ORIENTATION DEPENDENCE OF THE PIEZOELECTRIC PROPERTIES OF EPITAXIAL FERROELECTRIC THIN FILMS
Roytburd, Alexander L.
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There are both intrinsic piezoelectric response and extrinsic piezoelectric response in ferroelectric materials. The intrinsic piezoelectric response is due to the lattice deformation of a single-domain crystal, which can be characterized by tensors of piezoelectric constants. The extrinsic piezoelectric response depends on extrinsic sources of displacement under the electric field, which can be the movement of domain walls, phase boundaries, or even defects like grain boundaries or dislocations. Due to the elastic interaction between an epitaxial ferroelectric thin film and a substrate, the piezoelectric properties of an epitaxial ferroelectric film are different from those of bulk ferroelectric materials. This work is the first study on the general orientation dependence of the piezoelectric properties of epitaxial ferroelectric thin films, which includes both theoretical and experiment work on intrinsic and extrinsic piezoelectric properties of epitaxial ferroelectric films. A complete theoretical analysis of intrinsic piezoelectric responses in a single domain ferroelectric film, which are characterized by effective longitudinal,transverse and shear piezoelectric coefficients, is presented in this dissertation. On the part of extrinsic piezoelectric response, our recent work on the piezoelectric properties of epitaxial thick lead titanate zirconate (Pb(ZrxTi1-x)O3 with x=0.52) films with tetragonal distorted structures will be presented as an example. It is shown that(011) oriented epitaxial films had much enhanced piezoelectric responses as compared with those of (001) and (111) oriented films. Detailed structure analysis showed that instead of an interconnected 3-domain (3-D) architecture that is usually found in a (001) oriented thick film, the (011) films consisted of a dominant 2-domain (2-D) architecture, by which the pinning between neighboring domain walls is much reduced. This study demonstrate the possibility of achieving high extrinsic piezoelectric responses by optimizing the epitaxial relationship between the film and substrate with respect to the domain mobility, and should also be instructive to the design of ferromagnetic and ferroelastic thin film devices used for transducer applications.