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In situ X-ray characterization of piezoelectric ceramic thin films ing the intrinsic time Outlook scales of the processes In situ diffractometry studies offer responsible for the a quantitative way to characterize the electronic properties properties of piezoelectric materials and of ferroelectrics. In to begin to understand the fundamental epitaxial ferroelec- origin of these properties. The precision trics, polarization with which lattice parameters can be switching occurs by measured in X-ray studies allows piezo- a process in which electric coefficients to be extracted domains of the polar- quantitatively for thin-film materials, ization favored by the in the conventional case where the field nucleate and films expands normal to the surface and grow across the film. when adjacent areas cooperatively vary This process can be their in-plane structures. In more com- (Credit: Evans; U. Wisconsin-Madison.) imaged stroboscopi- plex systems, in situ probes allow the cally by using the properties of piezoelectrics to be studied large piezoelectric at high electric fields, very short pulse expansion that occurs times, and in systems with unusual when the polarization domain patterns. Further applications Figure 6. Disappearance of domain satellite reflections at switches as a marker of this approach will allow researchers Q = 0.08 Å–1 in a PbTiO /SrTiO superlattice in an applied for the transition. to better understand the relationship electric field. The decrease in the intensity of these satellites The images reveal between piezoelectric properties and33y occurs because the applied field drives the system out of the that domains in a crystallographic symmetry, for example, striped domain state and into a configuration with uniform PZT thin film nucle- in testing theoretical predictions of the polarization in the SrTiO3 and PbTiO3 layers. ate with character- role of distortions of oxygen octahedra pulses or very large electric fields. The istic spacings of several micrometers in superlattice materials.19 Advances ability to apply short-duration electric- and subsequently propagate into the in X-ray technology will allow these field pulses allows materials to be stud- unswitched material at a velocity of 40 studies to extend to shorter picosecond- ied in electric fields with magnitudes meters per second under electric fields scale times, and with improvements far larger than fields at which the film of 230 kilovolts per centimeter.6 in X-ray detectors, to probe less-well- would exhibit dielectric breakdown in Diffractometry probes are particu- ordered systems including polymers and steady state. These high fields can reach larly useful when the thin film has a other organic piezoelectrics. 2 to 3 megavolts per centimeter and lead complex domain pattern. For example, to strains of 2.0 percent in BiFeO and epitaxial superlattices consisting of Acknowledgments up to 2.7 percent in Pb(Zr,Ti)O .11, 15 alternating layers of dielectric and fer- The authors gratefully acknowledge3 These fields can be large enough that roelectric materials spontaneously form support from the Ceramics Program of3 the approximations that the strain and a nanometer-scale striped domain pat- the NSF Division of Materials Research electric field are small no longer apply. tern that results because of the weak through grants DMR-0705370 and For BiFeO , in particular, the effects interaction between adjacent ferroelec- DMR-1106050. The authors also thank that result from high fields are particu- tric layers.17 An applied electric field Pice Chen, Alexei Grigoriev, Ji Young3 larly large, as shown in Figure 5.11 In can favor stronger coupling, enough to Jo, and Dal-Hyun Do for collaborations this case, the electric field is applied drive the system into a single-polariza- and insightful discussions. along the pseudocubic 001 direction tion state. In this situation, diffraction of a BiFeO thin film, leading to a large information comes from the domains About the authors strain consistent with the rotation of themselves and from the piezoelectric Paul Evans is a professor at the3 the polarization and the possibility that strain induced by applied electric fields, University of Wisconsin–Madison. the system is approaching a structural as shown in Figure 6.19 Insight into the Rebecca Sichel-Tissot earned her rhombohedral-to-tetragonal phase mechanism of the electric-field-induced PhD in 2011 from the University of transition. Such phase transitions have transformation from the striped-domain Wisconsin–Madison and is presently been reported in thin films grown with state to the eventual uniform polariza- a postdoctoral researcher at Drexel varying degrees of epitaxial mismatch,16 tion state can be obtained either at University. Contact: evans@engr.wisc. but diffractometry probes have not yet long timescales using laboratory X-ray edu, rebecca.j.sichel@drexel.edu. been able to capture the transitions or diffractometry17 or at nanosecond their dynamics in situ. elapsed times using synchrotron-based References A further use of the time resolution techniques.18 1L.M. Shepard, “Advances in Processing of of in situ techniques is in understand- Ferroelectric Thin Films,” Am. Ceram. Soc. 22 www.ceramics.org | American Ceramic Society Bulletin, Vol. 92, No. 1


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