The effects of annealing and wake-up cycling on the ferroelectricity of zirconium hafnium oxide ultrathin films prepared by remote plasma atomic layer deposition

The crystalline phases and ferroelectric properties of ZrxHf1-xO2 (ZHO) ultrathin films (5.6-5.8 nm in thickness) prepared by remote plasma atomic layer deposition (RP-ALD) before and after post-annealing and wake-up cycling have been investigated in this study. For the films with high ZrO2 percentages, the plasma bombardment from RP-ALD was able to induce partial ferroelectric orthorhombic (o) crystallization during the film deposition stage. The as-deposited pure ZrO2 ultrathin film exhibited a fully developed ferroelectric polarization hysteresis after wake-up cycling. For the films containing the ferroelectric o-phase in the as-deposited state, post-annealing the films followed by a wake-up cycling procedure could greatly promote ferroelectric switching while maintaining low leakage. The ferroelectric properties of the ZHO ultrathin films were highly dependent on the ZrO2-to-HfO2 ratio and could be tailored by various combinations of post-annealing and wake-up cycling. An increase in the amount of HfO2 has been found to be detrimental to the ferroelectricity, mainly due to the high crystallization temperature of HfO2. For the post-annealed ZHO ultrathin films, the ferroelectricity was governed by the relative amounts of the ferroelectric o-phase and the non-ferroelectric monoclinic (m) phase. The Zr0.5H0.5O2 ultrathin film was the only composition which exhibited a large, stable polarization hysteresis after post-annealing but before wake-up cycling. The annealed Zr0.5H0.5O2 ultrathin film is believed to contain a suitable mix of o and m phases to produce loose grain boundaries, allowing the ferroelectric o-phase crystallites to switch freely under electrical loading.