Nanoscale cation motion in TaOx, HfOx and TiOx memristive systems

Abstract

A detailed understanding of the resistive switching mechanisms that operate in redox-based resistive random-access memories (ReRAM) is key to controlling these memristive devices and formulating appropriate design rules. Based on distinct fundamental switching mechanisms, two types of ReRAM have emerged: electrochemical metallization memories, in which the mobile species is thought to be metal cations, and valence change memories, in which the mobile species is thought to be oxygen anions (or positively charged oxygen vacancies). Here we show, using scanning tunnelling microscopy and supported by potentiodynamic current–voltage measurements, that in three typical valence change memory materials (TaOx, HfOx and TiOx) the host metal cations are mobile in films of 2 nm thickness. The cations can form metallic filaments and participate in the resistive switching process, illustrating that there is a bridge between the electrochemical metallization mechanism and the valence change mechanism. Reset/Set operations are, we suggest, driven by oxidation (passivation) and reduction reactions. For the Ta/Ta2O5 system, a rutile-type TaO2 film is believed to mediate switching, and we show that devices can be switched from a valence change mode to an electrochemical metallization mode by introducing an intermediate layer of amorphous carbon.

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Figure 1: Scanning tunnelling switching at negative tip voltages under UHV conditions.
Figure 2: Scanning tunnelling switching at positive tip voltages.
Figure 3: Scanning tunnelling switching on HfOx and TiOx.
Figure 4: EXAFS measurements at the Ta/Ta2O5 interface.
Figure 5: Reset behaviour of TaOx-based devices.
Figure 6: IV sweeps on TaOx-based devices.

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Acknowledgements

This study was financially supported in part by BMBF project no. 03X0140 and DFG priority programme SFB 917. B.Y. and K.K.A. also acknowledge financial support from the MIT MRSEC through the MRSEC Program of the National Science Foundation under award no. DMR-1419807.

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Contributions

I.V. conceived the idea and designed the study. A.W. and M.M. performed the STM experiments. M.L. conducted current–voltage measurements on TaOx devices. D.-Y.C. performed and interpreted the XAS measurements. K.S. prepared the samples for the STM experiments. V.R. performed IV sweeps on TaOx, HfOx and TiOx devices. T.H., K.K.A. and B.Y. contributed to interpretation of the STM results. A.W. and I.V. wrote the manuscript. R.W. and I.V. directed the research. All authors contributed to the discussion of the results and improved the text.

Corresponding author

Correspondence to Ilia Valov.

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The authors declare no competing financial interests.

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Wedig, A., Luebben, M., Cho, DY. et al. Nanoscale cation motion in TaOx, HfOx and TiOx memristive systems. Nature Nanotech 11, 67–74 (2016). https://doi.org/10.1038/nnano.2015.221

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