Synchrotron X-ray Microdiffraction Images of Polarization Switching in Epitaxial PZT Capacitors with Pt and SrRuO 3 Top
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Synchrotron X-ray Microdiffraction Images of Polarization Switching in Epitaxial PZT Capacitors with Pt and SrRuO3 Top Electrodes Dal-Hyun Do1, Dong Min Kim1, Chang-Beom Eom1, Eric M. Dufresne2, Eric D. Isaacs3, and Paul G. Evans1 1 Department of Materials Science and Engineering, University of Wisconsin, Madison, WI 53706 2 Department of Physics, University of Michigan, Ann Arbor, MI 48109 3 Center for Nanoscale Materials, Argonne National Laboratory, Argonne, IL 60439 ABSTRACT The evolution of stored ferroelectric polarization in PZT thin film capacitors was imaged using synchrotron x-ray microdiffraction with a submicron-diameter focused incident x-ray beam. To form the capacitors, an epitaxial Pb(Zr,Ti)O3 (PZT) thin film was deposited on an epitaxiallygrown conductive SrRuO3 (SRO) bottom electrode on a SrTiO3 (STO) (001) substrate. Polycrystalline SRO or Pt top electrodes were prepared by sputter deposition through a shadow mask and subsequent annealing. The intensity of x-ray reflections from the PZT film depended on the local ferroelectric polarization. With 10 keV x-rays, regions of opposite polarization differed in intensity by 26% in our PZT capacitor with an SRO top electrode. Devices with SRO electrodes showed just a 25% decrease in the remnant polarization after 107 switching cycles. In devices with Pt top electrodes, however, the switchable polarization decreased a by 70% after only 5×104 cycles. INTRODUCTION Ferroelectric oxides, including PZT, have excellent potential for electronic, photonic, and mechanical devices. One of the limitations of the use of PZT in memory devices is polarization fatigue, the reduction in the switchable polarization after repeated switching cycles. The development of polarization fatigue in operating devices can be influenced by the selection of electrode materials. PZT films with metallic oxide electrodes such as SRO, for instance, have shown little or no fatigue with 1010 or more switching cycles [1-3]. On the other hand, the use of elemental metal electrodes with PZT thin films leads to polarization fatigue within a relatively small number of cycles. Fatigue in capacitors with metal electrodes can be linked to interface defects caused by the migration of oxygen vacancies to the electrodes [3,4]. Since polarization fatigue is directly related to domain motion and is commonly thought to be caused by domain wall pinning or by the suppression of domain nucleation, it is useful to observe the behavior of ferroelectric domains. Techniques such as optical second harmonic microscopy [5], transmission electron microscopy (TEM) [6], and piezoresponse force microscopy (PFM) [7-9] have been used to investigate domain structure and motion. Among them, PFM, with 5-15 nm resolution, is the most common method for investigations of domain evolution. Since the scanned probe tip is often used as a top electrode, this approach can result in an inhomogeneous electric field and a potential drop between the tip and the sample surface due to a tip oxide [9]. Since the tip-sample interac
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