Electric field effect studies of atomically thin crystals of ferroelectric In₂Se₃

Ultrathin films of ferroelectric materials are important for applications, including non-volatile memories, electromechanical actuators, and sensors. For a ferroelectric film of transition metal oxide to possess electrically switchable polarization, a minimal film thickness is required. In₂Se₃ is a layered III–VI compound with the unit cell consisting of a quintuple layer (QL) of In and Se ions in a triangular lattice featuring covalent bonding within the unit cell but van der Waals bonding between neighboring unit cells. It was predicted recently that In₂Se₃ is ferroelectric down to single unit-cell thickness, possessing both in- and out-of-plane polarizations in the a- and b-phases of In₂Se₃ polymorphism, respectively. We have carried out electric field effect studies of atomically thin crystals of a-In₂Se₃ down to liquid helium temperatures. These 2D crystals were prepared by mechanical exfoliation and characterized by Raman spectroscopy, photoluminescence, and second harmonic generation measurements. We found that the crystals are easily gated to possess a finite electrical conductivity characterized by thermally activated behavior. Source-drain current vs. gate voltage measurements revealed a hysteresis loop in the transfer characteristic, typical for ferroelectric devices.