Built-in bias fields for retention stabilisation in hafnia-based ferroelectric tunnel junctions

Abstract

Ferroelectric tunnel junctions have recently attracted a lot of research interest, not only for memory operation but also for e.g. neuromorphic computing. Retention characteristics of these devices are of utmost importance in many applications. The authors have recently demonstrated the feasibility of double-layered ferroelectric tunnel junctions where the ferroelectric hafnium zirconium oxide acts as the memory layer and the dielectric aluminium oxide as the tunnelling barrier. The depolarisation field from the unscreened polarisation charges that is introduced by this structure causes serious limitations on the retention behaviour due to the unintentional back-switching of ferroelectric domains. This leads to a diminishing memory window over time. By using a built-in bias field from different work function metal electrodes, in this case platinum and titanium nitride with 0.85 eV difference, one can shift the polarisation hysteresis curve and counteract the electric depolarisation field. This leads to a more stable oncurrent over time while only marginally increasing the off-current degradation. Extrapolation to ten years shows a much larger memory window compared to the symmetric metal electrode structure.