Polarization Dependence and Relaxation of the Current in Polycrystalline Ferroelectric Pb(ZrTi)O 3 Film
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Polarization Dependence and Relaxation of the Current in Polycrystalline Ferroelectric Pb(ZrTi)O3 Film L. A. Delimova1, V. S. Yuferev1, A. V. Ankudinov2, E. V. Gushchina2, I. V. Grekhov1 1
Solid State Electronics Division, 2Solid State Physics Division, Ioffe Institute of the Russian Academy of Sciences, Polytekhnicheskaya 26, St.Petersburg, 194021, Russian Federation. ABSTRACT
Using Scanning Spreading Resistance Microscopy and direct current-voltage measurements, a long-relaxation transport current in polycrystalline PZT films is shown to depend on the polarization direction and voltage rise rate, the latter is typical for a capacitive current. The clockwise current hysteresis is observed at any polarization of the film. We suppose that the long current relaxation is due to recharge of traps, which participate in screening of polarization charges on PZT grain boundaries. The polarization charges response to applied bias for a short time, whereas the traps response to variation of the polarization charges takes much longer time. INTRODUCTION The relationship between electronic transport and ferroelectric polarization in oxide nanostructures is of great interest for possible application in high-density data storage. Great attention was given to ferroelectric tunnel junctions, where flipping the spontaneous polarization was predicted in [1-2] to modulate the height of the tunneling barrier providing the tunneling electroresistance effect. Using scanning probe microscopy techniques, the tunneling electroresistance effect was successfully demonstrated almost simultaneously on nanometerthick epitaxial BaTiO3 single-crystalline thin films [3], on a 30-nm-thick Pb(Zr0.2Ti0.8)O3 film [4], and in highly strained 3-nm-thick BaTiO3 film [5]. Besides the ferroelectric tunnel junctions, a diode-type conductance associated with the spontaneous polarization was found in thick, 70-90 μm, ferroelectric monodomain BiFeO3 crystals [6]. It was shown that the direction of the diode effect is switched when the electric polarization is flipped by an external bias: the current is larger when the polarization direction coincides with the applied bias. A study of origin and nature of the conductivity in ferroelectric oxides faces with the complexity in interpretation of the current measurements. It is associated with contributions of interfaces, ferroelectric domains, grain boundaries and traps recharge to the measured current, which result in appreciable components of transient and steady-state current. The transient current associated with the ferroelectric domains switching delays for a short time, which is far less than a second. However, experiments show that a response from PZT films to the applied bias produces a transient current, which can proceed seconds and tens of seconds both in epitaxially grown [7] and polycrystalline films [8]. The photovoltaic currents measured in PZT also demonstrate long relaxation (tens of seconds) to its steady-state magnitude [9-10]. In this paper, we study transport characteristics across polycrystal
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