Origin of Pinched Hysteresis Loop: Unified Model for Wake-Up and Imprint in Sub-5 nm Ferroelectric HZO

This study systematically investigates the origin of the wake-up effect in ferroelectric$\text{Hf}_{0.5} \text{Zr}_{0.5} \mathrm{O}_{2}$(HZO) films, particularly found prominent in sub-$\mathbf{5\ n m}$thickness. We examine and verify a model explaining the pinched loops, wake-up, and imprint behavior in ultra-thin HZO: (1) a pinched loop in pristine samples results from imprint with opposite directions of the net-zero polarization state; (2) this imprint-induced pinched loop is stronger in thinner films due to more trap sites for charge trapping; (3) wake-up can be considered as the recovery of imprint. We experimentally confirm this model by reproducing the pristine condition through properly initializing films to a net-zero-polarization state. We can observe the re-splitting of switching peaks by imprint at the net-zero polarization state with the more prominent re-splitting behavior in thinner HZO films, reproducing the pinched loop in sub-5nm films.