Lattice dynamics and dielectric properties of ferroelectric thin films for frequency agile devices

摘要

The lattice dynamical properties are important for a fundamental understanding of the tunability and loss mechanisms in thin films. Through Raman scattering and infrared ellipsometry measurements, we found that the soft mode in STO thin films is harder than that in bulk crystals, consistent with the reduced dielectric constant following the LST relation. The symmetry lowering in the STO films, evidenced by the observation of the symmetry-forbidden first-order optical phonon peaks in Raman scattering and the Fano asymmetry in the TO{sub}2 peak, suggests the existence of the local polar regions likely due to oxygen vacancies. Based on the Devonshire's model and the LST relation, the existence of local polar regions can give rise to the harder soft-mode frequency. It can also explain the higher dielectric loss in STO films because the quasi-Debye loss, which is much larger than the multi-phonon losses, but is active only in non-centrosymmetric crystals, can also contribute to the dielectric loss due to the symmetry lowering in STO thin films.