We present the concept of ferroelectric tunnel junctions ͑FTJs͒. These junctions consist of two metal electrodes separated by a nanometer-thick ferroelectric barrier. The current-voltage characteristics of FTJs are analyzed under the assumption that the direct electron tunneling represents the dominant conduction mechanism. First, the influence of converse piezoelectric effect inherent in ferroelectric materials on the tunnel current is described. The calculations show that the lattice strains of piezoelectric origin modify the currentvoltage relationship owing to strain-induced changes of the barrier thickness, electron effective mass, and position of the conduction-band edge. Remarkably, the conductance minimum becomes shifted from zero voltage due to the piezoelectric effect, and a strain-related resistive switching takes place after the polarization reversal in a ferroelectric barrier. Second, we analyze the influence of an internal electric field arising due to imperfect screening of polarization charges by electrons in metal electrodes. It is shown that, for asymmetric FTJs, this depolarizing-field effect also leads to a considerable change of the barrier resistance after the polarization reversal. However, the symmetry of the resulting current-voltage loop is different from that characteristic of the strain-related resistive switching. The crossover from one to another type of the hysteretic curve, which accompanies the increase of FTJ asymmetry, is described taking into account both the strain and depolarizing-field effects. It is noted that asymmetric FTJs with dissimilar top and bottom electrodes are preferable for the nonvolatile memory applications because of a larger resistance on/off ratio.

The polarization reversal in single-crystalline ferroelectric films has been investigated experimentally and theoretically. The hysteresis loops were measured for Pb(Zr0.52Ti0.48)O3 films with thicknesses ranging from 8 to 250 nm. These films were grown epitaxially on SrRuO3 bottom electrodes deposited on SrTiO3 substrates. The measurements using Pt top electrodes showed that the coercive field Ec increases drastically as the film becomes thinner, reaching values as high as Ec≈1200 kV/cm. To understand this observation, we calculated the thermodynamic coercive field Eth of a ferroelectric film as a function of the misfit strain Sm in an epitaxial system and showed that Eth strongly depends on Sm. However, the coercive field of ultrathin films, when measured at high frequencies, exceeds the calculated thermodynamic limit. Since this is impossible for an intrinsic coercive field Ec, we conclude that measurements give an apparent Ec rather than the intrinsic one. An enormous increase of apparent coercive field in ultrathin films may be explained by the presence of a conductive nonferroelectric interface layer.

The aim of this work is to investigate the electron transport through metal-ferroelectric-metal ͑MFM͒ junctions with ultrathin barriers in order to determine its dependence on the polarization state of the barrier. To that end, heteroepitaxial Pt/Pb(Zr 0.52 Ti 0.48 )O 3 /SrRuO 3 junctions have been fabricated on lattice-matched SrTiO 3 substrates. The current-voltage (I -V) characteristics of the MFM junctions involving a few-nanometer-thick Pb(Zr 0.52 Ti 0.48 )O 3 barriers have been recorded at temperatures between 4.2 K and 300 K. Typical I -V curves exhibit reproducible switching events at well-defined electric fields. The mechanism of charge transport through ultrathin barriers and the origin of the observed resistive switching effect are discussed.

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In SrRuO3/PbZr0.52Ti0.48O3/SrRuO3 multilayer thin films on SrTiO3 substrates the different lattice distortion behavior of the top and the bottom SrRuO3 film layer is found and characterized by means of transmission electron microscopy. The bottom SrRuO3 layer is compressively strained in the film plane by a constraint of the SrTiO3 substrate. In contrast, in the interface area of the top SrRuO3 layer, a lattice dilatation is measured not only in the film plane but also parallel to the film normal. The misfit strain, the lead interdiffusion and the oxygen concentration in this area are investigated and discussed as possible reasons for the unexpected lattice dilatation along the film normal direction.