Epitaxial La-doped HfO₂ Thin Films on YSZ (111): Ferroelectric Phase Stabilization via Strain Engineering

Hafnia-based materials have reignited interest in integrating ferroelectricity into modern electronics, primarily due to their compatibility with silicon technology. However, a key challenge remains the stabilization of the ferroelectric orthorhombic phase (o-FE, space group Pca2₁), which is metastable and does not appear in hafnia's temperature-pressure phase diagram. Consequently, the growth of the o-FE phase typically requires intricate processes involving both thermodynamic and kinetic factors. In conventional hafnia films thicker than 10 nm, substantial amounts of the monoclinic phase (P2₁/c)—the stable phase under ambient conditions—are often present. We will present results showing the successful growth of single-crystalline epitaxial films of Hf₀.₉La₀.₁O₂ (LHO) on yttrium-stabilized zirconia (YSZ) substrates, from the two-dimensional limit (one unit cell) up to 30 nm thickness, without any monoclinic phase. The stabilization of the o-FE phase is attributed to the compressive epitaxial strain imposed by the YSZ substrate, which is isomorphic to hafnia, and the incorporation of La enlarges the lattice constant of hafnia. These findings demonstrate the potential of strain engineering and dopant selection in stabilizing ferroelectric phases in hafnia-based thin films, opening pathways for their integration into next-generation electronic devices.