Universal insights into the origin of ferroelectricity in doped hafnia

Although doping stands out as a crucial method for inducing ferroelectricity in hafnia films, the mechanisms behind it remains controversial. Notably, the carriers introduced by heterovalent dopants have been largely overlooked. Here, through first-principles studies, we elucidate the key role played by the rich phase transition mechanism under carrier doping in understanding the origin of hafnia ferroelectricity. Electron doping orchestrates a metastable polar phase to stable antipolar phase transformation, driven by strong screening effects and the decay of nonpolar covalent bonds, resulting in the rarity of n-type dopants. Conversely, a delicate balanced interplay of weak screening effects and strengthened polar covalent bonds drives hole maintains robust ferroelectricity. However, the hole distribution pattern markedly affects the total energy difference across phases, culminating in a striking ground-state phase transition from nonpolar monoclinic phase to polar orthorhombic phase to nonpolar cube phase as the hole concentration increases. This phase transition is consistent with the variation trend of the highest phase fraction in hafnia films with the p-type dopants concentration. In addition, the thickness of hafnia ferroelectric films is easily stabilized on the 1 nm, unlike perovskite ferroelectrics, due to the hole enhanced polar distortion. Our theoretical analyses seamlessly align with experimental measurements, offering innovative perspectives for the preparation of robust hafnia-based ferroelectricity films through doping or interface engineering.