High Permittivity (1 – x)Ba(Zr_0.2Ti_0.8)O₃ – x(Ba_0.7Ca_0.3)TiO₃ (x = 0.45) Epitaxial Thin Films with Nanoscale Phase Fluctuations

Abstract

Epitaxial (1 - x)Ba(Ti0.8Zr0.2)TiO3 - x(Ba0.7Ca0.3)TiO3, x = 0.45 (BCZT 45), thin films have been deposited on (001) SrTiO3 (STO) and (001/100) SrLaAlO4 (SLAO) substrates by pulsed laser deposition. X-ray diffraction and high-resolution transmission electron microscopy (HRTEM) confirmed the epitaxial growth of the films. A high structural quality has been evidenced for the BCZT/STO films. Geometric phase analysis (GPA) associated with the HRTEM enabled us to obtain microstrain analysis and the in-plane and out-of-plane lattice parameter variation on different areas. Tetragonality ratio fluctuations at nanoscale level which are relevant for the existence of nanodomains have been evidenced on the BCZT/STO films. The in-plane dielectric constant has been measured on interdigital electrodes deposited by lift-off technique on the top of the films. High values of dielectric permittivity (>3000) combined with low dielectric loss (<0.01) are obtained for BCZT 45 film deposited on STO substrate, showing nearly constant values between 1 kHz and 10 MHz. The high dielectric permittivity of BCZT thin films was attributed to their high structural quality and to the loss of rotation stability of the polarization associated with the presence of nanodomains. This results into a divergence of fluctuations of polarization direction and a peak of dielectric susceptibility. The enhanced switching of such nanodomain configuration was probed by piezoforce microscopy, by writing and reading domains during topography scanning.

Impact of thickness variation on structural, dielectric and piezoelectric properties of (Ba,Ca)(Ti,Zr)O3 epitaxial thin films

“…Moreover, in all the thickness range, the values of the dielectric permittivity are much higher than in corresponding bulk materials, due to the previously evidenced peculiar nanostructure in partially relaxed epitaxial films 16 and the high crystallinity of the columnar growth in epitaxial films.…”

“…3a, were determined using conventional and off-axis diffraction, as described in our previous papers 16,22 . The evolution of these parameters evidences a strong anisotropy for the very thin film (35 nm); however it decreases rapidly when the thickness approaches 100 nm.…”

“…The in-plane dielectric permittivity and loss have been determined from measurements on different IDT, as described in our previous paper 16 .…”

“…The effective strain u eff in films at a temperature T varies with thickness h as 26,27 :where u(T ) and u(T G ) is the misfit strain at temperature T and at growth temperature T G , a F (T ) and a F (T G ) is the film lattice constant at T and T G temperatures, and h c is the critical film thickness related to the generation of dislocations. We have inserted an empiric correction factor β = 0.5 to take into account that at thicknesses of about 160–175 nm there is still a substantial effective strain, oscillating around 1.5–2% in the film, as previously measured by Geometrical Phase Analysis (GPA) microstrain analysis on HRTEM 16 . In order to obtain the misfit strain at growth temperature (T G = 700 °C), the lattice constants of the substrate (a S ) and BCTZ film (a F ) have been calculated by taking into account the room temperature bulk values (a S = 3.905 Å, a F ≈ 4.02 Å) and the linear thermal expansion coefficient α (α S ≈ 10 −5 K −1 , α F ≈ 1.2×10 −5 K −1 ).…”

“…Thus, in a previous work, we have evidenced the relationship between nanostructure and dielectric properties of BCTZ epitaxial thin films grown on different substrates 16 . We have also deposited BCTZ thin films on flexible polymeric substrates and demonstrated their biocompatibility for use in biomedical applications 17 .…”