Many metal–insulator–metal systems show electrically induced resistive switching effects and have therefore been proposed as the basis for future non-volatile memories. They combine the advantages of Flash and DRAM (dynamic random access memories) while avoiding their drawbacks, and they might be highly scalable. Here we propose a coarse-grained classification into primarily thermal, electrical or ion-migration-induced switching mechanisms. The ion-migration effects are coupled to redox processes which cause the change in resistance. They are subdivided into cation-migration cells, based on the electrochemical growth and dissolution of metallic filaments, and anion-migration cells, typically realized with transition metal oxides as the insulator, in which electronically conducting paths of sub-oxides are formed and removed by local redox processes. From this insight, we take a brief look into molecular switching systems. Finally, we discuss chip architecture and scaling issues.

http://chialvo.org/Curso/UBACurso/DIA7/Papers/Aono_NatMat_07_Ionic%20Switching%20Memory.pdf

Nanoionics-based resistive switching memories

Redox‐Based Resistive Switching Memories – Nanoionic Mechanisms, Prospects, and Challenges

This review covers important advances in recent years in the physics of thin-film ferroelectric oxides, the strongest emphasis being on those aspects particular to ferroelectrics in thin-film form. The authors introduce the current state of development in the application of ferroelectric thin films for electronic devices and discuss the physics relevant for the performance and failure of these devices. Following this the review covers the enormous progress that has been made in the first-principles computational approach to understanding ferroelectrics. The authors then discuss in detail the important role that strain plays in determining the properties of epitaxial thin ferroelectric films. Finally, this review ends with a look at the emerging possibilities for nanoscale ferroelectrics, with particular emphasis on ferroelectrics in nonconventional nanoscale geometries.

/pdf/physics-of-thin-film-ferroelectric-oxides-3zmqphtpv1.pdf

Physics of thin-film ferroelectric oxides

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.

https://iopscience.iop.org/article/10.1088/0034-4885/75/7/076502/pdf

Emerging memories: resistive switching mechanisms and current status.

Multilayer ceramic capacitor (MLCC) production and sales figures are the highest among fine-ceramic products developed in the past 30 years. The total worldwide production and sales reached 550 billion pieces and 6 billion dollars, respectively in 2000. In the course of progress, the development of base-metal electrode (BME) technology played an important role in expanding the application area. In this review, the recent progress in MLCCs with BME nickel (Ni) electrodes is reviewed from the viewpoint of nonreducible dielectric materials. Using intermediate-ionic-size rare-earth ion (Dy2O3, Ho2O3, Er2O3, Y2O3) doped BaTiO3 (ABO3)-based dielectrics, highly reliable Ni-MLCCs with a very thin layer below 2 µm in thickness have been developed. The effect of site occupancy of rare-earth ions in BaTiO3 on the electrical properties and microstructure of nonreducible dielectrics is studied systematically. It appears that intermediate-ionic-size rare-earth ions occupy both A- and B-sites in the BaTiO3 lattice and effectively control the donor/acceptor dopant ratio and microstructural evolution. The relationship between the electrical properties and the microstructure of Ni-MLCCs is also presented.

https://iopscience.iop.org/article/10.1143/JJAP.42.1/pdf

Base-Metal Electrode-Multilayer Ceramic Capacitors: Past, Present and Future Perspectives

The rate of the resistance degradation of doped SrTiO3 ceramics is investigated as a function of various external and material parameters. The effects of the mutually interrelated parameters dc voltage, dc electric field, and thickness of the dielectric are described by power laws. Electron microscopic potential contrast studies show a Maxwell-Wagner polarization leading to a concentration of the electric field at the grain boundaries during the degradation. Based on this finding, the voltage step per grain boundary, ΔΘgb, is introduced as a rate-determining parameter which allows an explanation of the influence of the grain size on the degradation rate as well as the difference in the power laws for ceramic and single-crystal samples.

dc Electrical Degradation of Perovskite-Type Titanates: I, Ceramics

A quantitative model for the dc voltage-induced resistance degradation is based on the defect structural material constants and a numerical solution of the general transport problem for the mixed-conducting perovskite-type titanates. During degradation, a demixing of the initially homogeneous oxygen vacancy concentration in nominally undoped and acceptor-doped systems occurs since the ionic transfer is (at least partially) blocked at the electrodes. Because of the specific defect structure, the concentration polarization of the oxygen vacancies leads to a pronounced increase of the p-conductivity in the anodic region and formation of an n-conducting cathodic region. The results of our phenomenological degradation studies concerning the influence of external parameters and material parameters as well as the time evolution of the field distribution and the electro-coloration are described by our simulation model with reasonable precision. The thermodynamic and kinetic aspects of the demixing process are discussed and compared to related phenomena.

dc Electrical Degradation of Perovskite‐Type Titanates: III, A Model of the Mechanism

The resistance degradation of Al-doped, Ni-doped, and Fedoped SrTiO3 single crystals is studied as a function of the temperature and the applied dc voltage. The individual influence of the acceptor dopants is discussed. Data of the mobility of oxygen vacancies are obtained from the determination of the time evolution of the electric field distribution between the electrodes, from electrocoloration studies, and from an examination of the thermal annealing after degradation. In a numerical calculation based on the defect chemistry and transport properties of titanates, the electrocoloration process is simulated and a qualitative explanation of the degradation process is presented.

dc Electrical Degradation of Perovskite‐Type Titanates: II, Single Crystals

The degradation phenomenon in dielectric ceramics denotes a long-term decrease of the high insulation resistance of the material under d.c. voltage stress at elevated temperatures. The influence of external parameters (such as temperature and field) and of material parameters (such as the microstructure and the doping) on the degradation process is described. Information about the local state of the material (i.e. the distribution of the specific conductivity and the oxidation states of dopants) and its time evolution is associated with calculations of the defect concentration. As a result, an improved model for explaining the degradation mechanism is proposed. The model suggests a de-mixing in the oxygen vacancy concentration between the electrodes and, subsequently, a significant increase in the electronic charge carrier concentrations.

Karl-Heinz Härdtl

Papers

dc Electrical Degradation of Perovskite-Type Titanates: I, Ceramics

dc Electrical Degradation of Perovskite‐Type Titanates: III, A Model of the Mechanism

dc Electrical Degradation of Perovskite‐Type Titanates: II, Single Crystals

Degradation of dielectric ceramics

Contributions to the high-temperature behaviour of YBa2Cu3O7−y