Cellular Domain Architecture of Stress-free Epitaxial Ferroelectric Films
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Cellular Domain Architecture of Stress-free Epitaxial Ferroelectric Films S. P. Alpay 1, A. L. Roytburd, V. Nagarajan, L. A. Bendersky 2, and R. Ramesh Department of Department of Materials and Nuclear Engineering, University of Maryland, College Park, MD 20742, U.S.A. 1 Department of Metallurgy and Materials Engineering and Institute of Materials Science, University of Connecticut, Storrs, CT 06269, U.S.A. 2 Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899, U.S.A. ABSTRACT Epitaxial ferroelectric films undergoing a cubic-tetragonal phase transformation relax internal stresses due to the structural phase transformation and the difference in the thermal expansion coefficients of the film and the substrate by forming polydomain structures. The most commonly observed polydomain structure is the c/a/c/a polytwin that relieves the internal stresses only partially. Relatively thicker films may completely reduce internal stresses if all three variants of the ferroelectric phase are brought together such that the film has the same in-plane size as the substrate. In this article, we provide experimental evidence on the formation of the 3-domain structure based on transmission electron microscopy in 450 nm thick (001) PbZr0.2Ti0.8O3 films on (001) SrTiO3 grown by pulsed laser deposition. X-ray diffraction studies show that the film is fully relaxed. Experimental data is analyzed in terms of a domain stability map. INTRODUCTION Polydomain formation in epitaxial ferroelectric films undergoing a cubic-tetragonal phase transformation is a mechanism that relaxes internal stresses that are a result of the lattice misfit due to the structural phase transformation and the difference in the thermal expansion coefficients of the film and the substrate. The polydomain structure consists of the three possible orientational domains of the tetragonal phase separated from each other by elastically compatible 90o domain walls. Figure 1a shows all possible orientational variants of the ferroelectric phase and possible polydomain structures due to a cubic-tetragonal transformation. Ferroelectric films are deposited at temperatures above the transition temperature and the internal stresses are usually completely relaxed by misfit dislocations at this temperature [1,2]. Thus, just before the structural phase transformation at TC the films are usually stress-free and tensile internal stresses develop below TC due to the phase transformation. Therefore, most common microstructure observed in epitaxial films PbTiO3 [2] and tetragonal Pb(Zr,Ti)O3 solid solutions [4] is the c/a/c/a polydomain consisting of alternating platelets of c-domains with the tetragonal axis perpendicular to the film-substrate interface and a-domains with the c-axis of the tetragonal film along [100] or [010] directions, called a1- and a2-domains, respectively (Figure 1b). The c/a/c/a polytwin structure relaxes the internal stresses only partially by reducing the biaxial stress state to a uniaxial one. The int
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