrier-free metal contacts. Parasitic contact resistance can significantly degrade device performance, including switching speed and power consumption. Some of the issues hindering the formation of ohmic contacts to 2D semiconductors include: weak van der Waals bonding to many metals leading to tunneling barriers, strong chemical bonding to metals that significantly alter the electronic states of the atomically-thin semiconductor, and Fermi-level pinning. New strategies to form low-resistivity ohmic contacts that eliminate Schottky/tunneling barriers are needed, such as phase engineering [1], Fermi-level depinning [2] and semimetal contacts [3].<br/><br/>In this talk, we focus on the insertion of a 2D electride between the metal and the 2D semiconductor, to promote charge transfer and eliminate the potential barrier to facilitate electron transport [4]. Layered electrides are ionic solids with loosely-bound conducting electrons located in the interstitial regions between the atomic layers, and on their surfaces when exfoliated to form 2D electrides. Using density-functional theory we explore the electronic properties of semiconductor-electride-metal heterojunctions, with an emphasis on monolayer MoS₂ as the 2D semiconductor, Ca₂N as the 2D electride and gold as the metal. An analysis of the charge transfer, exfoliation energy, band structure and electrostatic potential demonstrates that Ca₂N donates nearly all of its surface charge resulting in the metallization of the MoS₂ and a barrier-free contact. By comparison, the electride-free MoS₂-Au contact displays a large tunneling barrier and Fermi-level pinning. These findings suggest that introducing a layered electride in the 2D semiconductor-metal interface is a promising strategy towards achieving ultralow-resistivity, ohmic contacts.<br/><br/>[1] Kappera, Voiry, Yalcin, et al., Nat. Mater. <b>13</b>, 1128 (2014).<br/>[2] Farmanbar and Brocks, Phys. Rev. B <b>91</b>, 161304 (2015).<br/>[3] Shen, Su, Lin, et al., Nature <b>593</b>, 211 (2021).<br/>[4] Kaadou, Maassen, and Johnson, J. Phys. Chem. C <b>125</b>, 11656 (2021).<br/><br/>Acknowledgements: this research is supported by NSERC, SRC and Compute Canada.