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Queen Mary University of LondonQueen Mary University of London
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School of Engineering and Materials Science
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PhD Thesis: The elastic and electromechanical properties of ferroelectric thin films studied using spherical nanoindentation

Author: ROBERTS, Wei Lim

Year: 2007

Supervisor(s): Mike Reece

Pb[Zr1-xTix]O3(PZT) in thin film form is of interest for electronic applications including sensors, actuators, and cantilevers in micro-electro-mechanical systems (MEMS) and memory devices due to its piezoelectric and ferroelectric properties.

Elastic and electromechanical data for PZT films is necessary for MEMS design. In this study we have investigated the use of nanoindentation to measure the elastic and piezoelectric properties of PZTA films. The elastic properties of PZT films with a range of thicknesses, textures and compositions, were studied in both unpoled and poled films.

The characterisation of PZT films was performed with high spatial resolution suing spherical nanoindentation. The indentation modulus of the films was found to be similar for all of the films tested, regardless of composition. This is in contrast to the reported results for bulk PZT materials. The small change in the indentation stiffness with respect to composition is attributed to the fact that the nanoindentation technique measures the intrinsic stiffness of the PZT films. In order to explain the large variation in the stiffness of materials with different Zr/Ti ratios reported in the literature, we propose that it is due to a greater extrinsic contribution from domain wall movement in the test methods used to characterise bulk materials. XRD analysis was used to quantify the texture of the PZT films. A weighted averaging of the elastic stiffness was performed to predict the modulus of PZT films with mixed orientation.

Electromechanical experiments were performed using conductive indenter tips, which allowed the electrical displacements produced during indentation to be measured. The effective piezoelectrical coefficients deff were measured, and these were comparable to d33 coefficients measured by a flexure technique. Hysteric force-current and force-charge loops were observed and their behaviour fitted a Rayleigh relation. At relatively high indentation loads, irreversible depolarisation was observed. This study has shown that the nanoindentation technique can provide useful elastic data for MEMS device design. The technique can also be used to investigate the electromechanical response of the films.