Strain effect in PbTiO3/PbZr0.2Ti0.8O3 superlattices: From polydomain to monodomain structures

Ferroelectric symmetric superlattices consisting of alternating layers of PbTiO3 and PbZr0.2Ti0.8O3, were grown by pulsed laser deposition on SrTiO3 and SrRuO3-coated SrTiO3 substrates. The superlattices, with wavelengths K ranging from 20 angstrom to 200 angstrom, were analyzed using x-ray diffraction (theta - 2 theta diffraction scans, rocking curves, and reciprocal space mapping), high resolution transmission electron microscopy, and piezoforce scanning microscopy. For large-period superlattices, the strain is relieved by the formation of an a/c polydomain structure which propagates through the whole film. We investigate the influence of the wavelength on the a-domain volume fraction, the lattice parameters, the in-plane strain epsilon(xx), and the mosaicity of the samples. We show that with decreasing the wavelength, a reduction of the a-domain volume fraction is observed as well as a reduced tensile in-plane strain and a lower mosaicity. A concomitant improvement of the local ferroelectric response is detected. Below a critical wavelength of about 30 angstrom and a critical sample thickness of 500 angstrom, the formation of 90 degrees a/c domains is inhibited and the superlattices are completely c oriented. Thus the reduced wavelength induces compressive strain which dominates over the tensile clamping due to the thermal expansion mismatch between the substrate and the superlattice. This compressive strain favors a c-oriented structure in the PbTiO3/PbZr0.2Ti0.8O3 superlattices. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4767329]