Charge injection and ferroelectric domains state stability in La-doped Hf_0.5Zr_0.5O₂ thin film

Hafnia-based thin films are candidates for next-generation non-volatile memories and logic devices thanks to the robust nanoscale ferroelectricity. Probing the stability of the ferroelectric state is therefore an important step before device integration. Here, we present the study of ferroelectric domain stability in a 10 nm 2.3%mol La-doped Hf_0.5Zr_0.5O₂ (HZLO) film after top electrode (TiN) lift-off. Domains were written with applied fields between 1 and 3 MV/cm and probed using piezoresponse force microscopy and low energy electron microscopy. We distinguish two different processes. Domains of up or down polarization are created by bias. Simultaneously, charge is injected into the film. The Schottky barrier and internal field created by oxygen vacancies both appear to play a role in charge injection, which, in turn, can modulate the bias required for domain writing. The amount of injected charge depends on the band offsets of the HZLO with respect to the bottom TiN electrode and the tip and, possibly, the presence of a tetragonal, non-ferroelectric phase in the film. Trapped charges in HZLO film dissipate within two weeks, whereas ferroelectric domains are found to be stable over nine months.