  The wake-up phenomenon widely exists in hafnia-based ferroelectric
capacitors, which causes device parameter variation over time. Crystallization
at higher temperatures have been reported to be effective in eliminating
wake-up, but high temperature may yield the monoclinic phase or generate high
concentration oxygen vacancies. In this work, a unidirectional annealing method
is proposed for the crystallization of $\mathrm{Hf_{0.5}Zr_{0.5}O_2}$ (HZO)
superlattice ferroelectrics, which involves heating from the
$\mathrm{Pt/ZrO_2}$ interface side. Nanoscale $\mathrm{ZrO_2}$ is selected to
resist the formation of monoclinic phase, and the chemically inert Pt electrode
can avoid the continuous generation of oxygen vacancies during annealing. It is
demonstrated that $\mathrm{600^oC}$ annealing only leads to a moderate content
of monoclinic phase in HZO, and the TiN/HZO/Pt capacitor exhibits wake-up free
nature and a $2P_\mathrm{r}$ value of 27.4 $\mu\mathrm{C/cm^2}$. On the other
hand, heating from the $\mathrm{TiN/HfO_2}$ side, or using $\mathrm{500^oC}$
annealing temperature, both yield ferroelectric devices that require a wake-up
process. The special configuration of $\mathrm{Pt/ZrO_2}$ is verified by
comparative studies with several other superlattice structures and HZO
solid-state solutions. It is discovered that heating from the
$\mathrm{Pt/HfO_2}$ side at $\mathrm{600^oC}$ leads to high leakage current and
a memristor behavior. The mechanisms of ferroelectric phase stabilization and
memristor formation have been discussed. The unidirectional heating method can
also be useful for other hafnia-based ferroelectric devices.
