Vibrational fingerprints of ferroelectric HfO₂

Hafnia (HfO2) is a promising material for emerging chip applications due to its high-κ dielectric behavior, suitability for negative capacitance heterostructures, and scalable ferroelectricity together with silicon technology compatibility. The lattice dynamics along with phononic properties such as thermal conductivity and contraction as well as heat capacity are under-explored, primarily due to the absence of high-quality single crystal specimens. Herein, we report the vibrational properties of a series of HfO2 crystals stabilized with yttrium (chemical formula HfO2:xY, where x = 20, 12, 11, 8, and 0%) and compare our findings with a symmetry analysis and lattice dynamics calculations. The results reveal that incorporation of Y induces a resonance background in the Raman scattering spectrum, a weak metallic response in the infrared reflectance, and local symmetry breaking which activates otherwise silent vibrational features. Furthermore, we uncover a number of signature modes involving polar displacements as well as Hf-Hf dimer breathing modes that are connected with ferroelectric polarization in orthorhombic polar hafnia. This work provides a spectroscopic fingerprint for several different phases of HfO2 and, at the same time, paves the way for a detailed analysis of mode contributions to the high-κ dielectric and ferroelectric properties that are at the heart of emerging chip technologies.