Unraveling the origins of ferroelectricity in doped hafnia through carrier-mediated phase transitions

Doping is critical for inducing ferroelectricity in hafnia films, yet the underlying mechanisms remain debated. Here, through first-principles studies, we elucidate the pivotal role played by the complex phase transition mechanisms under carrier doping in understanding the origin of hafnia ferroelectricity. Specifically, electron doping orchestrates a metastable polar phase to stable antipolar phase transformation, driven by strong screening effects and weakened nonpolar covalent bonds, making n-type dopants rare. Conversely, weak screening effect and enhanced polar covalent bonding strengthen robust ferroelectricity, enabling significant ground-state phase transitions from the monoclinic to the polar orthorhombic phase and finally to the cubic phase under hole doping, a phenomenon prevalent in hafnia-based films doped with p-type dopants. Furthermore, this hole-enhanced polar distortion also results in an inverse size effect in hafnia ferroelectric films, unlike perovskite ferroelectrics. Our findings offer new insights into the preparation of robust hafnia-based ferroelectric films through doping or interface engineering.