Content area
Full Text
ARTICLES
Unit-cell scale mapping of ferroelectricity and tetragonality in epitaxial ultrathin ferroelectric lms
CHUN-LIN JIA1,2*, VALANOOR NAGARAJAN3, JIA-QING HE1,2, LOTHAR HOUBEN1,2, TONG ZHAO4, RAMAMOORTHY RAMESH4, KNUT URBAN1,2 AND RAINER WASER1
1Institute of Solid State Research, Research Centre Jlich, D-52425 Jlich, Germany
2Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons (ER-C), Research Centre Jlich, D-52425 Jlich, Germany
3School of Materials Science and Engineering, University of New South Wales, Sydney NSW, Australia
4Dept of Materials Science and Dept of Physics, University of California, Berkeley, California 94720, USA *e-mail: [email protected]
Published online: 17 December 2006; doi:10.1038/nmat1808
Typically, polarization and strain in ferroelectric materials are coupled, leading to the generally accepted direct relation between polarization and unit-cell tetragonality. Here, by means of high-resolution transmission electron microscopy we map, on the unit-cell scale, the degree of tetragonality and the displacements of cations away from the centrosymmetry positions in an ultrathin epitaxial PbZr0.2Ti0.8O3 lm on a SrRuO3 electrode layer deposited on a SrTiO3 substrate. The lattice is highly tetragonal at the centre of the lm, whereas it shows reduced tetragonality close to the interfaces. Most strikingly, we nd that the maximum o-centre displacements for the central area of the lm do not scale with the tetragonality. This challenges the fundamental belief in a strong polarizationtetragonality coupling in PbTiO3-based ferroelectrics, at such thicknesses. Furthermore, a systematic reduction of the atomic displacements is measured at the interfaces, suggesting that interface-induced suppression of the ferroelectric polarization plays a critical role in the size eect of nanoscale ferroelectrics.
Epitaxial ultrathin (a few nanometres thick) lms of ferroelectric oxides, such as Pb(Zr,Ti)O3 (PZT) and BaTiO3 (BTO), have attracted considerable attention with respect to potential applications in nanoelectronic high-density memory devices1,2.
The magnitude and stability of the switchable ferroelectric polarization are the central gures of merit for such devices. How the polarization scales with reduced thickness, particularly when the lm thickness becomes smaller than typically 10 nm, is now an intensely debated topic310. This so-called size eect represents one of the most intriguing phenomena in the area of thin-lm ferroelectrics.
In ultrathin epitaxial PbTiO3 (PTO) lms8 deposited on conducting Nb-doped SrTiO3 (STO) substrates, the decrease in measured polarization is commensurate with a corresponding decrease in the tetragonality and is in line with the traditionally accepted model of...