Investigation into the dielectric behavior of ferroelectric superlattices formed by pulsed laser deposition

Abstract

In an attempt to reproduce the functional properties associated with relaxor electroceramics, pulsed laser deposition has been used to fabricate thin-film capacitor structures in which the dielectric layer is composed of a superlattice of Ba_0.8Sr_0.2TiO₃ and Ba_0.2Sr_0.8TiO₃. The properties of the capacitors were investigated as a function of superlattice periodicity. The dielectric constant was enhanced at stacking periodicities of a few unit cells, consistent with relaxor behavior. However, enhancement of the dielectric constant was found to be associated with high dielectric loss. Analysis of the imaginary permittivity as a function of frequency shows that fine-scale superlattices conform to Maxwell–Wagner behavior. This suggests that the observed enhancement of the real part of the dielectric constant is an artefact produced by carrier migration. A comparison of this data with that already published on dielectric superlattices suggests that previous claims of an enhancement in dielectric constant may also be due to the Maxwell–Wagner effect. The onset of Maxwell–Wagner behavior was attributed to increasing density of defect zones associated with discontinuities in the superlattice structures. In an attempt to exaggerate the influence of such zones, deliberate delays between deposition of successive dielectric layers were introduced. This resulted in reproduction of several features normally associated with relaxors: enhancement of dielectric constants by over an order of magnitude; strong frequency dispersion around and below T_m; migration of T_m with frequency. However, these features were again associated with relatively high loss.