Infrared phonons in monoclinic Hf0.5Zr0.5O2 thin films

Infrared (IR) reflectance spectra of monoclinic Hf0.5Zr0.5O2 (HZO) films with thickness in the range of 10–90 nm deposited on different cuts of Al2O3 and yttria-stabilized zirconia (YSZ) substrates were measured in the frequency region 30–4000 cm−1. Several phonons belonging to HZO were observed even for the 10 nm films. X-ray diffraction measurements were used to determine the crystal structure of the films and their orientation with respect to the substrates. The phonon spectra of HZO films grown on the anisotropic (11¯02) R-cut Al2O3 substrates exhibit strong anisotropy. Comparing them with the spectra of the films on isotropic YSZ substrates, it allows us to assign the symmetry and polarization of the phonons in HZO films. Fitting the IR spectra by the classical Lorentz model, we obtain the phonon parameters, their response functions, and their contribution to static permittivity. We also measured the IR reflectance of HfO2, ZrO2 and HZO ceramics and compared their phonon parameters with those of the corresponding thin films.