Role of defects in unconventional moiré ferroelectrics

The low defect density and atomically planar surfaces of exfoliated hexagonal boron nitride (hBN) make it a material of choice for encapsulation of graphene devices. Such encapsulation protects graphene from extrinsic disorder, enabling ultra-high mobilities, micrometre-scale ballistic transport and low charge inhomogeneities in the order of 10¹⁰ cm^-2. When the crystallographic axes of graphene and hBN are aligned, the system forms high-quality moiré superlattices. Some of these superlattices revealed a peculiar type of ferroelectric behaviour, in which the effect of one gate can be selectively screened. This effect has been reproduced in a variety of devices, including some for which no alignment between hBN and graphene was found, suggesting that hBN defects could play a role in the observed ferroelectricity. Other experiments suggested that the debated ferroelectricity could find origin in excitons trapped at moiré sites.



Here we used photocurrent measurements to clarify the role of defects and moiré sites into the observed hysteretic behaviour.