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.

The resistive switching mechanism of 20-to 57-nm-thick TiO 2 thin films grown by atomic-layer deposition was studied by current-voltage measurements and conductive atomic force microscopy. Electric pulse-induced resistance switching was repetitively ͑Ͼ a few hundred times͒ observed with a resistance ratio ӷ10 2 . Both the low-and high-resistance states showed linear log current versus log voltage graphs with a slope of 1 in the low-voltage region where switching did not occur. The thermal stability of both conduction states was also studied. Atomic force microscopy studies under atmosphere and high-vacuum conditions showed that resistance switching is closely related to the formation and elimination of conducting spots. The conducting spots of the low-resistance state have a few tens times higher conductivity than those of the high-resistance state and their density is also a few tens times higher which results in a ϳ10 3 times larger overall conductivity. An interesting finding was that the area where the conducting spots do not exist shows a few times different resistance between the low-and high-resistance state films. It is believed that this resistance change is due to the difference in point defect density that was generated by the applied bias field. The point defects possibly align to form tiny conducting filaments in the high-resistance state and these tiny conducting filaments gather together to form stronger and more conducting filaments during the transition to the low-resistance state.

This review article summarized the recent understanding of resistance switching (RS) behavior in several binary oxide thin film systems. Among the various RS materials and mechanisms, TiO(2) and NiO thin films in unipolar thermo-chemical switching mode are primarily dealt with. To facilitate the discussions, the RS was divided into three parts; electroforming, set and reset steps. After short discussions on the electrochemistry of 'electrolytic' oxide materials, the general and peculiar aspects of these RS systems and mechanism are elaborated. Although the RS behaviors and characteristics of these materials are primarily dependent on the repeated formation and rupture of the conducting filaments (CFs) at the nanoscale at a localized position, this mechanism appears to offer a basis for the understanding of other RS mechanisms which were originally considered to be irrelevant to the localized events. The electroforming and set switching phenomena were understood as the process of CF formation and rejuvenation, respectively, which are mainly driven by the thermally assisted electromigration and percolation (or even local phase transition) of defects, while the reset process was understood as the process of CF rupture where the thermal energy plays a more crucial role. This review also contains several remarks on the outlook of these resistance change devices as a semiconductor memory.

Issues in the circuitry, integration, and material properties of the two-dimensional (2D) and three-dimensional (3D) crossbar array (CBA)-type resistance switching memories are described. Two important quantitative guidelines for the memory integration are provided with respect to the required numbers of signal wires and sneak current paths. The advantage of 3D CBAs over 2D CBAs (i.e., the decrease in effect memory cell size) can be exploited only under certain limited conditions due to the increased area and layout complexity of the periphery circuits. The sneak current problem can be mitigated by the adoption of different voltage application schemes and various selection devices. These have critical correlations, however, and depend on the involved types of resistance switching memory. The problem is quantitatively dealt with using the generalized equation for the overall resistance of the parasitic current paths. Atomic layer deposition is discussed in detail as the most feasible fabrication process of 3D CBAs because it can provide the device with the necessary conformality and atomic-level accuracy in thickness control. Other subsidiary issues related to the line resistance, maximum available current, and fabrication technologies are also reviewed. Finally, a summary and outlook on various other applications of 3D CBAs are provided.

Electroforming effects on the composition, structure, and electrical resistance of Pt/ TiO 2 / Pt switching cells are investigated. The correlation between the electroforming procedure and the resulting bipolar switching behavior is discussed. The dependence of electroforming behavior on atmosphere is also identified, from which we define symmetric or asymmetric electroforming. The symmetry of electroforming is a key factor determining the resulting bipolar switching characteristics. From the experimental results we suggest a possible mechanism for electroforming in Pt/ TiO 2 / Pt in terms of the formation of oxygen gas and vacancies in the vicinity of the anode.

In this Review, memristors are examined from the frameworks of both von Neumann and neuromorphic computing architectures. For the former, a new logic computational process based on the material implication is discussed. It consists of several memristors which play roles of combined logic processor and memory, called stateful logic circuit. In this circuit confi guration, the logic process fl ows primarily along a time dimension, whereas in current von Neumann computers it occurs along a spatial dimension. In the stateful logic computation scheme, the energy required for the data transfer between the logic and memory chips can be saved. The non-volatile memory in this circuit also saves the energy required for the data refresh. Neuromorphic (cognitive) computing refers to a computing paradigm that mimics the human brain. Currently, the neuromorphic or cognitive computing mainly relies on the software emulation of several brain functionalities, such as image and voice recognition utilizing the recently highlighted deep learning algorithm. However, the human brain typically consumes ≈10-20 Watts for selected "human-like" tasks, which can be currently mimicked by a supercomputer with power consumption of several tens of kilo-to megawatts. Therefore, hardware implementation of such brain functionality must be eventually sought for power-effi cient computation. Several fundamental ideas for utilizing the memristors and their recent progresses in these regards are reviewed. Finally, material and processing issues are dealt with, which is followed by the conclusion and outlook of the fi eld. These technical improvements will substantially decrease the energy consumption for futuristic information technology.

Electric-pulse-induced resistive switching of 43nm thick TiO2 thin films grown by metalorganic chemical vapor deposition was studied by current-voltage (I-V) and constant voltage-time measurements. The resistance ratio between the two stable states of the film constitutes approximately 1000. The allowed current level and voltage step width during the sweep mode I-V measurements influenced switching parameters, such as the switching voltage, time before switching, and resistance values. However, it was clearly observed that the power imparted to the film controlled mainly switching. The required power for successful switching was almost invariant irrespective of other measurement variables.

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