Effects of poling, and implications for metastable phase behavior in barium strontium titanate thin film capacitors

Barium strontium titanate (Ba0.5Sr0.5TiO3–BST) thin film capacitor structures were made using pulsed laser deposition, and their functional properties were monitored with varying temperature. It was found that poling at low temperature could induce distinct differences in the behavior of the dielectric constant and loss tangent, on heating. In relatively thick BST films (>∼650nm), poling the sample at 80K produced a change from a single broad anomaly to one in which three distinct anomalies could be observed. The temperatures of these anomalies (∼140, ∼200, and ∼260K) were close to those known to be associated with phase transitions in bulk. Monitoring changes in polarization loops with temperature confirmed the likelihood that the dielectric anomalies observed were indeed the result of phase transitions in the films. Unusually, though, when the films were poled at 150K, and then cooled to 80K prior to collection of dielectric data on heating, the dielectric anomaly around 140K was completely suppressed. The lack of a phase transition was confirmed by monitoring depolarization currents in the sample. It is suggested that poling has therefore allowed the phase state that existed at 150K to persist metastably down to 80K. For relatively thin BST films (<∼400nm), poling at 80K only induced two distinct anomalies in the dielectric response (at ∼200 and ∼290K). Nevertheless, poling-related metastability could again be observed: when the samples were poled at 250K and then cooled to 80K prior to data collection on heating, the anomaly at ∼200K was completely suppressed. These experiments suggest that metastable phase behavior could be commonplace in thin film ferroelectrics.