Yttrium: A Highly Efficient Dopant for Ferroelectric HfO₂

The ferroelectric phase of HfO₂ is metastable, and its stabilization in thin films strongly depends on both doping and microstructure. Optimally doped films exhibit high ferroelectric polarization; however, it typically decreases rapidly with increasing film thickness. In contrast to these usual results, a few exceptions report ferroelectricity in thick films with specific dopants, such as Y or La, crystallized under conditions that favor granular microstructure. Disentangling the roles of microstructure and doping on the thickness-dependent stability of ferroelectricity remains essential, and identifying highly effective dopant atoms is of high relevance. In this work, Y-doped epitaxial films of various thicknesses up to about 100 nm are prepared to determine phase evolution and ferroelectric polarization. Films deposited on SrTiO₃(001) and SrTiO₃(110) substrates exhibit robust ferroelectric response across the entire thickness range, in contrast to equivalent La-doped films. Coexistence of monoclinic (paraelectric) and orthorhombic (ferroelectric) phases is observed, with columnar grains revealed by scanning transmission electron microscopy, demonstrating that the microstructure of epitaxial HfO₂ films can be preserved beyond 10 nm. The observed columnar grain structure indicates that the robustness of ferroelectricity in Y-doped films results from the high effectiveness of Y, supporting its use in devices requiring thick ferroelectric layers.