Two-dimensional electron gas on the surface of alkali-earth metal based electrides: Assistance to overcome tunneling barriers in ohmic contacts

van der Waals (vdW) stacking of two-dimensional (2D) metals and 2D semiconductors has attracted significant interest in metal-semiconductor junctions (MSJs). Unfortunately, the vdW gap always leads to large tunneling barriers even in ohmic contacts. Herein, by constructing 2D electrides possessing sufficient electron gas at the surface, the formation of quasibonds at MSJ interface is expected to overcome the challenge of contact resistance induced by vdW gap. Specifically, 2D $\mathrm{C}{\mathrm{a}}_{2}X{Y}_{2}$ ($X=\mathrm{Ti},\mathrm{Zr},\mathrm{Hf}$; $Y=\mathrm{N},\mathrm{P}$) electrides possess ultralow work functions ranging from 3.28 to 3.90 eV, accompanied by nearly free electrons on the surface, rendering them efficient electron donors. Taking typical 2D semiconductor $\mathrm{Mo}{\mathrm{S}}_{2}$ to contact $\mathrm{C}{\mathrm{a}}_{2}X{Y}_{2}$, the ohmic contact and complete tunneling effect can be achieved. Application of a modest bias voltage yields a noticeable current density of about $0.6\textmu{}\mathrm{A}/{\mathrm{\AA{}}}^{2}$. Moreover, these MSJs exhibit superior environmental stability with bromine terminated. Our work not only offers a series of promising 2D electrides, but also paves the way for advancing the progress of 2D electronic and optoelectronic devices.