In this paper, we review the recent progress in the resistive random access memory (ReRAM) technology, one of the most promising emerging nonvolatile memories, in which both electronic and electrochemical effects play important roles in the nonvolatile functionalities. First, we provide a brief historical overview of the research in this field. We also provide a technological overview and the epoch-making achievements, followed by an account of the current understanding of both bipolar and unipolar ReRAM operations. Finally, we summarize the challenges facing the ReRAM technology as it moves toward the beyond-2X-nm generation of nonvolatile memories and the so-called beyond complementary metal-oxide-semiconductor (CMOS) device.

The resistance switching properties in $\mathrm{Pt}∕\mathrm{Ni}–\mathrm{O}∕\mathrm{Pt}$ and $\mathrm{Pt}∕\mathrm{Co}–\mathrm{O}∕\mathrm{Pt}$ synthesized by the magnetron sputtering have been investigated. The oxygen partial pressure during sputtering and the post-thermal process are crucial to forming of the trilayer. By investigating x-ray photoemission spectroscopy spectra, the increase of initial resistance in Ni–O was caused by the variation of the stoichiometry, while that in the Co–O was accompanied by the phase transformation between CoO and ${\mathrm{Co}}_3{\mathrm{O}}_4$ . The resistance switching in $\mathrm{Pt}∕\mathrm{Co}–\mathrm{O}∕\mathrm{Pt}$ and $\mathrm{Pt}∕\mathrm{Ni}–\mathrm{O}∕\mathrm{Pt}$ exhibits the analogous electrode area and temperature dependences. As a result of the $I\text{-}V$ measurements at the elevated temperature, the assistance of Joule heating in the reset process is implied.

Structural and resistance switching properties were investigated in the CoO resistance random access memory (RRAM) with the Ta electrode. The intermediate layer consisting of Co and Ta oxides was confirmed at the interface by the transmission electron microscopy and electron energy loss spectroscopy. The great affinity with oxygen in Ta together with a high resistivity of the Ta oxide improves the operational performance of RRAM. The controllability of the resistance after forming and the low-current operation property were substantially improved by using the load resistor connected in series with CoO RRAM with the Ta electrode. The reset current less than 0.2 mA and the switching speed faster than 20 ns were demonstrated.

The switchable rectifier built with $\text{Pt}/{\text{TiO}}_x/\text{Pt}$ was proposed and the reproducible rectification switching was demonstrated. The rectification switching is considered to be originated from the electrical control of the oxygen vacancy concentration at the interfaces between Pt and ${\text{TiO}}_x$ and resultant variations in the Schottky barrier height. The electrode area dependence of the current conduction after the rectification switching revealed that this switching occurs at the almost entire interface region. The reproducible switching in the $\text{Pt}/{\text{TiO}}_x/\text{Pt}$ rectifier is demonstrated by changing the polarity of the pulse voltage, evidencing the excellent functionality of the engineered metal/oxide interface.

The $\mathrm{Pt}∕\mathrm{Ti}{\mathrm{O}}_x∕\mathrm{Pt}$ trilayer with electrically asymmetrical interface have been synthesized by means of the reactive sputtering technique followed by the oxygen annealing. The initial current-voltage characteristics in the $\mathrm{Pt}∕\mathrm{Ti}{\mathrm{O}}_x∕\mathrm{Pt}$ trilayer cell have rectifying behavior originated from the Schottky junction formed between $\mathrm{Ti}{\mathrm{O}}_x$ and Pt top electrode layer. The series connection of $\mathrm{Pt}∕\mathrm{Ti}{\mathrm{O}}_x∕\mathrm{Pt}$ trilayer cells brings about the control of the reset and set voltages depending on the resistance of the connected Schottky diode, which is the demonstration of the resistance switching in the resistance random access memory with the one diode and one resistance structure using Schottky barrier diode.

We have successfully achieved high speed (~50 ns) unipolar operation in RRAM devices comprised of titanium oxynitride (TiON) combined with a control resistor connected in series. For unipolar switching, programming and erasing pulses can be the same width, typically, a few tens of nano-seconds. This enables high speed and high density cross-point RRAM memory arrays. In addition, we demonstrate how switching characteristics can be controlled by a series resistor