and the ionization behavior becomes obvious with the increase of interface O
concentration and/or temperature; (3) the CF in the Pt/a-TaO x /Pt resistive switch
consists of Ta-Ta bonding structures, but not O vacancies; (4) the mobility of Ta ions
in the CF region of Pt/a-TaO x /Pt device is comparable to that of O ions, and thus
could actively participate in the switching process of a-TaO x based devices.
For the atomistic understanding on the switching mechanism of the (a-TaO x )
based resistive switches, several issues still remain as future tasks. For example, the
atomistic details on the effects of moisture on the Cu diffusion [5] have not been
clarified yet. In fact, our preliminary simulation, where we compared the Cu
diffusion behaviors between bare and water-adsorbed a-Ta 2 O 5 surfaces, suggests
that the Cu diffusion can be faster on the water-adsorbed surface than the bare one
[11]. Theoretical studies based on a more realistic model, however, would be
desirable.
For tackling with such issues, density functional calculations are still heavy even
on the current supercomputers. So development of methods that have both reliability
and computational efficiency is highly desired. For this purpose, the interatomic
potentials constructed by combining the density functional calculations and
machine-learning techniques have attracted much attention recently. We applied
such approaches to Cu diffusion in the a-Ta 2 O 5 [54] and Li diffusion in a-Li 3 PO 4
[55], but extensive investigation using these approaches remains as a future task.
Acknowledgments We thank Prof. Tinkun Gu, Prof. Tomofumi Tada for collaboration in the
early stage of the present work. SW also thanks Prof. Shu Yamaguchi, Prof. Tsuyoshi Hasegawa,
Dr. Tohru Tsuruoka, Dr Toshi Sakamoto and Dr Naoki Banno for fruitful discussion. This work was
partially supported by CREST-JST “Atom transistor”, Low Power Electronics Association and
Projects, the grant-in-aid for Innovation Area “Computics” by MEXT, Japan, and Global COE
program “Global COE for Mechanical Systems Innovation” by MEXT, Japan.
References
1. Wang, J.Y., Li, L.Z., Huyan, H., Pan, X.Q., Nonnenmann, S.: Highly uniform resistive
switching in HfO 2 films embedded with ordered metal nanoisland arrays. Adv. Funct. Mater.
29, 1808430 (2019)
2. Zhou, G.D., Duan, S.K., Li, P., Sun, B., Wu, B., Yao, Y.Q., Yang, X.D., Han, J.J., Wu, J.G.,
Wang, G., Liao, L.P., Lin, C.Y., Hu, W., Xu, C.Y., Liu, D.B., Chen, T., Chen, L.J., Zhou, A.K.,
Song, Q.L.: Coexistence of negative differential resistance and resistive switching memory at
room temperature in TiO x modulated by moisture. Adv. Electron. Mater. 4, 1700567 (2018)
3. Prakash, A., Jana, D., Maikap, S.: TaO x -based resistive switching memories: Prospective and
challenges. Nanoscale Res. Lett. 8, 418 (2013)
4. Lee, M.J., Lee, C.B., Lee, D., Lee, S.R., Chang, M., Hur, J.H., Kim, Y.B., Kim, C.J., Seo, D.H.,
Seo, S., Chung, U., Kim, K.: A fast, high-endurance and scalable non-volatile memory device
made from asymmetric Ta 2 O 5Àx /TaO 2Àx bilayer structures. Nat. Mater. 10, 625–630 (2011)
5. Valov, L., Tsuruoka, T.: Effects of moisture and redox reactions in VCM and ECM resistive
switching memories. J. Phys. D. Appl. Phys. 51, 413001 (2018)
6. Xiao, B., Gu, T.K., Tada, T., Watanabe, S.: Conduction paths in Cu/amorphous- Ta 2 O 5 /Pt
atomic switch: First-principles studies. J. Appl. Phys. 115, 034503 (2014)
122
S. Watanabe and B. Xiao
concentration and/or temperature; (3) the CF in the Pt/a-TaO x /Pt resistive switch
consists of Ta-Ta bonding structures, but not O vacancies; (4) the mobility of Ta ions
in the CF region of Pt/a-TaO x /Pt device is comparable to that of O ions, and thus
could actively participate in the switching process of a-TaO x based devices.
