J. Rodrı́guez Contreras

Theoretical current-voltage characteristics of ferroelectric tunnel junctions

We present the concept of ferroelectric tunnel junctions (FTJs). 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 current-voltage 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 the 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 non-volatile memory applications because of a larger resistance on/off ratio.

Resistive switching in metal–ferroelectric–metal junctions

The aim of this work is to investigate the electron transport through metal–ferroelectric–metal (MFM) junctions with ultrathin barriers in order to determine its dependence on the polarization state of the barrier. To that end, heteroepitaxial Pt/Pb(Zr0.52Ti0.48)O3/SrRuO3 junctions have been fabricated on lattice-matched SrTiO3 substrates. The current–voltage (I–V) characteristics of the MFM junctions involving a few-nanometer-thick Pb(Zr0.52Ti0.48)O3 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.

Coercive field of ultrathin Pb(Zr0.52Ti0.48)O3 epitaxial films

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 fie...

Surface treatment effects on the thickness dependence of the remanent polarization of PbZr0.52Ti0.48O3 capacitors

In this letter, we report on the thickness dependence of the remanent polarization of Pt/PbZr0.52Ti0.48O3/SrRuO3 capacitors. Two different patterning techniques were used to fabricate the capacitors. For lift-off processed capacitors, the remanent polarization decreased with decreasing thickness. Ion-beam-etched capacitors, however, showed a constant remanent polarization for all PbZr0.52Ti0.48O3 film thicknesses down to 23 nm. Remarkably, this constant remanent polarization for ion-beam-etched capacitors corresponds to the spontaneous polarization expected for a stress-free bulk PbZr0.52Ti0.48O3 crystal.

Lattice strain and lattice expansion of the SrRuO3 layers in SrRuO3/PbZr0.52Ti0.48O3/SrRuO3 multilayer thin films

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.

On a Novel Ferro Resistive Random Access Memory (FRRAM): Basic Model and First Experiments

We report on a novel non-volatile memory concept for resistive information storage. The particular device structure consists of a conductive ferroelectric/non-ferroelectric 2-layer sequence. Resistive switching is observed, by applying a voltage pulses. Our model predicts that the switching correlates with a change of the potential barrier height inside the structure. It may also explain resistive switching in systems consisting only of one ferroelectric layer and by assuming the presence of non-ferroelectric interface layers. The operation of the device is demonstrated for the PZT (48/52) system with a SrRuO3 bottom electrode and a Pt top electrode. The simulated and measured I-V curves are in good agreement.

Current transport in ramp-type junctions with engineered interface

The transport properties of “interface-engineered” edge-type YBa2Cu3O7 Josephson junctions are investigated in detail. We have investigated the dependence of the current–voltage characteristics on external magnetic field, temperature, and microwave irradiation and compare them to the resistively shunted junction model. The temperature dependence of the critical current and the normal resistance allows us to draw conclusions to the transport of quasiparticles and Cooper pairs in the investigated “interface-engineered” junctions. We have studied the properties of junctions for which La doped YBa2Cu3O7 is used for the superconducting electrodes. We will propose a model for the undoped and the La doped case which takes into account a barrier which consists of a series connection of a normal conducting layer and an insulator, containing superconducting microconstrictions.

Introduction and characterization of interfacial defects in SrRuO3/BaTiO3/SrRuO3 multilayer films

Abstract A Ruddlesden–Popper-type planar fault was introduced at the SrRuO 3 /BaTiO 3 interface of a SrRuO 3 /BaTiO 3 /SrRuO 3 heterofilm system using different processing conditions for the individual film layer. This fault occurs continuously and homogeneously along the interface, forming an extra Sr-rich sub-nanometer layer. The structure of the fault and the lattice behavior in the interface area were characterized on an atomic scale by properly imaging all types of atomic columns, especially the pure oxygen columns, by means of spherical-aberration-corrected high-resolution transmission electron microscopy. Information on local interdiffusion and lattice strain at the interface was obtained by quantitative evaluation of the atomic resolution images.

Effect of the magnetic-field orientation on the modulation period of the critical current of ramp-type Josephson junctions

We have investigated the dependence of the critical current IC on the value and orientation of an externally applied magnetic field H for interface-engineered YBa2Cu3O7−x ramp-type Josephson junctions. The results are compared with measurements of Nb ramp-type junctions with a PdAu interlayer. The IC versus H dependences are similar to Fraunhofer patterns and their modulation period changes several orders of magnitude with the orientation of the magnetic field. For both junction types, the dependence of the modulation period on the orientation of the magnetic field can be well described by the change of the relevant projection of the junction area and the influence of flux-focusing. Therefore the features of the IC(H) curves have to be attributed to the ramp geometry and not to specific properties of the superconducting material.

Imprint in ferroelectric Pb(Zr,Ti)O3 thin films with thin SrRuO3 layers at the electrodes

Abstract The imprint behavior of ferroelectric Pb(Zr,Ti)O3 (PZT) films with thin SrRuO3 (SRO) layers at the platinum electrodes was investigated. Different models (defect dipole alignment, bulk screening and interface screening) are discussed which are usually applied to explain the imprint effect in ferroelectric materials. Based on our experimental results for the material under investigation, we suggest that the interface screening mechanism is the dominant mechanism which is responsible for the imprint scenario in ferroelectric thin films.