CogniGron Seminar: Horatio Cox (University College London) - "Measuring the Ionic Diffusion in Silicon Oxide Memristors and Harnessing their Frequency Response"

A promising route to neuromorphic systems is using resistance switching in oxide-based, intrinsic ReRAM devices. During operation, under an applied field, oxygen is driven across the metal-oxide-metal (MIM) stacks modulating the resistance between the electrodes. The electrode is thought to act as an oxygen reservoir enabling the reversible exchange of oxygen with the high resistance oxide. How effective the electrodes are as a reservoir is a significant factor in determining the cycling endurance and stability of devices. Here we apply state of the art analysis techniques and develop new measurement methodologies to investigate the role of oxygen diffusion, impurities, and microstructure in ReRAM operation.

Firstly, a novel Secondary Ion Mass Spectrometry (SIMS) technique allows us for the first time to observe the movement of 16O across electrically biased silicon oxide (SiOx) ReRAM stacks, measuring bulk concentration changes in a continuous profile with unprecedented sensitivity. Applying this to three devices, using different electrode materials, we systematically examine the exchange of oxygen across the oxide-metal interface. A clear link is revealed between the thermodynamics and microstructure of the electrode material with the scale and nature of this diffusion across the oxide-metal interface.

Secondly, Quantitative measurements of the thin film compositions in ReRAM devices reveal oxidised electrodes and high concentrations of impurities, in particular hydrogen, through every device analysed, even those manufactured industrially. This provides further compelling evidence that hydrogen is involved in ReRAM operation.

Finally, memristors are often simply employed as variable conductances. Nevertheless, their complex frequency response, which remains largely unstudied, offers radical new ways to extend the capabilities of memristive computation by exploiting the frequency and phase domains. The devices exhibit rich dynamics, which hint at exciting new ways to encode and process information in the frequency domain.

H.R.J. Cox1*, M. Buckwell2, W.H. Ng1, D.J. Mannion1, A. Mehonic1, Matthew K. Sharpe2, Callum McAleese2, Jeevan Dulai2, Richard Smith2, Jonathan England2, S. Fearn3 & A.J. Kenyon1

1: Department of Electronic and Electrical Engineering, University College London, Torrington Place, London WC1E 7JE, UK.

2: England Ion Beam Centre, Advanced Technology Institute, University of Surrey, UK, Guildford, GU2 7XH, United Kingdom

3: Department of Materials, Imperial College London, South Kensington Campus, London SW7 2AZ, UK