Giant Self-Polarization in FeRAM Element Based on Sol-Gel PZT Films
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Giant Self-Polarization in FeRAM Element Based on Sol-Gel PZT Films L. A. Delimova1, E. V. Guschina2, V. S. Yuferev1, I. V. Grekhov1, N. V. Zaiceva3, N. V. Sharenkova3, D. S. Seregin4, K. A. Vorotilov4, A. S. Sigov4 1 Solid-State Electronics Division, 2Solid State Physics Division, 3Division of Physics of Dielectric and Semiconductors, Ioffe Institute, Polytekhnicheskaya 26, St.Petersburg, 194021, Russian Federation. 4 Electronics Department, Moscow State Technical University of Radioengineering, Electronics and Automation, Vernadskogo 78, Moscow, 119454, Russian Federation. ABSTRACT Integrated ferroelectric capacitors Pt/PZT/Pt/Ti/SiO2/Si with sol-gel deposited PZT films are studied. The (111) textured polycrystalline films are shown to have nonconductive PZT grain boundaries. The short-circuited photocurrents measured under illumination of the films by light with the quantum energy of 2.7 eV indicate the polarization inside the film directed from the top to the bottom electrode. Using the modified method of depolarization hysteresis loops, we found a non-switchable part of polarization which was measured to be -16 µC/cm2 and directed from the top to the bottom electrode. We consider this result to be a giant self-polarization and explain it in terms of flexoelectricity caused by lattice mismatch between the PZT and bottom Pt layers. The strain gradient across the PZT film thickness is estimated from the in-plane lattice constants measured in Pt and PZT films to be ~103cm-1, which can produce the downward flexoelectric polarization of ~14 µC/cm2, coinciding well with the measured one. Nonsymmetrical depolarization loops are found in the films when the polarization switching itself becomes more difficult under the negative or positive driving voltage. We show experimentally how depolarization with compensating bias or film illumination can affect the film polarization switching. INTRODUCTION Integrated ferroelectric thin-film capacitors Pt/Pb(Zr,Ti)O3(PZT)/Pt/Ti/SiO2/Si are the basic element of nonvolatile ferroelectric random access memory [1]. Contrast to an ideal system, to switch completely the polarization in real thin films, the operating voltage might be twice the coercive voltage VC, since an interface defect layer with low dielectric constant reduces the field inside the ferroelectric layer [2]. The defect layer hampers screening the polarization charge by electrode free carriers. Due to this, a depolarization field is generated inside the film that leads to a decrease in the polarization [3]. The real PZT capacitors show leakage currents [46] and transient long-relaxed currents [6,7] also contributing to the charge state at the film interfaces. Therefore, the internal electric fields and charge transport phenomena in thin ferroelectric films are important issues, which determine the performance, stability, and reliability of the ferroelectric memory cell. Here, we study the grain boundaries conduction and the polarization dependences of photocurrents, measured under illumination of the shortcircuited films
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