Nanoscale cation motion in TaOx, HfOx and TiOx memristive systems


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.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

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.


  1. 1

    Baek, I. G. et al. Highly scalable non-volatile resistive memory using simple binary oxide driven by asymmetric unipolar voltage pulses. IEDM Tech. Dig. 587–590 (2004).

  2. 2

    Waser, R. & Aono, M. Nanoionics-based resistive switching memories. Nature Mater. 6, 833–840 (2007).

    CAS  Article  Google Scholar 

  3. 3

    Kaeriyama, S. et al. A nonvolatile programmable solid–electrolyte nanometer switch. IEEE J. Solid-State Circ. 40, 168–176 (2005).

    Article  Google Scholar 

  4. 4

    Strukov, D. B. & Likharev, K. K. CMOL FPGA: a reconfigurable architecture for hybrid digital circuits with two-terminal nanodevices. Nanotechnology 16, 888–900 (2005).

    CAS  Article  Google Scholar 

  5. 5

    Foelling, S., Tuerel, O. & Likharev, K. Single-electron latching switches as nanoscale synapses. Proc. IJCNN‘01, 216–221 (2001).

  6. 6

    Pickett, M. D., Medeiros-Ribeiro, G. & Williams, R. S. A scalable neuristor built with Mott memristors. Nature Mater. 12, 114–117 (2013).

    CAS  Article  Google Scholar 

  7. 7

    Jo, S. H. et al. Nanoscale memristor device as synapse in neuromorphic systems. Nano Lett. 10, 1297–1301 (2010).

    CAS  Article  Google Scholar 

  8. 8

    Ohno, T. et al. Short-term plasticity and long-term potentiation mimicked in single inorganic synapses. Nature Mater. 10, 591–595 (2011).

    CAS  Article  Google Scholar 

  9. 9

    Torrezan, A. C., Strachan, J. P., Medeiros-Ribeiro, G. & Williams, R. S. Sub-nanosecond switching of a tantalum oxide memristor. Nanotechnology 22, 485203 (2011).

    Article  Google Scholar 

  10. 10

    Hasegawa, T., Terabe, K., Tsuruoka, T. & Aono, M. Atomic switch: atom/ion movement controlled devices for beyond von-Neumann computers. Adv. Mater. 24, 252–267 (2012).

    CAS  Article  Google Scholar 

  11. 11

    Waser, R., Bruchhaus, R. & Menzel, S. in Nanoelectronics and Information Technology (ed. Waser, R.) 683–710 (Wiley, 2012).

  12. 12

    Szot, K., Speier, W., Bihlmayer, G. & Waser, R. Switching the electrical resistance of individual dislocations in single-crystalline SrTiO3 . Nature Mater. 5, 312–320 (2006).

    CAS  Article  Google Scholar 

  13. 13

    Yang, Y. et al. Electrochemical dynamics of nanoscale metallic inclusions in dielectrics. Nature Commun. 5, 4232 (2014).

    CAS  Article  Google Scholar 

  14. 14

    Park, G.-S. et al. In situ observation of filamentary conducting channels in an asymmetric Ta2O5–x/TaO2–x bilayer structure. Nature Commun. 4, 2382 (2013).

    Article  Google Scholar 

  15. 15

    Valov, I. et al. Atomically controlled electrochemical nucleation at superionic solid electrolyte surfaces. Nature Mater. 11, 530–535 (2012).

    CAS  Article  Google Scholar 

  16. 16

    Nayak, A., Tsuruoka, T., Terabe, K., Hasegawa, T. & Aono, M. Switching kinetics of a Cu2S-based gap-type atomic switch. Nanotechnology 22, 235201 (2011).

    Article  Google Scholar 

  17. 17

    Nayak, A. et al. Rate-limiting processes determining the switching time in a Ag2S atomic switch. J. Phys. Chem. Lett. 1, 604–608 (2010).

    CAS  Article  Google Scholar 

  18. 18

    Chen, Y. L. et al. Scanning tunneling microscopy/spectroscopy studies of resistive switching in Nb-doped SrTiO3 . J. Appl. Phys. 112, 023703 (2012).

    Article  Google Scholar 

  19. 19

    Yang, J. J., Strukov, D. B. & Stewart, D. R. Memristive devices for computing. Nature Nanotech. 8, 13–24 (2013).

    CAS  Article  Google Scholar 

  20. 20

    Yang, J. J. et al. High switching endurance in TaOx memristive devices. Appl. Phys. Lett. 97, 232102 (2010).

    Article  Google Scholar 

  21. 21

    Balatti, S., Larentis, S., Gilmer, D. C. & Ielmini, D. Multiple memory states in resistive switching devices through controlled size and orientation of the conductive filament. Adv. Mater. 25, 1474–1478 (2013).

    CAS  Article  Google Scholar 

  22. 22

    Kroeger, F. A. The Chemistry of Imperfect Crystals (North-Holland, 1973).

