Low-resistivity Metal/2D-Semiconductor Contacts through Electrene Insertion

Abstract

Achieving low-resistance metal contacts to 2D semiconductors is crucial for improving the efficiency of nanodevices, through reduced power dissipation, higher speed, and enhanced carrier injection efficiency. Although transition-metal dichalcogenides (TMDCs), including MoS2, are promising for electronic applications, their performance is limited by high contact resistance due to strong Fermi-level pinning and the presence of a tunnelling barrier. In this thesis, we explore computationally a strategy utilizing two-dimensional (2D) alkaline-earth sub-pnictide electrenes with a general formula of M2X (M = Ca, Sr, Ba; X = N, P, As, Sb) as an intermediate material between the TMDC and metal. Electrenes possess one excess electron per formula unit, resulting in the formation of 2D sheets of charge on their surfaces. This charge can be readily donated when interfaced with a TMDC semiconductor, thereby lowering its conduction band below the Fermi level and eliminating Schottky and tunnelling barriers. Density-functional theory (DFT) calculations were performed for Cu,Au/electrene/MoS2 heterojunctions involving all stable M2X electrenes. To identify the material combinations that provide the most effective Ohmic contacts, the charge transfer, band structure, and electrostatic potential were analyzed. Ca2N was found to be the most promising electrene due to its high surface charge density. The electrene insertion strategy was also tested for MoSe2, MoTe2, WS2, and WSe2, and all were found to form Ohmic contacts with the metal in the presence of Ca2N as the interfacial layer. Finally, the non-equilibrium Green's function formalism, combined with DFT, was used to simulate charge injection into selected monolayer TMDCs using Ca2N as a contact material. For direct comparison, conventional metal (Cu/MoS2) and semi-metal (Sb/MoS2 and Bi/MoS2) contacts were also studied using the same computational framework. Strong charge transfer from Ca2N to the TMDC enables effectively barrier-free charge injection, yielding contact resistance values approaching the quantum limit. In particular, Ca2N/MoS2 exhibits a contact resistance of ~15 Ohm-micrometers, comparable to the values computed for state-of-the-art Sb/MoS2 and Bi/MoS2 contacts. However, in contrast to conventional metals and semi-metals that predominantly exhibit edge-dominated injection, Ca2N/TMDC interfaces support distributed, areal charge injection across the contacted region. Electrene materials therefore offer a promising route toward barrier-free contacts in 2D electronics, despite challenges associated with their synthesis and integration.