Phase diagrams, dielectric response, and piezoelectric properties of epitaxial ultrathin (001) lead zirconate titanate films under anisotropic misfit strains

JOURNAL OF APPLIED PHYSICS 107, 114105 2010
Q. Y. Qiu and V. Nagarajan
School of Materials Science and Engineering, University of New South Wales, Sydney,
New South Wales 2052, Australia
S. P. Alpay
Materials Science and Engineering Program, Department of Chemical, Materials, and Biomolecular
Engineering and Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269, USA
Received 16 December 2009; accepted 13 March 2010; published online 3 June 2010
We develop a nonlinear thermodynamic model to predict the phase stability of ultrathin epitaxial
001 -oriented ferroelectric PbZr1−xTixO3 PZT films with x=1.0, 0.9, 0.8, and 0.7 on substrates
which induce anisotropic in-plane strains. The theoretical formalism incorporates the relaxation by
misfit dislocations at the film deposition temperature, the possibility of formation of ferroelectric
polydomain structures, and the effect of the internal electric field that is generated due to incomplete
charge screening at the film-electrode interfaces and the termination of the ferroelectric layer. This
analysis allows the development of misfit strain phase diagrams that provide the regions of stability
of monodomain and polydomain structures at a given temperature, film thickness, and composition.
It is shown that the range of stability for rotational monodomain phase is markedly increased in
comparison to the same ferroelectric films on isotropic substrates. Furthermore, the model finds a
strong similarity between ultrathin PbTiO3 and relatively thicker PZT films in terms of phase
stability. The combinations of the in-plane misfit strains that yield a phase transition sequence that
results in a polarization rotation from the c-phase polarization parallel to the 001 direction in the
film to the r-phase, and eventually to an in-plane polarization parallel to the 110 direction the
aa-phase is determined to be the path with the most attractive dielectric and piezoelectric
coefficients resulting in enhancements of 10 to 100 times in the dielectric permittivity and
piezoresponse compared to bulk tetragonal ferroelectrics of the same PZT composition. © 2010
American Institute of Physics. doi:10.1063/1.3386465