Synchrotronstrahlungs-Experimente zu Skalierungseffekten und ungewöhnlichen Phasen epitaktischer perowskitischer Schichten
Zusammenfassung der Projektergebnisse
Ferroelectric thin films are the subject of intensive experimental and theoretical research due to their numerous applications in present and future electronics. The current trend towards downscaling of electronic devices makes the investigations of size effects on the physical properties of such films especially important. For a better understanding of the nature of such effects in ferroelectric films, it is important to analyse both the polarization and lattice parameters precisely in a wide range of film thicknesses and compare the results to state-of-the-art theoretical calculations. In particular, the possible existence of a thickness limit for the existence of ferroelectricity in perovskite oxide films such as BaTiO3 or Pb(ZrxTi1-x)O3 is a topical question. High-quality BaTiO3 films with thicknesses from several nanometers to 0,175 microns were grown on SrTiO3 substrates by high-pressure sputtering. The films were designed to grow in registry (epitaxially) with the crystal structure of the substrate, leading to well-defined mechanical stresses in the films that leads to unusual behaviour compared to 3D crystals. The structure of the films was analyzed by x-ray diffraction, and the lattice parameters were determined as a function of the film thickness. To study the electrical properties, capacitors with SrRuO3 electrodes were produced and polarization-voltage hysteresis loops were measured at frequencies from 1 to 30 kHz. To study ultrathin layers, also thin films in wedge geometry were deposited across SrTiO3 substrates. BaTiO3 films were found to be fully strained by the SrTiO3 substrate up to a thickness of about 30 nanometers. The observed thickness effect on the lattice parameters and the electrical polarization was analyzed in the light of strain and depolarizing-field effects using a thermodynamic theory. The theoretical predictions were found in reasonable agreement with the measured thickness dependences, although the maximum experimental values of the polarization and the lattice parameter in growth direction exceeded the theoretical estimates. The electrical measurements on wedges performed at low temperatures (77 K) showed that, despite progressive reduction in polarization with decreasing film thickness, even a BaTiO3 film as thin as 3.5 nanometers retains a large permanent electrical polarization. The experimental and theoretical studies hence indicate that even ultrathin films containing only a few unit cells in the thickness direction may remain ferroelectric under appropriate electrical and mechanical boundary conditions and could be used for storage purposes.
Projektbezogene Publikationen (Auswahl)
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Polarization states of polydomain epitaxial Pb(Zr1-xTix)O3 thin films and their dielectric properties, Phys. Rev. B 73, 214103 (2006)
V. G. Kukhar, N. A. Pertsev, H. Kohlstedt, and R. Waser
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Recent developments in thermodynamic theory of ferroelectric thin films, in: Ferroelectric Thin Films XIII, edited by R. Ramesh, J.P. Maria, M. Alexe, and V. Joshi, Mater. Res. Soc. Symp. Proc. 902E, Art. T-08-05 (Warrendale, PA, 2006)
N.A. Pertsev
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Elastic stabilization of a single-domain ferroelectric state in nanoscale capacitors and tunnel junctions, Phys. Rev. Lett. 98, 257603 (2007)
N.A. Pertsev, H. Kohlstedt
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Polarization and lattice strains in epitaxial BaTiO3 films grown by high-pressure sputtering, J. Appl. Phys. 101, 114106 (2007)
A. Petraru, N.A. Pertsev, H. Kohlstedt, U. Poppe, R. Waser, A. Solbach, U. Klemradt
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Wedgelike ultrathin epitaxial BaTiO3 films for studies of scaling effects in ferroelectrics, Appl. Phys. Lett. 93, 072902 (2008)
A. Petraru, H. Kohlstedt, U. Poppe, R. Waser, A. Solbach, U. Klemradt, J. Schubert, W. Zander, N.A. Pertsev
