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.

The development of a resistance switching (RS) memory cell that contains rectifi cation functionality in itself, highly reproducible RS performance, and electroforming-free characteristics is an impending task for the development of resistance switching random access memory. In this work, a two-layered dielectric structure consisting of HfO 2 and Ta 2 O 5 layers, which are in contact with the TiN and Pt electrode, is presented for achieving these tasks simultaneously in one sample confi guration. The HfO 2 layer works as the resistance switching layer by trapping or detrapping of electronic carriers, whereas the Ta 2 O 5 layer remains intact during the whole switching cycle, which provides the rectifi cation. With the optimized structure and operation conditions for the given materials, excellent RS uniformity, electroforming-free, and selfrectifying functionality could be simultaneously achieved from the Pt/Ta 2 O 5 / HfO 2 /TiN structure.

The biomimetic characteristics of the memristor as an electronic synapse and neuron have inspired the advent of new information technology in the neuromorphic computing. The application of the memristors can be extended to the artificial nerves on condition of the presence of electronic receptors which can transfer the external stimuli to the internal nerve system. In this work, nociceptor behaviors are demonstrated from the Pt/HfO /TiN memristor for the electronic receptors. The device shows four specific nociceptive behaviors; threshold, relaxation, allodynia, and hyperalgesia, according to the strength, duration, and repetition rate of the external stimuli. Such nociceptive behaviors are attributed to the electron trapping/detrapping to/from the traps in the HfO layer, where the depth of trap energy level is ≈0.7 eV. Also, the built-in potential by the work function mismatch between the Pt and TiN electrodes induces time-dependent relaxation of trapped electrons, providing the appropriate relaxation behavior. The relaxation time can take from several milliseconds to tens of seconds, which corresponds to the time span of the decay of biosignal. The material-wise evaluation of the electronic nociceptor in comparison with other material, which did not show the desired functionality, Pt/Ti/HfO /TiN, reveals the importance of careful material design and fabrication.

Limiting the location where electron injection occurs at the cathode interface to a narrower region is the key factor for achieving a highly improved RS performance, which can be achieved by including Ru Nanodots. The development of a memory cell structure truly at the nanoscale with such a limiting factor for the electric-field distribution can solve the non-uniformity issue of future ReRAM.

Various array types of 1-diode and 1-resistor stacked crossbar array (1D1R CA) devices composed of a Schottky diode (SD) (Pt/TiO 2 /Ti/Pt) and a resistive switching (RS) memory cell (Pt/TiO 2 /Pt) are fabricated and their performances are investigated. The unit cell of the 1D1R CA device shows high RS resistance ratio ( ≈ 10 3 at 1.5 V) between low and high resistance state (LRS and HRS), and high rectifi cation ratio ( ≈ 10 5 ) between LRS and reverse-state SD. It also shows a short RS time of < 50 ns for SET (resistance transition from HRS to LRS), and ≈ 600 ns for RESET (resistance transition from LRS to HRS), as well as stable RS endurance and data retention characteristics. It is experimentally confi rmed that the selected unit cell in HRS (logically the "off " state) is stably readable when it is surrounded by unselected LRS (logically the "on" state) cells, in an array of up to 32 × 32 cells. The SD, as a highly non-linear resistor, appropriately controls the conducting path formation during the switching and protects the memory element from the noise during retention.