Enhancing Ferroelectrics using Strain

Darrell G. Schlom
Pennsylvania State University

Using epitaxy and the misfit strain imposed by an underlying substrate, we have strained ferroelectric thin films to percent levels far beyond where they would crack in bulk. Under such strains, the ferroelectric properties of SrTiO3 and BaTiO3 are dramatically enhanced. Although SrTiO3 is normally not ferroelectric at any temperature, predictions based on thermodynamic analysis imply that a biaxial strain of order 1% will shift its paraelectric-to-ferroelectric transition temperature (Tc) to the vicinity of room temperature. Such strains are also predicted to elevate the Tc of BaTiO3 by comparable amounts. In practice, the synthesis of uniformly strained ferroelectric films is challenging. Epitaxial ferroelectric films are often grown to thicknesses greatly exceeding their critical values, resulting in undesirable relaxation toward a zero-strain state by the introduction of dislocations. Dislocation densities of ~1011cm-2 are common in epitaxial ferroelectric films grown on lattice-mismatched substrates, and the resulting inhomogeneous strain smears out the ferroelectric phase transition. Our approach to controlling the properties of ferroelectric SrTiO3 and BaTiO3 films centers on the development of new substrates (DyScO3 and GdScO3) that enable the growth of uniformly strained films below, or at least far closer to, the critical thickness for relaxation. Our results1,2 demonstrate not only the largest strain-induced shift in Tc ever achieved, but also manifest a paradigm shift in how to manipulate the properties of ferroelectric thin films. Strain is a viable alternative to the traditional method of chemical substitutions for shifting Tc by large amounts. These strained SrTiO3 and BaTiO3 films have better structural perfection (narrower rocking curve widths) than SrTiO3 and BaTiO3 single crystals. An unexpected surprise is that the strained SrTiO3 films exhibit a frequency dependence of their dielectric constant consistent with relaxor ferroelectricity.

1 J.H. Haeni, P. Irvin, W. Chang, R. Uecker, P. Reiche, Y.L. Li, S. Choudhury, W. Tian, M.E. Hawley, B. Craigo, A.K. Tagantsev, X.Q. Pan, S.K. Streiffer, L.Q. Chen, S.W. Kirchoefer, J. Levy, and D.G. Schlom, Nature 430 (2004) 758-761.

2 K.J. Choi, M. Biegalski, Y.L. Li, A. Sharan, J. Schubert, R. Uecker, P. Reiche, Y.B. Chen, X.Q. Pan, V. Gopalan, L.-Q. Chen, D.G. Schlom, and C.B. Eom, Science 306 (2004) 1005-1009.