Experimental and computational evidence of defect centres in amorphous titanium dioxide and their confluence in resistive switching*

This work identifies dominant defect centres in amorphous titanium dioxide (a-TiO₂) films and their role in resistive random access memories with the help of combined experimental and theoretical studies. X-ray absorption spectroscopy reveals a significant decrease in peak splitting between t_2g and e_g sub-bands at the Ti-L edge, which is attributed to lower valence of Ti-ions. Deconvolution of the Ti-2p X-ray photoelectron spectrum also suggests the existence of Ti³⁺ state along with oxygen vacancies in a-TiO₂. Further, the role of oxygen vacancies as intrinsic electron trapping centres in such system is recognised by low-temperature electron paramagnetic resonance measurements. The same has been verified using ab-initio density functional theory based simulations for a-TiO₂ phase, generated using ab-initio molecular dynamics simulations. The partial charge density and Bader charge analysis were done, in particular, to manifest the localized character of the excess electrons ejected during the creation of oxygen vacancies. Such electronic localisations facilitate local high conducting regions in the amorphous matrix, which in turn leads to forming free resistive switching behaviour in Pt/a-TiO₂/Pt devices.