Redox-based resistive memory cells

Promising candidates for tomorrow’s information technology

  • TeaserTeaser
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  • Fig. 1: A device with TaOx electrolyte, commonly classified as VCM cell, shows ECM-type switching behaviour after a thin amorphous carbon layer was introduced between the oxide and the tantalum electrode (reprinted with permission from [3]).
  • Fig. 2: In a TaOx-based cell, both the diffusion of oxygen vacancies and the movement of tantalum ions can contribute to the filament formation.

Redox-based resistive random-access memory (ReRAM) cells are currently considered to be very promising candidates for the information technology of tomorrow combining outstanding performance with energy-saving operation. Recent experiments now provided new insight into the operating principle of these cells. Thus, the current understanding of ReRAM cells could be improved significantly, as an important step was towards the large-scale commercial application of the devices.

What are redox-based resistive memory cells?
Redox-based resistive memory cells are data storage devices which consist of a solid electrolyte layer embedded between two metal electrodes. The originally high electrical resistance of this electrolyte layer can be reduced significantly by applying a high voltage. That sets the cells into an ON state, which can be reset to an electrically non-conductive OFF state by applying a high voltage of opposite polarity. ON and OFF state represent the bit values 1 and 0, hence the cells can be used as digital data storage devices.
In contrast to the conventional flash memory cells, the resistive switching in ReRAM devices is not only based on a change of the electronic structure of the storage element. It is rather determined by redox and transport processes of ionic defects in the solid electrolyte layer, which also lead to changes in the atomic configuratoin and the valence state (“redox-based” switching) [1]. As a result of the defect reactions and diffusion, an electron-conducting filament can form percolating the whole layer and setting the cell into an ON state. This fibre-shaped filament can be (partially) dissolved, resulting in an OFF state of the cell, and reformed again.

Advanteages of the ReRAM Technology
The ReRAM technology is nowadays considered to be a very promising alternative to the conventional storing technologies whose potential for development, in particular with regard to the miniaturizability, will soon be exhausted. It is expected that the ReRAM technology allows for significantly smaller cell sizes, low power consumption and high switching speed.

Furthermore, the technology offers the possibility to simulate the behaviour of synapses and therefore implement concepts of neuromorphic computing.
First ReRAM products were brought to the market by Adesto Technologies and Panasonic for niche applications. Further development aiming at the application for example in mobile devices such as smartphones or wearable electronics is currently under progress. For the technology to be used on a large scale, improvement in the reproducibility of the switching behaviour is required. To achieve this, a better understanding of the processes occurring in the cells is still necessary.

Different Types
So far, mainly two types of ReRAM cells have been considered as most promising [2]: In electrochemical metallization (ECM) cells, a metal filament is formed via the diffusion of cationic defects, that is, metal ions. The metal ions are dissolved in redox reactions from electrochemically active electrode (e.g., silver). As electrolyte, materials are used which offer a sufficiently high metal ion mobility, for example oxides such as SiOx or higher metal chalcogenides.
In valence change memory (VCM) cells, the electrolyte typically consists of a transition metal oxide such as TaOx. To date, the resistive switching in VCM cells was considered to be based on a shift of anionic defects (oxygen vacancies). The electronic conductivity in an oxygen-deficient filament, created by oxygen vacancy diffusion, is significantly increased in respect to the surrounding matrix. As electrodes, typically an inert electrode (e.g., platinum) and an electrode from a metal with high oxygen affinity (e.g., tantalum) are used. The latter withdraws oxygen from the electrolyte and thus creates the required oxygen vacancies within the TaOx.
Extensive electrical measurements now proved that also in typical VCM cells, an ECM mechanism can be realized [3,4]. This was enabled by the introduction of a thin amorphous carbon layer between the electrolyte and the reactive electrode. The modified cells showed completely different switching characteristics. The current-voltage curves (Figure 1) exhibited sharp transitions between the different resistance states and asymmetrical switching voltages and currents, typical features of an ECM cell. Thus, it was possible to completely change the switching mechanism of the cells via a relatively simple modification step.
It is noteworthy that ECM-type switching can only be expected if the cations are mobile in the electrolyte layer. Indications for a high mobility of Ta ions in TaOx already exist in literature (e.g., ref. [5]). The importance of the cation mobility for the resistive switching mechanism has however not been investigated in more detail. For the cells with carbon interlayer, it is expected that the carbon layer prevents the removal of oxygen from the electrolyte and thus the creation of oxygen vacancies. Therefore, the possibility for a cation-based switching is created also in VCM cells.
Further evidence of cation-based switching in VCM cells was gained from scanning tunnelling microscopy experiments: A cation-based switching mechanism could be implemented in simple TaOx, HfOx and TiOx model systems. The samples were pre-treated to reproduce the structure of the oxide films used in the electrical measurements. Therefore, the switching mechanisms are expected to be comparable. Finally, investigations by means of X-ray absorption spectroscopy provided evidence of structural changes in TaOx possibly favouring cation diffusion.

Our investigations show that it is not always possible to strictly distinguish between ECM and VCM cells. Rather, both switching mechanisms can be found in one and the same cell (Figure 2). This finding represents fundamentally new insight into the switching behaviour of resistive memory cells. It provides the basis for further improvement of ReRAM cells in respect to their large-scale application in the information technology of tomorrow.


Anja Wedig1, Michael Luebben1 and Ilia Valov1,2

1Peter Gruenberg Institute, Juelich Research Centre, Juelich, Germany
2Institute for Materials in Electrical Engineering II, RWTH Aachen University, Aachen, Germany

Dr. Ilia Valov

Electronic Materials (IEM)
Peter Grünberg Institut

Related Articles:

[1] R. Waser, M. Aono: Nanoionics-based resistive switching memories, Nat. Mater. 6, 833 (2007).  

[2] R. Waser (Ed.), Nanoelectronics and Information Technology (3rd edition), Wiley-VCH, Weinheim, 2012.

[3] A. Wedig, M. Lübben, D.-Y. Cho, M. Moors, K. Skaja, V. Rana, T. Hasegawa, K. K. Adepalli, B. Yildiz, R. Waser, I. Valov: Nanoscale cation motion in TaOx, HfOx and TiOx memristive systems Nat. Nanotechnol. (2015). 

[4] M. Lübben, P. Karakolis, V. Ioannou-Sougleridis, P. Normand, P. Dimitrakis, I. Valov: Graphene-Modified Interface Controls Transition from VCM to ECM Switching Modes in Ta/TaOx Based Memristive Devices, Adv. Mater. (2015). 

[5] B. Verkerk, P. Winkel, D. G. de Groot, Phillips Res. Rep. 13, 506 (1958)

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