NSLS-II Seminar

Under biaxial strain the ferroelectric properties of many ferroelectric materials have been predicted to be enhanced. Using reactive MBE, strained epitaxial films of BaTiO3 and SrTiO3 were grown on rare earth scandate substrates. Under these conditions strain levels of ~1% are obtained. At these strain levels bulk oxide materials would typically fracture, but not films thinner than the Griffith crack criterion. Prior researchers have grown epitaxial films of these materials to thicknesses greatly exceeding their critical thicknesses (~4 nm for BaTiO3 growth on SrTiO3). This results in a high dislocation density and an inhomogeneous strain that can smear out the ferroelectric phase transition and reduce the ferroelectric properties of the material. Our approach limits this by developing new substrates, DyScO3 and GdScO3, that allow for the growth of uniformly strained films below or near the critical thickness. This technique allows our strained SrTiO3 and BaTiO3 films to have even better structural perfection (narrower rocking curve widths) than bulk single crystals of these materials.
Modeling of ferroelectrics under these strain levels predicts dramatic shifts in the transition temperature and enhancement of the polarization. Indeed, in our strained SrTiO3, a material that is normally not ferroelectric at any temperature, a ferroelectric state was induced with a transition near room temperature. These films also exhibit the large dielectric constant (~20,000) normally seen at very low temperature (<4K) in bulk SrTiO3 at room temperature. An unexpected surprise is that the strained SrTiO3 films exhibit a frequency dependence of their dielectric constant consistent with relaxor ferroelectricity.
Our strained BaTiO3 not only shows the largest strain induced shift of the ferroelectric transition ever reported, but also a two to three fold increase of the remanent polarization. The strained BaTiO3 films offer a lead-free ferroelectric alternative to Pb(Zr,Ti)O3, w