H. Kohlstedt

Emerging memories: resistive switching mechanisms and current status

The resistance switching behaviour of several materials has recently attracted considerable attention for its application in non-volatile memory (NVM) devices, popularly described as resistive random access memories (RRAMs). RRAM is a type of NVM that uses a material(s) that changes the resistance when a voltage is applied. Resistive switching phenomena have been observed in many oxides: (i) binary transition metal oxides (TMOs), e.g. TiO(2), Cr(2)O(3), FeO(x) and NiO; (ii) perovskite-type complex TMOs that are variously functional, paraelectric, ferroelectric, multiferroic and magnetic, e.g. (Ba,Sr)TiO(3), Pb(Zr(x) Ti(1-x))O(3), BiFeO(3) and Pr(x)Ca(1-x)MnO(3); (iii) large band gap high-k dielectrics, e.g. Al(2)O(3) and Gd(2)O(3); (iv) graphene oxides. In the non-oxide category, higher chalcogenides are front runners, e.g. In(2)Se(3) and In(2)Te(3). Hence, the number of materials showing this technologically interesting behaviour for information storage is enormous. Resistive switching in these materials can form the basis for the next generation of NVM, i.e. RRAM, when current semiconductor memory technology reaches its limit in terms of density. RRAMs may be the high-density and low-cost NVMs of the future. A review on this topic is of importance to focus concentration on the most promising materials to accelerate application into the semiconductor industry. This review is a small effort to realize the ambitious goal of RRAMs. Its basic focus is on resistive switching in various materials with particular emphasis on binary TMOs. It also addresses the current understanding of resistive switching behaviour. Moreover, a brief comparison between RRAMs and memristors is included. The review ends with the current status of RRAMs in terms of stability, scalability and switching speed, which are three important aspects of integration onto semiconductors.

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.

Ultrathin epitaxial ferroelectric films grown on compressive substrates: Competition between the surface and strain effects

The mean-field Landau-type theory is used to analyze the polarization properties of epitaxial ferroelectric thin films grown on dissimilar cubic substrates, which induce biaxial compressive stress in the film plane. The intrinsic effect of the film surfaces on the spontaneous polarization is taken into account via the concept of the extrapolation length. The theory simultaneously allows for the influence of the misfit strain imposed on the film lattice by a thick substrate. Numerical calculations are performed for PbTiO3 and BaTiO3 films under an assumption of the polarization reduction in surface layers. The film mean polarization is calculated as a function of film thickness, temperature, and misfit strain. It is shown that the negative intrinsic size effect is reduced in epitaxial films due to the in-plane compression of the film lattice. At room temperature, strong reduction of the mean polarization may take place only in ultrathin films (thickness ∼1 nm). Theoretical predictions are compared with the...

Misfit dislocations in nanoscale ferroelectric heterostructures

We present a quantitative study of the thickness dependence of the polarization and piezoelectric properties in epitaxial (001) PbZr0.52Ti0.48O3 films grown on (001) SrRuO3-buffered (001) SrTiO3 substrates. High-resolution transmission electron microscopy reveals that even the thinnest films (∼8nm) are fully relaxed with a dislocation density close to 1012cm−2 and a spacing of approximately 12 nm. Quantitative piezoelectric and ferroelectric measurements show a drastic degradation in the out-of-plane piezoelectric constant (d33) and the switched polarization (ΔP) as a function of decreasing thickness. In contrast, lattice-matched ultrathin PbZr0.2Ti0.8O3 films that have a very low dislocation density show superior ferroelectric properties. Supporting theoretical calculations show that the variations in the strain field around the core of the dislocation leads to highly localized polarization gradients and hence strong depolarizing fields, which result in suppression of ferroelectricity in the vicinity of ...

Size effects in ultrathin epitaxial ferroelectric heterostructures

In this letter we report on the effect of thickness scaling in model PbZr0.2Ti0.8O3(PZT)∕SrRuO3 heterostructures. Although theoretical models for thickness scaling have been widely reported, direct quantitative experimental data for ultrathin perovskite (<10nm) films in the presence of real electrodes have still not been reported. In this letter we show a systematic quantitative experimental study of the thickness dependence of switched polarization in (001) epitaxial PZT films, 4to80nm thick. A preliminary model based on a modified Landau Ginzburg approach suggests that the nature of the electrostatics at the ferroelectric–electrode interface plays a significant role in the scaling of ferroelectric thin films.

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...

Probing Cu doped Ge0.3Se0.7 based resistance switching memory devices with random telegraph noise

The ultimate sensitivity of any solid state device is limited by fluctuations. Fluctuations are manifestations of the thermal motion of matter and the discreteness of its structure which are also inherent ingredients during the resistive switching process of resistance random access memory (RRAM) devices. In quest for the role of fluctuations in different memory states and to develop resistive switching based nonvolatile memory devices, here we present our study on random telegraph noise (RTN) resistance fluctuations in Cu doped Ge0.3Se0.7 based RRAM cells. The influence of temperature and electric field on the RTN fluctuations is studied on different resistance states of the memory cells to reveal the dynamics of the underlying fluctuators. Our analysis indicates that the observed fluctuations could arise from thermally activated transpositions of Cu ions inside ionic or redox “double-site traps” triggering fluctuations in the current transport through a filamentary conducting path. Giant RTN fluctuation...

Method to distinguish ferroelectric from nonferroelectric origin in case of resistive switching in ferroelectric capacitors

We present investigations on the resistive switching effect in SrRuO3∕PbZr0.2Ti0.8O3∕Pt ferroelectric capacitors. Using a conductive atomic force microscope, the out-of-plane piezoelectric response and the capacitive and resistive current were simultaneously measured as a function of applied bias voltage. We observed two independent switching phenomena, one attributed to the ferroelectric switching process and the other to resistive switching.We show that I-V curves alone are not sufficient in ferroelectric materials to clarify the underlying switching mechanism and must be used with sufficient caution.

0-π josephson tunnel junctions with ferromagnetic barrier

We fabricated high quality Nb/Al2O3/Ni(0.6)Cu(0.4)/Nb superconductor-insulator-ferromagnet-superconductor Josephson tunnel junctions. Using a ferromagnetic layer with a steplike thickness, we obtain a 0-pi junction, with equal lengths and critical currents of 0 and pi parts. The ground state of our 330 microm (1.3lambda(J)) long junction corresponds to a spontaneous vortex of supercurrent pinned at the 0-pi step and carrying approximately 6.7% of the magnetic flux quantum Phi(0). The dependence of the critical current on the applied magnetic field shows a clear minimum in the vicinity of zero field.