Perovskite Ba0.5Sr0.5TiO3
(BST) thin films, with thickness of about 350 nm, have been epitaxially grown on (001)
LaAlO3
substrates by pulsed-laser deposition. The good crystallography and epitaxy characteristics
were confirmed using x-ray diffraction and transmission electron microscopy (TEM). The
dielectric properties of the BST thin films were measured with a planar capacitor
configuration in the temperature range of 77–300 K. The capacitance–temperature
characteristics, measured with no and several different DC biases, reveal that
the BST films are in the paraelectric state at room temperature, with a very
good dielectric tunability. However, a butterfly-shaped curve, typical for a
ferroelectric material, was obtained from room temperature capacitance–voltage
(C–V) measurements, suggesting a faint ferroelectric-like effect for the BST thin films. Careful dielectric
property characterizations showed significant temperature and frequency dependence, probably
indicating a behaviour of Maxwell–Wagner-type dielectric relaxation. As a result, the weak
C–V
hysteresis effect in our paraelectric BST thin films is believed to be ascribable both to
oxygen vacancies and to the presence of other space charges, trapped at the grain
boundaries and/or at the substrate/dielectric film interface, which give rise to
local polar regions in the thin-film samples. Furthermore, this explanation is
supported by cross-sectional TEM and off-axis electron holographic observations.
Ferroelectric and superconductor bilayers of Ba0.1Sr0.9TiO3 (BSTO)∕YBa2Cu3O7−δ (YBCO), with different YBCO film thicknesses, have been fabricated in situ by pulsed-laser deposition on 1.2° vicinal LaAlO3 substrates. The dielectric properties of BSTO thin films were measured with a parallel-plate capacitor configuration in the temperature range of 77–300K. We observed a strong dependence of the dielectric properties of BSTO thin films on the thickness of the YBCO layer. As the YBCO-film thickness increases, the temperature of the dielectric permittivity maximum of BSTO thin films shifts to higher values, and the leakage current and dielectric loss increase drastically, while the dielectric constant and dielectric tunability decrease remarkably. The results are explained in terms of the transformation in the growth mode of the YBCO layer from two-dimensional step flow to three-dimensional island that leads to significant deterioration in the dielectric properties of BSTO thin films. We propose that improved dielectric properties could be obtained by reasonably manipulating the growth mode of the YBCO layer in the multilayer structures.
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