  Ferroelectric tunnel junctions (FTJs), which consist of two metal electrodes
separated by a thin ferroelectric barrier, have recently aroused significant
interest for technological applications as nanoscale resistive switching
devices. So far, most of existing FTJs have been based on perovskite-oxide
barrier layers. The recent discovery of the two-dimensional (2D) van der Waals
ferroelectric materials opens a new route to realize tunnel junctions with new
functionalities and nm-scale dimensions. Due to the weak coupling between the
atomic layers in these materials, the relative dipole alignment between them
can be controlled by applied voltage. This allows transitions between
ferroelectric and antiferroelectric orderings, resulting in significant changes
of the electronic structure. Here, we propose to realize 2D antiferroelectric
tunnel junctions (AFTJs), which exploit this new functionality, based on
bilayer In$_2$X$_3$ (X = S, Se, Te) barriers and different 2D electrodes. Using
first-principles density functional theory calculations, we demonstrate that
the In$_2$X$_3$ bilayers exhibit stable ferroelectric and antiferroelectric
states separated by sizable energy barriers, thus supporting a non-volatile
switching between these states. Using quantum-mechanical modeling of the
electronic transport, we explore in-plane and out-of-plane tunneling across the
In$_2$S$_3$ van der Waals bilayers, and predict giant tunneling
electroresistance (TER) effects and multiple non-volatile resistance states
driven by ferroelectric-antiferroelectric order transitions. Our proposal opens
a new route to realize nanoscale memory devices with ultrahigh storage density
using 2D AFTJs.
