Yttrium: A Highly Efficient Dopant for Ferroelectric HfO.

The ferroelectric phase of HfO 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 SrTiO(001) and SrTiO(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 HfO 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.