Vibrational fingerprints of ferroelectric HfO2

Abstract

Hafnia (HfO2) is a promising material for emerging chip applications due to its high-κ dielectric behavior, suitability for negative capacitance heterostructures, scalable ferroelectricity, and silicon compatibility. The lattice dynamics along with phononic properties such as thermal conductivity, contraction, and heat capacity are under-explored, primarily due to the absence of high quality single crystals. 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. We untangle the effects of Y by testing our calculations against the measured Raman and infrared spectra of the cubic, antipolar orthorhombic, and monoclinic phases and then proceed to reveal the signature modes of polar orthorhombic hafnia. This work provides a spectroscopic fingerprint for several different phases of HfO2 and paves the way for an analysis of mode contributions to high-κ dielectric and ferroelectric properties for chip technologies.

Simulation of XRD, Raman and IR spectrum for phase identification in doped HfO2 and ZrO2

“…To explore the stability of the phases, the calculation of all phonon modes in the first Brillouin zone is necessary, expressed in the phonon band structure diagram. The calculation of the phonon band structure for the most discussed phases P2 1 /c, Pca2 1 , Pbca, and P4 2 /nmc for HfO 2 was done in Fan et al (2022) using the PBEsol density functional. The calculation for all further phases contained in Table 1 was done in Antunes et al (2022) proving the dynamical stability except for the c-phase and the cII-phase.…”

“…The calculation of the Raman spectrum for polycrystalline, ferroelectric ZrO 2 has first been done in Materano et al (2022) and for monocrystalline, ferroelectric HfO 2 in Fan et al (2022). The spectra for the other conventionally considered phases have been calculated earlier by Zhou et al (2014).…”

“…The calculation of the dielectric functions for ZrO 2 and HfO 2 phases was not carried out before this work. Infrared modes of the ferroelectric phase were previously calculated by Fan et al (2022) and by Materlik et al (2015). Exemplary dielectric functions for other crystals have been performed by Ratnaparkhe and Lambrecht (2020) and Huang and Lambrecht (2016).…”

“…Materano et al (2022) performed the first Raman measurements on stacks of thin ZrO 2 films with stabilizing interlayer, and compared the data to simulated polycrystalline spectra of m-, t-and oIII-phase to identify the ferroelectric phase. Thicker crystalline samples with ferroelectric HfO 2 prepared with a special floating zone technique have been investigated recently in Fan et al (2022) who compared the data with simulated oriented crystal spectra of c-, t-, oI-and oIIIphase. In past work, experimental data from ceramic samples of HfO 2 and ZrO 2 have been collected from Hirata et al (1994), Arashi (1992).…”

“…Reflectance measurements need a sufficiently thick sample, which is available for crystalline samples. Fan et al (2022) performed recently such experiments on relatively thick samples of different crystal phases of HfO 2 . Thin samples can be investigated with the Attenuated Total Reflection (ATR) spectroscopy, which can measure the absorptivity of very thin films.…”