Threshold switching or current-controlled negative differential resistance (CC-NDR) in strongly correlated oxides is of considerable technological interest for applications such as a memory selector elements and voltage-controlled oscillators . A particular focus in recent years has been the development of coupled relaxation oscillators for neuromorphic computing applications, based on the fact that these have been shown to emulate the functionality of computational neurons . Threshold switching in NbOx thin films has been attributed to two main mechanisms, namely: a thermally induced metal-insulator phase transformation (MIT)  or Poole-Frenkel (PF) conduction , with more recent studies showing that both mechanisms can play a role .
In this study we fabricated simple metal-insulator-metal (MIM) capacitor structures, comprising: TiN (50 nm) /NbOx (70 nm)/Pt (50 nm) heterostructures, with top Pt contacts of 15-150 µm diameter. The electroformed devices were found to exhibit a combination of volatile threshold switching and non-volatile bipolar resistive switching, and therefore comprised a selector/memory (1S1M) structure. The threshold and hold voltages were found to be independent of niobium oxide thickness and device area, suggesting that the switching volume is localized both laterally and vertically. This was confirmed by finite element modelling using both PF and MIT switching mechanisms. Specifically, the models predicted a significant increase in threshold voltage with increasing film thickness when switching occurred along the full length of the filamentary conduction path. Only by limiting the switching region to a fixed volume near the metal/oxide interface could the experimental results be reproduced. Experiments also showed that the threshold current decreased with increasing film thickness. Both finite element and equivalent circuit models suggest that this arises from a parallel resistance, most likely associated with a halo-region surrounding the metallic filament. These experimental results and model predictions are used to develop a comprehensive model of filamentary threshold switching in NbOx films.
1. S. Li, X. Liu, S. K. Nandi, D. K. Venkatachalam, and R. G. Elliman, Appl. Phys. Lett. 106, 212902 (2015).
2. M. D. Pickett, G. Medeiros-Ribeiro, and R. S. Williams, Nat. Mater., 12, 114–117 (2013).
3. M.D. Pickett, and R.S. Williams, Nanotechnology, 23(21): 215202. (2012)
4. S. Slesazeck, H. Mähne, H. Wylezich, A. Wachowiak, J. Radhakrishnan, A. Ascoli, R. Tetzlaff, and T. Mikolajick, RSC Advances, 5, 102318 (2015)
5. S. Kumar, Z. Wang, N. Davila, N. Kumari, K.J. Norris, X. Huang, J.P. Strachan, D. Vine, A.L.D. Kilcoyne, Y. Nishi, and R.S. Williams, 8, 658 (2017)