Experimental and Theoretical Investigation into the Dielectric Behaviour of Ferroelectric Thin Film Superlattices
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Experimental and Theoretical Investigation into the Dielectric Behaviour of Ferroelectric Thin Film Superlattices J. M. Gregg, D. O’Neill, G. Catalan and R. M. Bowman Condensed Matter Physics and Materials Science Division School of Maths and Physics, Queen’s University Belfast Belfast BT7 1NN, U. K. ABSTRACT Pulsed laser deposition has been used to fabricate thin-film capacitor structures in which the dielectric layer is composed of a superlattice of Ba0.8Sr0.2TiO3 and Ba0.2Sr0.8TiO3. The properties of the capacitors were investigated as a function of superlattice periodicity. The dielectric constant was significantly enhanced, and temperature migration of the peak in dielectric constant as a function of frequency was observed, at stacking periodicities of a few unit cells. However, such ‘relaxor-like’ features were found to be associated with high dielectric loss. Analysis of the imaginary permittivity as a function of frequency showed that fine-scale superlattices conform to Maxwell-Wagner behaviour, indicating that the observed features may be an artefact of increased carrier mobility. Modelling showed that both dielectric enhancement and frequency relaxation could readily be reproduced by Maxwell-Wagner formalism. INTRODUCTION There is strong evidence that the physics of relaxor behaviour is intimately linked to nanoscale chemical heterogeneity [1- 4]. A number of investigations have used growth of thin film ceramic heterostructures to see whether artificial nanolayering of different ferroelectric materials can reproduce the beneficial properties of relaxors [5-11]. In many cases enhanced dielectric constants have been observed. Notably Erbil et al. [5] claimed a relative dielectric constant of 420,000 at 1kHz and at room temperature. Qu et al. [6] have also seen evidence that nanolayer heterostructures show the migration of Tm seen in relaxors, and nearly all work shows broad temperature dependence of dielectric response. However, there are serious inconsistencies in the body of research produced to date. The extent of dielectric enhancement varies dramatically, and is found to be a maximum at very different scales of heterostructure. Crucially, dielectric losses are not fully reported, or are high in much of the work. In an attempt to rationalise such inconsistencies the authors report from a study examining thin film capacitor structures in which the dielectric layer is a superlattice of Ba0.8Sr0.2TiO3 and Ba0.2Sr0.8TiO3. EXPERIMENTAL DETAILS Thin film capacitors were made by Pulsed Laser Deposition (PLD) as follows: SrRuO3 lower electrodes were deposited onto single crystal {100} MgO substrates under 0.15mbar of oxygen with a substrate temperature of 775oC. Ba0.8Sr0.2TiO3 / Ba0.2Sr0.8TiO3 superlattices were then deposited under the same conditions, before cooling to 650oC for a 15 min post-deposition anneal under 900 mbar O2. The total thickness of all the dielectric superlattices was maintained at 250 ± 15 nm. The specimen was then removed from the PLD system and two gold electrodes (~2 mm2) were deposited
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