For the atomistic understanding on the switching mechanism of the (a-TaO x )
based resistive switches, several issues still remain as future tasks. For example, the
atomistic details on the effects of moisture on the Cu diffusion [5] have not been
clarified yet. In fact, our preliminary simulation, where we compared the Cu
diffusion behaviors between bare and water-adsorbed a-Ta 2 O 5 surfaces, suggests
that the Cu diffusion can be faster on the water-adsorbed surface than the bare one
[11]. Theoretical studies based on a more realistic model, however, would be
desirable.
For tackling with such issues, density functional calculations are still heavy even
on the current supercomputers. So development of methods that have both reliability
and computational efficiency is highly desired. For this purpose, the interatomic
potentials constructed by combining the density functional calculations and
machine-learning techniques have attracted much attention recently. We applied
such approaches to Cu diffusion in the a-Ta 2 O 5 [54] and Li diffusion in a-Li 3 PO 4
[55], but extensive investigation using these approaches remains as a future task.
Acknowledgments We thank Prof. Tinkun Gu, Prof. Tomofumi Tada for collaboration in the
early stage of the present work. SW also thanks Prof. Shu Yamaguchi, Prof. Tsuyoshi Hasegawa,
Dr. Tohru Tsuruoka, Dr Toshi Sakamoto and Dr Naoki Banno for fruitful discussion. This work was
partially supported by CREST-JST “Atom transistor”, Low Power Electronics Association and
Projects, the grant-in-aid for Innovation Area “Computics” by MEXT, Japan, and Global COE
program “Global COE for Mechanical Systems Innovation” by MEXT, Japan.
References
1. Wang, J.Y., Li, L.Z., Huyan, H., Pan, X.Q., Nonnenmann, S.: Highly uniform resistive
switching in HfO 2 films embedded with ordered metal nanoisland arrays. Adv. Funct. Mater.
29, 1808430 (2019)
2. Zhou, G.D., Duan, S.K., Li, P., Sun, B., Wu, B., Yao, Y.Q., Yang, X.D., Han, J.J., Wu, J.G.,
Wang, G., Liao, L.P., Lin, C.Y., Hu, W., Xu, C.Y., Liu, D.B., Chen, T., Chen, L.J., Zhou, A.K.,
Song, Q.L.: Coexistence of negative differential resistance and resistive switching memory at
room temperature in TiO x modulated by moisture. Adv. Electron. Mater. 4, 1700567 (2018)
3. Prakash, A., Jana, D., Maikap, S.: TaO x -based resistive switching memories: Prospective and
challenges. Nanoscale Res. Lett. 8, 418 (2013)
4. Lee, M.J., Lee, C.B., Lee, D., Lee, S.R., Chang, M., Hur, J.H., Kim, Y.B., Kim, C.J., Seo, D.H.,
Seo, S., Chung, U., Kim, K.: A fast, high-endurance and scalable non-volatile memory device
made from asymmetric Ta 2 O 5Àx /TaO 2Àx bilayer structures. Nat. Mater. 10, 625–630 (2011)
5. Valov, L., Tsuruoka, T.: Effects of moisture and redox reactions in VCM and ECM resistive
switching memories. J. Phys. D. Appl. Phys. 51, 413001 (2018)
6. Xiao, B., Gu, T.K., Tada, T., Watanabe, S.: Conduction paths in Cu/amorphous- Ta 2 O 5 /Pt
atomic switch: First-principles studies. J. Appl. Phys. 115, 034503 (2014)
122
S. Watanabe and B. Xiao