  Recent discovery of ultrathick $\mathrm{MoSi_2N_4(MoN)_n}$ monolayers open up
an exciting platform to engineer 2D material properties via intercalation
architecture. Here we computationally investigate a series of ultrathick
MA$_2$N$_4$(M'N) monolayers (M, M' = Mo, W; A = Si, Ge) under both homolayer
and heterolayer intercalation architectures in which the same and different
species of transition metal nitride inner core layers are intercalated by outer
passivating nitride sublayers, respectively. The MA$_2$N$_4$(M'N) monolayers
are thermally, dynamically and mechanically stable with excellent mechanical
strength and metallic properties. Intriguingly, the metallic states around
Fermi level are localized within the inner core layers. Carrier conduction
mediated by electronic states around the Fermi level is thus spatially
insulated from the external environment by the native outer nitride sublayers,
suggesting the potential of MA$_2$N$_4$(M'N) in back-end-of-line (BEOL) metal
interconnect applications. Nitrogen vacancy defect at the outer sublayers
creates `punch through' states around the Fermi level that bridges the carrier
conduction in the inner core layers and the outer environment, forming a
electrical contact akin to the `vias' structures of metal interconnects. We
further show that MoSi$_2$N$_4$(MoN) can serve as a quasi-Ohmic contact to 2D
WSe$_2$. These findings reveal the promising potential of ultrathick
MA$_2$N$_4$(MN) monolayers as metal electrodes and BEOL interconnect
applications.
