Functional Behaviour of Thin Film Dielectric Superlattices
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Functional Behaviour of Thin Film Dielectric Superlattices J. M. Gregg, M. H. Corbett, D. O’Neill, G. Catalan and R. M. Bowman Department of Pure and Applied Physics The Queen’s University of 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 a superlattice. The properties of two superlattice systems were investigated as a function of superlattice wavelength (Λ) – one based on barium strontium titanate and the other on lead-based relaxor electroceramics. In both systems the dielectric constant was significantly enhanced at stacking wavelengths of a few unit cells. However, the dielectric enhancement seen in the barium strontium titanate superlattices was found to be due to Maxwell-Wagner effects, whereas in the relaxor superlattices Maxwell-Wagner behaviour was not evident; rather, the dielectric enhancement was associated with the onset of polar coupling around Λ ~ 10nm. INTRODUCTION Significant interest in dielectric superlattices has developed over the last decade, fuelled by the possibility of functional properties in superlattices being superior to those of compositionally equivalent solid solutions. Experimentally, dielectric constants have frequently been observed to increase on decreasing superlattice wavelength (Λ), with the most dramatic study claiming a relative dielectric constant of 420,000 [1]. Other effects include reduced temperature dependence of dielectric properties and potentially dramatically enhanced polarisation [2]. Interesting physics has also emerged, with inter-layer coupling occurring at relatively small Λ [3]. Such results have encouraged modelling of dielectric superlattices in which coupling is considered. Unfortunately, there are serious inconsistencies in the body of research produced to date. The extent of dielectric enhancement on decreasing Λ varies dramatically, and is found to be a maximum at very different scales of heterostructure; dielectric losses are not fully reported, or are high in much of the work. In an attempt to rationalise such inconsistencies the authors here report on the properties of two superlattice systems: Ba0.2Sr0.8TiO3 / Ba0.8Sr0.2TiO3 and Pb(Mg1/3Nb2/3)O3 / [0.2Pb(Zn1/3Nb2/3)O3-0.8BaTiO3]. EXPERIMENTAL DETAILS Thin film capacitors were made by pulsed laser deposition (PLD): SrRuO3 lower electrodes were deposited onto single crystal {100} MgO substrates under 0.15mbar of oxygen at a substrate temperature of 775-800oC. Superlattices were then deposited (BST-based at 775oC, and relaxor-based at 630oC), before post-deposition annealing under 900 mbar O2. Specimens were then removed from the PLD system and two gold electrodes (~2 mm2) were deposited by thermal evaporation through a hard mask. Dielectric testing was performed making contact to two upper electrodes (two capacitors in series) and using Hewlett Packard LCR meters in conjunction with Oxford Instruments cryostats. Polarisation loops were measured using a Radiant Technologies C6.7.1
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