The origin of the weak ferroelectric-like hysteresis effect in paraelectricBa0.5Sr0.5TiO3 thin films grownepitaxially on LaAlO3

Abstract

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 characteristicswere confirmed using x-ray diffraction and transmission electron microscopy (TEM). Thedielectric properties of the BST thin films were measured with a planar capacitorconfiguration in the temperature range of 77–300 K. The capacitance–temperaturecharacteristics, measured with no and several different DC biases, reveal thatthe BST films are in the paraelectric state at room temperature, with a verygood dielectric tunability. However, a butterfly-shaped curve, typical for aferroelectric material, was obtained from room temperature capacitance–voltage(C–V) measurements, suggesting a faint ferroelectric-like effect for the BST thin films. Careful dielectricproperty characterizations showed significant temperature and frequency dependence, probablyindicating a behaviour of Maxwell–Wagner-type dielectric relaxation. As a result, the weakC–V hysteresis effect in our paraelectric BST thin films is believed to be ascribable both tooxygen vacancies and to the presence of other space charges, trapped at the grainboundaries and/or at the substrate/dielectric film interface, which give rise tolocal polar regions in the thin-film samples. Furthermore, this explanation issupported by cross-sectional TEM and off-axis electron holographic observations.