Lattice dynamical properties of SrTiO[sub 3] thin films

Abstract

We report the experimental studies of the vibrational spectra of SrTiO 3 films grown by pulsed laser deposition. Raman scattering, with and without bias electric field, and Fourier-transform far-infrared ellipsometry between 30 and 700 cm −1 have been utilized for investigation of the phonon behavior. These results are discussed in connection with the low-frequency dielectric properties of the SrTiO 3 films. In the films, the soft mode reveals hardening compared to that in bulk crystals. This observation is in agreement with the Lyddane-Sachs-Teller formalism. The existence of local polar regions is proposed as an important factor determining the dielectric properties of ferroelectric thin films.

Polar Phonon Behaviour in Polycrystalline Bi-Doped Strontium Titanate Thin Films

“…According to reports on the lattice dynamics of ST films, the soft mode of the films is usually much harder than that in the bulk materials at low temperature [ 14 , 15 , 16 , 17 , 18 , 19 ]. Correspondingly, the phonon contribution to the low-temperature dielectric permittivity is strongly suppressed in ST films compared to bulk ST. For epitaxial films deposited by pulsed laser deposition (PLD), it was explained by influence of some specific local polar regions occurring in ST thin films during the deposition process [ 15 ]. For polycrystalline films, porosity and eccentricity of the grain boundaries are suggested as the main reasons for such behaviour [ 18 ].…”

“…For polycrystalline films, porosity and eccentricity of the grain boundaries are suggested as the main reasons for such behaviour [ 18 ]. Sirenko et al reported that at high temperatures the soft-mode frequency in the film and the bulk are close at zero electric field, indicating that either the density of the local polar regions is low or the polarization of these regions is weak or small [ 15 ]. However, whereas the electric-field hardening of the soft mode vanishes in ST crystals above T ≈ 80 K [ 20 ], it persists in ST thin films until high temperatures.…”

“…However, whereas the electric-field hardening of the soft mode vanishes in ST crystals above T ≈ 80 K [ 20 ], it persists in ST thin films until high temperatures. This fact was attributed to the polarization by the electric field of the easily polarisable local regions around oxygen vacancies, which are expected to increase the dielectric loss in thin films [ 15 ]. At the same time, Ostapchuk et al reported that in contrast to polycrystalline ST films, a 300 nm thick ST film, quasiepitaxially grown on a (0001) sapphire substrate with a perfect (111) orientation, displays a ferroelectric phase transition near 125 K induced in the film plane by a tensile residual stress [ 18 ].…”