Atomic switch: Development of resistive switching memories based on nanoscalemigration of cations in oxide thin films
Tohru Tsuruoka, Tsuyoshi Hasegawa, Kazuya Terabe, and Masakazu Aono
International Center for Materials Nanoarchitectonics (MANA)
National Institute for Materials Science (NIMS)
Namiki 1-1, Tsukuba, Ibaraki 305-0044 Japan

Abstract
Resistive switching memories based on the formation and dissolution of a metal filament in simple
metal/insulator/metal (MIM) structures are attractive because of their potential high scalability,
low-power consumption, and ease of operation. From the standpoint of the operation mechanism,
this type of memory devices can be referred to as a gapless-type atomic switch.

Here, we present

our recent results of MIM structured atomic switches consisting of thin layers of inorganic andorganic oxides, focusing on their switching characteristics and mechanisms.

As the downscaling of flash and dynamic random access memories approaches the physicallimits, new solutions for volatile/nonvolatile storage are being investigated at the research level.
Among several emerging technologies, cation-migration-based resistive memory is one of the most
attractive alternatives to the current memory technologies. Several years ago, a conceptually novelswitching device, called an ‘atomic switch’, was proposed, whose resistance across a nanometer gap
is controlled by migration of metal ions and solid-state electrochemical reactions under electricalbias.[1] Similar resistive switching has been observed for simple metal/insulator/metal (MIM) cells,
in which a thin layer of metal-ion conductor is sandwiched between an electrochemically active
electrode (usually Cu and Ag) and an inert metal electrode (for example, Pt). Because migration of
metal ions plays a crucial role on the switching behavior, we call this MIM structured cell a‘gapless-type atomic switch’. In the beginning, mixed electronic and ionic conductors such as
Ag2S and Cu2S were employed,[2] but then oxide materials were used as the metal-ion conductor inorder to increase the threshold voltages of resistive switching above the operating voltage ofcomplementary metal-oxide-semiconductor devices.[3]mechanism is not yet fully understood.

However, their resistive switching

A complete understanding of the mechanism is very

important for the development of this type of cells for commercial electronic devices.
To address this issue, we have investigated current-voltage (I-V) and current-time characteristics
of cross-point structured Cu/Ta2O5/Pt cells.[4] The cells were initially SET from the OFF state to
the ON state when a relatively higher positive bias was applied to the Cu electrode, as shown by theblue curve in Fig. 1a. This first SET operation corresponds to the first formation of a metal
filament between the electrodes by inhomogeneous nucleation and subsequent growth of Cu on thePt electrode, based on the migration of Cu ions into Ta2O5 layer from the Cu electrode. After theformation of the metal filament, the cells exhibited bipolar switching behavior with the repetition ofSET and RESET (from the ON state to the OFF state) operations, as shown by the red curves. TheSET process corresponds to the reformation of the metal filament on a remaining Cu by nucleation(or, electrodeposition) as shown in Figs. 1b to 1e, while the RESET process is attributed to thethermal dissolution of the metal filament due to the Joule-heating-assisted oxidation followed by thediffusion of Cu ions under the concentration gradient and the applied electric field as illustrated inFigs. 1f to 1i.
Since the SET and RESET processes are based on thermochemical reactions, the switching
behavior should depend on ambient temperature.

We performed I-V measurements as a function oftemperature to determine the validity of our switching model.[5] The SET and RESET voltages
decreased in magnitude with increasing temperature, as shown in Fig. 2a.

From calculations of thenucleation rate of Cu nuclei based on the classical nucleation theory, it was found that the observed
temperature variation of the SET voltage is determined by supersatuation in the vicinity of the Ptelectrode and migration speed of Cu ions in the Ta2O5 layer, which are controlled by the applicationof positive bias. The ON resistance increased with increasing temperature (Fig. 2b), which was theopposite behavior expected given the SET and the current compliance (represented by thegray-shaded area). This discrepancy is probably due to varied growth speed of a Cu nucleus after
nucleation at different temperatures or in different supersaturation conditions. The RESET voltage
is determined by the size of the metal filament formed at a given temperature and its thermal
stability against the surrounding temperature.

