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Ferroelectricity is very attractive for nonvolatile memories since it allows non-volatility paired with a field driven switching mechanism enabling a very low-power write operation. Non-volatile memories based on ferroelectric lead-zirconium-titanate (PZT) (see fig. la) are available on the market for more than a quarter of a century now [1]. Yet they are limited to niche applications due to the compatibility issues of the ferroelectric material with CMOS processes and the associated limited scalability [2]. The discovery of ferroelectricity in doped hafnium oxide has revived the activities towards a variety of scalable ferroelectric nonvolatile memory devices

To overcome the fundamental limit of the transistor subthreshold swing of 60 mV/dec at room temperature, the use of negative capacitance (NC) in ferroelectric materials was proposed [1]. Due to the recent discovery of ferroelectricity in CMOS compatible HfO₂ and ZrO₂ based thin films [2], [3], the promise of ultra-low power steep-slope devices seems within reach. However, concerns have been raised about switching-speed limitations and unavoidable hysteresis in NC devices [4], [5]. Nevertheless, it was shown that NC effects without hysteresis can be observed in fast pulsed voltage measurements on ferroelectric/dielectric capacitors [6], which was recently confirmed using ferroelectric Hf₀.₅ Zr₀.₅ O₂[7], [8]. While in these works only the integrated charge after each pulse was studied, here we investigate for the first time if the transient voltage and charge characteristics are also hysteresis-free.

Ferroelectrics are theoretically an ideal solution for low write power nonvolatile memories. However, the complexity of ferroelectric perovskites has hindered the scaling of such devices to competitive feature sizes. The discovery of ferroelectricity in hafnium oxide solved this issue. Ferroelectric memories in three variants, capacitor based ferroelectric RAM, ferroelectric field effect transistors and ferroelectric tunneling junctions have become competitors for future memory solutions again. In this paper, the basics and current status of hafnium oxide based ferroelectric memory devices is described and recent results are shown.

The use of ferroelectric negative capacitance (NC) has been proposed as a promising way to reduce the power dissipation in nanoscale devices [1]. According to single-domain (SD) Landau theory, a hysteresis-free NC state in a ferroelectric might be stabilized in the presence of depolarization fields below a certain critical film thickness tF, SD. However, it is well-known that depolarization fields will cause the formation of domains in ferroelectrics to reduce the depolarization energy [2], which is rarely considered in the literature on NC [3]. The improvident use of SD Landau theory to model NC devices seems to be the main reason for the large discrepancy between experimental data and the current theory [4]. Here, we will show by simulation how anti-parallel domain formation can strongly limit the stability of the NC state in a metal-ferroelectric-insulator-metal (MFIM) structure, which is schematically shown in Fig. 1.

Resistive switching devices with Nb2O5 as a switching layer are treated with argon ion irradiation, which generates defects in the oxide layer that support the electroforming step. To distinguish between the effects of layer thinning by sputtering and that of defect generation, devices with different thicknesses of deposited oxide are investigated. It is found that the defect-rich interfaces allow the formation of thick oxides at low forming voltages, and therefore, the effects of the ion irradiation are comparable to the use of reactive electrodes.

Summarizing, metal-insulator-metal devices consisting of one insulating Nb2O5 layer were irradiated with krypton ions to form a metallic NbOx sublayer in order to introduce threshold switching. Two effects were identified that induce this metallic NbOx layer: preferential sputtering at the sample surface and interface mixing at the bottom electrode. These krypton irradiated devices can be operated either as a pure threshold switch or as a combination of both, threshold switch and memory element. The presented fabrication method enables costefficient device manufacturing, since ion irradiation could be structured easily using well established lithography methods. Thus, the threshold switch can be formed in defined areas, e.g. the intersection of top and bottom electrode in cross bar arrays.

The Virtual Institute MEMRIOX establishes a joint research initiative in the field of ion-tailored oxide-based memristive elements, to be pursued within a novel and unique combination of core competences from the Helmholtz centers Dresden-Rossendorf and Jülich and their university partners in Dresden, Freiberg, Jena, San Diego, and Zürich. A nanoscale memristive element may prove the concept of the ultimate future non-volatile memory cell with a resistance set directly by electric currents. The Virtual Institute aims at stepping beyond the established layer-by-layer control of intrinsic defects during the synthesis of memristive elements. The project is financed by the Initiative and Networking Funds of the Helmholtz Association (VH-VI-442).

Die Anmeldung beschreibt ein elektrisches Element das Einzel- oder Mehrkomponentensysteme umfasst, wobei die Komponenten des Einzel- und Mehrkomponentensystems teilweise oder vollständig aus thermochromem Material mit anisotropen oder isotropen Dielektrizitätseigenschaften bestehen und in sich strukturiert ausgeführt sein können. Diese Elemente können zum lokalen Heizen, als elektronische oder optische Schalter oder zur Magnetfelderzeugung genutzt werden.

