Two-Dimensional Electrons at Aligned and Twisted Mirror Twin Boundaries in van der Waals Ferroelectrics

Semiconducting transition metal dichalcogenides (MX₂) occur in two polytypes, distinguished by antiparallel (2H) and parallel (3R, rhombohedral) consecutive stacking of monolayer crystals. When stacked, few-layer and bulk 3R-MX₂ are ferroelectric, hosting a substantial out-of-plane polarization dictated by stacking. Because this unconventional ferroelectricity arises from the transverse shifting of adjacent layers rather than a traditional displacive mechanism, it has been the subject of significant experimental and theoretical interest. In bilayer samples, it permits ultrafast, fatigue-resistant switching through sliding, while lattice reconstruction at small-angle twisted interfaces leads to the formation of polar domains with nontrivial topology, controllable through the application of an out-of-plane electric field.

Here, we examine the effect of this polarization at c-axis mirror twin boundaries (mTBs), between adjacent domains with reversed built-in electric fields and demonstrate the accumulation of carriers at opposite interfaces, leading to n-type and p-type mTBs, with typical carrier density ∼10¹³cm⁻². We analyse the stability of such charge accumulation layers and find that n-doped mTBs are more stable as two-dimensional systems, whereas hole accumulation is more stable at external surfaces of ferroelectric domains. We also propose that assembling pairs of mono-twin films with a ‘magic’ twist angle θ* at the interface between these twins leads to a moiré pattern commensurate with the accumulated charge density, which has potential for promoting a regime of strongly correlated states of electrons such as Wigner crystallization, and we specify the values of θ* for homo- and heterostructures of various TMDs.