Double-Layer La: HfO 2 Ferroelectric Capacitors for Multi-Level Memory Applications

A symmetric ferroelectric multi-layer stack, comprising two La:HfO2 ferroelectrics separated by a HfO2 dielectric layer, is proposed to achieve 2-bit (4 states) per cell functionality for non-volatile memory applications. A significant remanent polarization (42.5 μC/cm2) is demonstrated in a uniformly-doped single-layer (SL) La:HfO2 ferroelectric capacitor. The introduction of an intermediate HfO2 dielectric layer in the double-layer (DL) configuration leads to an increased presence of the monoclinic (M) phase, which induces a characteristic double-peak in the displacement current, enabling a robust multi-level switching behavior. Despite this structural modification, the DL La:HfO2 ferroelectric capacitor maintains a relatively high remanent polarization of 18.5 μC/cm2. The presence of the ferroelectric orthorhombic (O) phase in both SL and DL La:HfO2 ferroelectric capacitors is confirmed via TEM, while the phase composition variation between M and O phases is further validated by XRD analysis across different stack configurations. Notably, the DL La:HfO2 ferroelectric capacitor exhibits improved tolerance to the variation in Write voltage compared to the SL La:HfO2 ferroelectric capacitor, highlighting its suitability for reliable multi-level memory applications. Furthermore, all four states in the DL La:HfO2 ferroelectric capacitor remain distinctly separated after extrapolating data retention to 10 years. However, the endurance of the double-peak configuration is limited; peak merging becomes evident after 50k voltage sweeping cycles due to the ongoing phase to phase evolution.