BiFeO3 thin films have been deposited on Pt/sapphire and Pt/Ti/SiO2/Si substrates with pulsed laser deposition using the same growth conditions. Au was sputtered as the top electrode. The microscopic structure of the thin film varies by changing the underlying substrate. Thin films on Pt/sapphire are not resistively switchable due to the formation of Schottky contacts at both the top and the bottom interfaces. However, thin films on Pt/Ti/SiO2/Si exhibit an obvious resistive switching behavior under forward bias. The conduction mechanisms in BiFeO3 thin films on Pt/sapphire and Pt/Ti/SiO2/Si substrates are discussed to understand the different resistive switching behaviors.

Nonvolatile resistive switching has been observed in an Au/BiFeO3/Pt structure, where a Schottky contact and an Ohmic contact were formed at the Au/BiFeO3 and BiFeO3/Pt interface, respectively. By changing the polarity of the external voltage, the Au/BiFeO3/Pt is switched between two stable resistance states with a resistance ratio larger than two orders of magnitude. Based on a systematic investigation of its electrical properties with an emphasize on its transport characteristics, a model associated with the redistribution of oxygen vacancies and the formation of an electron hopping path is proposed, which agrees well with our experimental observations. In the present work we found that the electron hopping can be controlled and utilized to realize bipolar resistive switching, which is promising for future high density memory devices.

Current-perpendicular-to-plane (CPP) magnetic tunnel junctions (MgO-MTJ) have been prepared using electron beam lithography as well as argon ion beam etching. A tantalum hardmask was utilized for pattern transfer. The size of the elliptical nanopillars could be decreased down to 90nm x 150 nm while preserving a TMR ratio of 92.5 %. TEM images proof the absence of redepositions at the MgO layer edge and give an insight into the interface quality. Magnetization switching was performed using either static magnetic fields and/or dc current (spin torque). The nanopillars could be characterized at temperatures ranging from 5 K to 150 K and room temperature. The analysis of magnetization dynamics included single-shot time-resolved magnetoresistance measurements as well as dc current induced oscillations of the free-layer magnetization.

BiFeO3 thin films were grown on Pt/c-sapphire substrates by pulsed laser deposition with different growth rates. With increasing growth rate the leakage current is decreased and the conduction mechanism changes from bulk-limited Poole-Frenkel emission to interface-limited Schottky emission. In the present work, we show that only the growth rate of the BiFeO3 films close to the metal contacts has to be increased in order to reduce the leakage current and to observe saturated polarization-electric field hysteresis loops.

BiFeO3 thin films have been grown on Pt/Ti/SiO2/Si substrates with pulsed laser deposition using Au as the top electrode. The resistive switching property of the Au/BiFeO3/Pt stack has been significantly improved by carefully tuning the oxygen pressure during the growth, and a large switching ratio of ∼4500 has been achieved. The deposition pressure modifies the concentration of oxygen vacancies and the rectifying behavior of the Au/BiFeO3 junction, and consequently influences the resistive switching behavior of the whole stack. The switching takes place homogeneously over the entire electrode, and shows a long-term retention.

Resistive switching in oxides has attracted increasing attention due to the potential application for nonvolatile memory devices [1,2]. Resistive switching has been observed usually in a metal-insulator-metal (MIM) capacitor-like structure, which can be set and reset to low resistance state and high resistance state by applying external voltages with opposite polarities. In an asymmetric MIM structure where a Schottky contact and an Ohmic contact are formed at the two interfaces, respectively, it is generally believed that the Schottky interface dominates the bipolar resistive switching behavior. BiFeO3 thin films have been grown on Pt/Ti/SiO2/Si substrates with pulsed laser deposition. RF sputtered Au has been used for the top electrode. The transport properties of the BiFeO3 thin films have been previously demonstrated to be sensitive to the interface [3]. In the present work, an interface-related resistive switching behavior with large switching ratio of ~300 has been observed in the Au/BiFeO3/Pt structure [4]. The different polarities of the external voltage induce an electron trapping or detrapping process, and consequently change the depletion layer width below the Au Schottky contact, which is revealed by capacitance-voltage measurements and by long-term low/high resistance state capacitance transient measurements at zero bias. The resistive switching shows a long term retention and non-destructive read-out character, which is proved by pulsed voltage measurements. A dynamic equilibrium process involving the extension of the depletion region can be used to explain the good retention in the Au/BiFeO3/Pt structure. The present work can help to further understand the physical origin of bipolar switching in BiFeO3 and in other thin film oxides with electron trapping centers. [1] K. Terabe et al., Nature 433, 47 (2005). [2] R. Waser and M. Aono, Nature Mater. 6, 833 (2007). [3] Y. Shuai et al., J. A