Emergence of unconventional ferroelectric phase in ultrathin Hf 0.5 Zr 0.5 O 2 films

Unlike perovskite-based ferroelectrics, whose polarization typically diminishes at reduced thickness, doped hafnia films retain robust ferroelectricity down to 1-nanometer thickness. Accurate structural determination is crucial for understanding this behavior, as ferroelectricity in ultrathin hafnia-based films has been attributed to enhanced polar distortion in their crystal structures. However, precise identification is challenging because of structural similarities among various polymorphs. Here, we experimentally identify the emergence of an unconventional ferroelectric Pmn2 1 orthorhombic phase in 1.5-nanometer-thick Hf 0.5 Zr 0.5 O 2 films directly grown on silicon substrates. Structural and spectroscopic analyses clearly distinguish the experimentally observed Pmn2 1 phase from the conventional Pca2 1 phase typically reported in thicker hafnia films. Furthermore, we find that substantial expansion of the out-of-plane lattice dimension at ultrathin scales drives the stabilization of the Pmn2 1 structure. The experimental identification of this unconventional phase provides crucial insights into the structural evolution underlying the distinct thickness-dependent ferroelectric properties of ultrathin hafnia films.