Tuning the Transport Properties of Artificial Synapses: A DFT-Supported Experimental Approach

Defect engineering is an essential aspect of resistive switching (RS) memories. This work illustrates cationic dopant (Ni) controlled RS characteristics in the thin film of anatase titanium dioxide (A-TiO₂). A significant increase in ON/OFF ratio to 10³ with a gradual change in resistance state is accomplished and such RS characteristics are highly expedient for neuromorphic computing applications. Detailed investigations by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) as well as density functional theory (DFT)-based first-principles calculations establish the role of Ni in memristive operation. In particular, TEM results show an evolution of two different Ni-concentration regions within the films that help to reduce the host Ti-site from its +4 valence state, as obtained from Ti-2p XPS analysis. This claim is further verified by DFT calculations, showing a change in the charge density near the Ni-site form the Bader charge analysis and electron localisation function (ELF) plots. Moreover, the role of those two Ni-concentration regions in regulating transport properties of the Au/ Ni:A-TiO₂ /Pt devices similar to the biological synapse is discussed.