The origin of the weak ferroelectric-like hysteresis effect in paraelectric Ba0.5Sr0.5TiO3 thin films grown epitaxially on LaAlO3

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.