Piezoelectric and dielectric tunabilities of ultra-thin ferroelectric heterostructures
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V. Nagarajan School of Materials Science and Engineering, University of New South Wales, Sydney NSW 2052, Australia (Received 18 November 2005; accepted 22 March 2006)
The scaling of the piezoelectric and dielectric constants with film thickness in ultra-thin ferroelectric heterostructures is investigated. Epitaxial (001) PbZr0.2Ti0.8O3 films ranging in thickness from 5 nm to 30 nm with top and bottom SrRuO3 electrodes were grown onto (001) SrTiO3 substrates via pulsed laser deposition. Piezoelectric and dielectric measurements were performed using an atomic force microscope. The remnant value of the out of plane piezoresponse (d33) decreases from 60 pm/V for the 30 nm film to just 7 pm/V for the 5 nm film. This systematic decline in d33 is accompanied by a corresponding increase in the coercive field. The d33 loops show a systematic increase in tilt towards the applied field axis as function of reducing thickness coupled with a decrease in piezoelectric tunability. The small-signal relative dielectric response in the direction normal to the film-substrate interface decreases from 140 for a 50 nm film to just 60 for a 8 nm film. A similar drop is also observed in the dielectric tunability, from ∼17% to approximately −2% at an electric field of 750 kV/cm with the film thickness decreasing from 50 nm to 8 nm. We show that these observations cannot be explained using a straightforward application of a modified Landau-Devonshire thermodynamic model that incorporates the internal stresses due to the lattice and thermal expansion mismatch between the film and the substrate. We attribute this behavior to degradation in the polarization due to an intrinsic finite size effect. I. INTRODUCTION
In recent years there has been intense research on the size scaling of ferroelectric nanostructures.1 In particular, thickness scaling for ultra-thin films has been investigated via novel structural measurements,2–5 pyroelectric measurements,6 measurement of the polarization,7,8 piezoresponse scanning force microscopy9,10 and a variety of theoretical models.11–16 More recently tunneling effects have also been explored17 and a few reports demonstrate conductance switching in ultra-thin films as well.18–20 In addition to the polarization and the structure, ferroelectric parameters that require attention are the scaling of the piezoelectric and dielectric response. Although there have been studies on dielectric and piezoelectric constants of films few tens of nanometers thick,21 information for films in the “ultra-thin” regime (
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