Impact of Wet Annealing on Ferroelectric Phase Formation and Phase Transition of HfO2–ZrO2 System

Wet annealing of the ZrO2 thin film can promote martensitic phase transition, leading to the formation of the ferroelectric orthorhombic (o-) phase. However, the present remanent polarization does not achieve the corresponding material potential of ferroelectric ZrO2. In this study, to enhance the ferroelectricity of ZrO2 thin films, the o-phase formation mechanism by wet annealing was presented considering the thermodynamics by systematically investigating the impact of wet annealing on the HfO2–ZrO2 solid solution for promoting phase transition. The structural analysis results indicated that wet annealing at low and high temperatures can promote the martensitic transition of Zr-rich Hf1–xZrxO2 and Hf-rich Hf1–xZrxO2, respectively. In addition, the Zr content exhibiting strong ferroelectricity increased from 0% (HfO2) to 66%. We performed a quantitative estimation of the phase deconvolution through polarization-electric field simulations, incorporating the depolarization field model. According to the change in the phase fraction by wet annealing, all results can be qualitatively and clearly understood based on the phase diagram of the HfO2–ZrO2 system. The impact of wet annealing pertains to the film-thinning effect, which tends to increase the thermodynamic driving force. Finally, we discuss possible solutions to suppress the film-thinning effect by introducing OH ions and improving ZrO2 ferroelectricity.