Emergence of unconventional ferroelectric phase in ultrathin Hf_0.5Zr_0.5O₂ 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 Pmn2₁ orthorhombic phase in 1.5-nanometer-thick Hf_0.5Zr_0.5O₂ films directly grown on silicon substrates. Structural and spectroscopic analyses clearly distinguish the experimentally observed Pmn2₁ phase from the conventional Pca2₁ 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 Pmn2₁ 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.