These results are consistent with the proposed

switching model.
The effects of moisture on the operational characteristics were also examined.[6]Figures 3a and3b plot the SET and RESET voltages of Cu/Ta2O5/Pt and Cu/SiO2/Pt cells as a function of ambientpressure. Both the SET and RESET voltages of the Cu/Ta2O5/Pt cell were nearly constant, but theCu/SiO2/Pt cell exhibited a two-fold increase in magnitude of those voltages with a reduction in theambient pressure.

FT-IR measurements indicated the presence of a certain amount of residual

water in both the Ta2O5 and SiO2 layer, which is absorbed from the ambient air.

From the results

obtained, we concluded that the ionization of Cu at the anode interface is attributed to chemical
oxidation via residual water.

The moisture absorption also results in the formation of ahydrogen-bond network at grain boundaries in the oxide layers, and Cu ions are likely to migratealong the grain boundaries, as illustrated in Fig. 3c. Due to strong hydrogen bonds in the Ta2O5layer, the operational voltages of the Cu/Ta2O5/Pt cell are independent of the ambient H2O pressure.
However, the switching behavior of the Cu/SiO2/Pt cell is significantly influenced by ambientconditions because of weak hydrogen bonds in the SiO2 layer.
We also demonstrated that the atomic switch can be realized using solid polymer electrolytes(SPE).[7] MIM cells were fabricated with a mixture of polyethylene oxide (PEO) and AgClO4.

Figure 4a shows a typical resistance-voltage (R-V) curve of an Ag/PEO+AgClO4/Pt cell with anAgClO4 concentration of 3 wt%.

The cell exhibited bipolar switching behavior with a high
5ON/OFF resistance ratio of about 10 . From the results of thermal, transport, and electrochemicalmeasurements, it was concluded that the observed switching originates from the formation anddissolution of a metal filament inside the SPE film caused by electrochemical reactions, similar toinorganic electrolyte-based atomic switches, as illustrated in the schematics of Fig. 4a.

Moreover,the cell showed high programming speed (< 1s) and long retention time (> 1 week) at this earlystage, as seen in Fig. 4b. Now we are trying to fabricate the cell on a flexible substrate using an
inkjet technique.

Atomic switches are a new type of nanoionics devices that are operated by transport of metal ionsand solid-state electrochemical reactions on a nanometer scale. Because of the unique operation
mechanism, they show novel characteristics such as nonvolatility, low-power consumption, low ONresistance, and high ON/OFF resistance ratio. Much research effort is still needed to explore thepotential of resistive switching effect and develop for practical device applications.

[1] K. Terabe, T. Hasegawa, T. Nakayama, and M. Aono, Nature 433, 47-50 (2005).
[2] T. Sakamoto, H. Sunamura, H. Kawaura, T. Hasegawa, T. Nakayama, and M. Aono, Appl. Phys.
Lett. 82, 3032-4 (2003).
[3] T. Sakamoto, K. Lister, K. Banno, T. Hasegawa, K. Terabe, and M. Aono, Appl. Phys. Lett. 91,092110 (2007).
[4] T. Tsuruoka, K. Terabe, T. Hasegawa, and M. Aono, Nanotechnology 21, 425205 (2010).
[5] T. Tsuruoka, K. Terabe, T. Hasegawa, and M. Aono, Nanotechnology 22, 254013 (2011).
[6] T. Tsuruoka, K. Terabe, T. Hasegawa, I. Valov, R. Waser, and M. Aono, Adv. Func. Mater. (inpress).
[7] S. Wu, T. Tsuruoka, K. Terabe, T. Hasegawa, J. P. Hill, K. Ariga, and M. Aono, Adv. Func. Mater.
21, 93-99 (2011).