First-Principles Investigation of MXene/MoSi2N4 van der Waals Heterostructures: Strong Fermi Level Pinning Effect Resulting in Ohmic Contact with Low Contact Resistance

High-performance semiconductor devices require ohmic contact (OhC) with low contact resistance, which are widely associated with weak Fermi level pinning (FLP) effects. However, the FLP effect does not play a completely negative role in the metal/semiconductor contact. Especially for functionalized MXenes, it is necessary to systematically study whether the FLP effect occurs at the interface of different functional groups and the influence of the FLP effect on MXene/semiconductor interfaces. In this article, we select functionalized MXenes (Mn+1XnT2, M = Ti–Ta; n = 1 and 2; X = C and N; T = F, O, and OH) as electrode materials for the MoSi2N4 monolayer and investigate their interfacial properties by using first-principles calculations. Our results indicate that OH-MXenes are promising electrode materials for the MoSi2N4 monolayer. In OH-MXene/MoSi2N4 van der Waals heterostructures (VHTs), the strong FLP effect fixes the Fermi level in conduction bands, resulting in the ohmic contact. A small van der Waals (vdW) gap brings low tunneling barriers and contact resistances, which improve the electron injection efficiency. However, F-MXenes will bring a controllable Schottky contact (ShC) and O groups form Ohmic contact (OhC) or quasi-OhC with a strong FLP effect and high contact resistance, when contacting the MoSi2N4 monolayer. Therefore, it is suggested to avoid the presence of O functional groups in the experiment. Our work first correlates the interface properties of MXenes/semiconductors with the FLP effect and provides a useful insight, which will provide a theoretical guidance on how to select the dominant functional group experimentally and find efficient two-dimensional (2D) semiconductor devices.