  Some van der Waals (vdW) materials exhibit ferroelectricity, making them
promising for novel non-volatile memories (NVMs) such as ferroelectric diodes
(FeDs). CuInP2S6 (CIPS) is a well-known vdW ferroelectric that has been
integrated with graphene for memory devices. Here we demonstrate FeDs with
self-rectifying, hysteretic current-voltage characteristics based on vertical
heterostructures of 10-nm-thick CIPS and graphene. By using vdW indium-cobalt
top electrodes and graphene bottom electrodes, we achieve high
electroresistance (on- and off-state resistance ratios) of ~10^6, on-state
rectification ratios of ~2500 for read/write voltages of 2 V/0.5 V and maximum
output current densities of 100 A/cm^2. These metrics compare favourably with
state-of-the-art FeDs. Piezoresponse force microscopy measurements show that
stabilization of intermediate net polarization states in CIPS leads to stable
multi-bit data retention at room temperature. The combination of two-terminal
design, multi-bit memory, and low-power operation in CIPS-based FeDs is
potentially interesting for compute-in-memory and neuromorphic computing
applications.
