Origin of Ferroelectricity in Epitaxial Si-Doped HfO₂ Films

HfO₂-based unconventional ferroelectric materials were recently discovered and have attracted a great deal of attention in both academia and industry. The growth of epitaxial Si-doped HfO₂ films has opened up a route to understand the mechanism of ferroelectricity. Here, we used pulsed laser deposition to grow epitaxial Si-doped HfO₂ films in different orientations of N-type SrTiO₃ substrates. Polar nanodomains can be written and read using piezoforce microscopy, and these domains are reversibly switched with a phase change of 180°. Films with different thicknesses displayed a coercive field E_c and a remnant polarization P_r of approximately 4-5 MV/cm and 8-32 μC/cm², respectively. X-ray diffraction and high-resolution transmission electron microscopy (HRTEM) results identified that the as-grown Si-doped HfO₂ films have strained fluorite structures. The ABAB stacking mode of the Hf atomic grid observed by HRTEM clearly demonstrates that the ferroelectricity originates from the noncentrosymmetric Pca2₁ polar structure. Combined with soft X-ray absorption spectra, the results showed that the Pca2₁ ferroelectric crystal structure manifested as an O sublattice distortion by the effect of the interface strain and Si dopant interactions, resulting in a nanoscaled ferroelectric ordered state because of further crystal splitting.