Poling Sequence-Dependent Tunneling Electroresistance in HfO₂-Based Ferroelectric Tunnel Junctions

The current–voltage (I–V) characteristics and ON/OFF ratio in hafnium oxide (HfO2)-based ferroelectric tunnel junctions (FTJs) were investigated under different poling sequences. When −5 V poling pulse is applied prior to +5 V pulse (−5 V-poling-first operation), both ON-state and OFF-state show relatively low currents, whereas the ON/OFF ratio is more than doubled, as compared to the reverse poling sequence (+5 V-poling-first operation, i.e., + 5 V pulse applied prior to −5 V). Interestingly, the ON-state I–V curves exhibit the Ohmic behavior, while the OFF-state curves are nonlinear that can be described by direct tunneling across a barrier, regardless of the poling sequence. The poling sequence-dependent tunneling electroresistance in our FTJs is explained by the evolution of domain structure in the ferroelectric films driven by the poling pulse, as supported by both I–V measurements and data fitting. This work provides a guidance to modulate the performance of FTJs, and further help understand the structure–property relationship of HfO2-based ferroelectric memories at the nanoscale.