Dielectric characteristics of ferroelectric films prepared by aerosol deposition in THz range

The dielectric properties of lead titanate [PbTiO₃] and lanthanum-modified lead zirconate titanate [PLZT, Pb_0.91La _0.09-(Zr_0.65Ti_0.35)O₃] films, prepared by aerosol deposition (AD), were measured in the frequency range from 100 GHz to 10 THz and in the kHz range. THz time-domain spectroscopy was applied for measurement in the wavenumber range from 4 cm^-1 to 70 cm ^-1, and far-infrared Fourier transform measurements from 70 cm ^-1 to 700 cm^-1 was carried out. We observed that the dielectric constants change smoothly from kHz to THz ranges for the as-deposited PbTiO₃ film, indicating that the as-deposited PbTiO₃ film exhibits no dielectric dispersion due to domain wall motions. The value of the real part of the dielectric constants in the kHz range increased with annealing, suggesting the appearance of Debye relaxation by annealing. We found that the real parts of the dielectric constants of the AD-PLZT films in the kHz range were one order of magnitude smaller than those of bulk PLZT ceramics, while the dielectric responses in the THz range, which were determined by phonon modes, were comparable to those of bulk ceramic PLZT. The structure of AD-PLZT film, which consists of grains of about 20 nm in size, may affect the motion of polar nanoregions, resulting in a low dielectric response in the low frequency range.