Real-Time In Situ Optical Tracking of Oxygen Vacancy Migration in Memristors

In recent years, resistive switches have been widely developed because of their low power consumption, nanosecond-scale response and logic-in-memory applications. The switching mechanism of valence change memories involves the migration, accumulation and rearrangement of oxygen vacancies within a dielectric medium to change the electrical conductivity, triggered by an external applied potential. The ability to look deep inside materials to unveil how morphological changes characterise the functioning of active devices has been vital. However, current technologies are often destructive and invasive and despite significant research efforts, a microscopic picture of how exactly the mobile species actuate device switching is still under debate [1, 2]. In-situ characterization techniques that can visualize oxygen vacancy migration in real-time and at nanoscale resolution would be hugely valuable to achieve reliable and predictable nanodevices at the wafer scale.<br/><br/>We develop a novel non-destructive fully-optical technique [3,4] to observe in-situ and in real-time the oxygen vacancy migration within a switching material under ambient conditions, otherwise invisible to conventional electron microscopy techniques. The formation of oxygen-vacancy-induced filaments in the switching material is captured within a tightly-confined plasmonic hotspot [5]. This allows the optical measurements to be intimately linked to the electrical and material properties, hence accessing switching dynamics directly. Different optical resonances are independently sensitive to either build-up of oxygen vacancies or formation of O₂ nano-reservoirs at the interfaces. Our spectroscopy resolves the movement of several hundred oxygen vacancies even before any electrical transport change is observed. We find that oxygen vacancies build up at the material interfaces, and that O₂ bubbles of a few nm-size forming at interfaces are enough to cause long term degradation [4].<br/><br/><b>References</b><br/>[1] Yang, J.J. et al, (2013) <i>Nature Nanotech,</i> 8, 13.<br/>[2] Sun, W. et al, (2019) <i>Nat Comm, </i>10, 3453.<br/>[3] Di Martino, G. et al, (2016) <i>Small</i>, 12, 10, 1334<br/>[4] Di Martino, G. et al, (2020) Nature Electronics, 3, 687<br/>[5] Baumberg, J.J. et al, (2019) <i>Nat. Mater.</i> 18, 668.