Cumulative polarization in conductive interfacial ferroelectrics

Ferroelectricity in atomically thin bilayer structures has been recently predicted¹ and measured^2-4 in two-dimensional materials with hexagonal non-centrosymmetric unit-cells. The crystal symmetry translates lateral shifts between parallel two-dimensional layers to sign changes in their out-of-plane electric polarization, a mechanism termed 'slide-tronics'⁴. These observations have been restricted to switching between only two polarization states under low charge carrier densities^5-12, limiting the practical application of the revealed phenomena¹³. To overcome these issues, one should explore the nature of polarization in multi-layered van der Waals stacks, how it is governed by intra- and interlayer charge redistribution and to what extent it survives the addition of mobile charge carriers¹⁴. To explore these questions, we conduct surface potential measurements of parallel WSe₂ and MoS₂ multi-layers with aligned and anti-aligned configurations of the polar interfaces. We find evenly spaced, nearly decoupled potential steps, indicating highly confined interfacial electric fields that provide a means to design multi-state 'ladder-ferroelectrics'. Furthermore, we find that the internal polarization remains notable on electrostatic doping of mobile charge carrier densities as high as 10¹³ cm^-2, with substantial in-plane conductivity. Using density functional theory calculations, we trace the extra charge redistribution in real and momentum spaces and identify an eventual doping-induced depolarization mechanism.