    Google Scholar 

  23. 23

    Terabe, K., Hasegawa, T., Nakayama, T. & Aono, M. Quantized conductance atomic switch. Nature 433, 47–50 (2005).

    CAS  Article  Google Scholar 

  24. 24

    Chien, T., Santos, T. S., Bode, M., Guisinger, N. P. & Freeland, J. W. Controllable local modification of fractured Nb-doped SrTiO3 surfaces. Appl. Phys. Lett. 95, 163107 (2009).

    Article  Google Scholar 

  25. 25

    Valov, I. et al. Atomic scale and interface interactions in redox-based resistive switching memories. ECS Trans. 64, 3–18 (2014).

    CAS  Article  Google Scholar 

  26. 26

    Davies, J. A., Domeij, B., Pringle, J. P. S. & Brown, F. The migration of metal and oxygen during anodic film formation. J. Electrochem. Soc. 112, 675–680 (1965).

    CAS  Article  Google Scholar 

  27. 27

    Verkerk, B., Winkel, P. & de Groot, D. G. On the mechanism of anodic oxidation of tantalum. Phillips Res. Rep. 13, 506–508 (1958).

    CAS  Google Scholar 

  28. 28

    Whitton, J. L. The measurement of ionic mobilities in the anodic oxides of tantalum and zirconium by a precision sectioning technique. J. Electrochem. Soc. 115, 58–61 (1968).

    CAS  Article  Google Scholar 

  29. 29

    Venkatu, D. A. & Poteat, L. E. Diffusion of titanium in single crystal rutile. Mater. Sci. Eng. 5, 258–262 (1970).

    CAS  Article  Google Scholar 

  30. 30

    Khalil, N. & Leach, J. The anodic-oxidation of valve metals 1. Determination of ionic transport numbers by alpha-spectrometry. Electrochim. Acta 31, 1279–1285 (1986).

    CAS  Article  Google Scholar 

  31. 31

    Habazaki, H. et al. Ionic transport in amorphous anodic titania stabilised by incorporation of silicon species. Corrosion Sci. 44, 1047–1055 (2002).

    CAS  Article  Google Scholar 

  32. 32

    Akse, J. R. & Whitehurst, H. B. Diffusion of titanium in slightly reduced rutile. J. Phys. Chem. Solids 39, 457–465 (1978).

    CAS  Article  Google Scholar 

  33. 33

    Hu, C. et al. Highly controllable and stable quantized conductance and resistive switching mechanism in single-crystal TiO2 resistive memory on silicon. Nano Lett. 14, 4360–4367 (2014).

    CAS  Article  Google Scholar 

  34. 34

    Shibuya, K., Dittmann, R., Mi, S. & Waser, R. Impact of defect distribution on resistive switching characteristics of Sr2TiO4 thin films. Adv. Mater. 22, 411–414 (2010).

    CAS  Article  Google Scholar 

  35. 35

    Linn, E., Rosezin, R., Kügeler, C. & Waser, R. Complementary resistive switches for passive nanocrossbar memories. Nature Mater. 9, 403–406 (2010).

    CAS  Article  Google Scholar 

  36. 36

    Muenstermann, R., Menke, T., Dittmann, R. & Waser, R. Coexistence of filamentary and homogeneous resistive switching in Fe-doped SrTiO3 thin-film memristive devices. Adv. Mater. 22, 4819–4822 (2010).

    CAS  Article  Google Scholar 

  37. 37

    Tsuruoka, T. et al. Effects of moisture on the switching characteristics of oxide-based, gapless-type atomic switches. Adv. Funct. Mater. 22, 70–77 (2012).

    CAS  Article  Google Scholar 

  38. 38

    Tsuruoka, T., Terabe, K., Hasegawa, T. & Aono, M. Temperature effects on the switching kinetics of a Cu-Ta2O5-based atomic switch. Nanotechnology 22, 254013 (2011).

    Article  Google Scholar 

  39. 39

    Tsuruoka, T., Terabe, K., Hasegawa, T. & Aono, M. Forming and switching mechanisms of a cation-migration-based oxide resistive memory. Nanotechnology 21, 425205 (2010).

    CAS  Article  Google Scholar 

  40. 40

    Tappertzhofen, S., Waser, R. & Valov, I. Impact of counter electrode material on the redox processes in resistive switching memories. ChemElectroChem 1, 1287–1292 (2014).

    CAS  Article  Google Scholar 

  41. 41

    Van den Hurk, J. et al. Physical origins and suppression of Ag dissolution in GeSx-based ECM cells. Phys. Chem. Chem. Phys. 16, 18217–18225 (2014).

    CAS  Article  Google Scholar 

Download references


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.

Author information




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.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Supplementary information

Supplementary information

Supplementary information (PDF 1476 kb)

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Wedig, A., Luebben, M., Cho, DY. et al. Nanoscale cation motion in TaOx, HfOx and TiOx memristive systems. Nature Nanotech 11, 67–74 (2016).

Download citation

Further reading


Find nanotechnology articles, nanomaterial data and patents all in one place. Visit Nano by Nature Research