Interface-Dependent Transport and Resistive Switching in TiO₂ and TiO₂:Co Nanotubes for Resistive Memories

In this work, TiO₂ and TiO₂:Co nanotubes were fabricated via electrochemical anodization, using Ti (99.99% purity) and Ti/Co foils as the anode and cathode. Cobalt was deposited onto the Ti foils using the DC magnetron sputtering technique under a working pressure of 2.5 × 10^-2 Torr. The resulting nanotubes exhibited wall nodes and an average length of 308.6 ± 13.07 nm for TiO₂ and 92.63 ± 2.846 nm for TiO₂:Co. The synthesized structures were characterized by X-ray diffraction (XRD), identifying anatase as the predominant phase, accompanied by an amorphous halo. Two types of MSM devices were fabricated to study bulk and surface conduction: a transverse configuration (TE/(TiO₂, TiO₂:Co)/Ti), with top electrodes (TE) of Al or Au, and a coplanar configuration (Al/TiO₂/Al). Surface topography and surface-potential variations were investigated using Atomic Force Microscopy (AFM) and Kelvin Probe Force Microscopy (KPFM), respectively. The device behavior is mainly governed by the TE/TiO₂ junction due to the absence of an energy barrier at the TiO₂/Ti interface. All samples exhibit asymmetric I-V behavior with higher conduction under positive bias. The Au/TiO₂/Ti device showed the lowest resistance among the Ti BE structures, displaying Schottky behavior with low reverse current leakage and a shift in the zero-current crossing depending on the scan direction. Barrier heights near the zero-current crossing, calculated via the thermionic emission model, were 0.91 eV and 0.70 eV for the reverse and forward directions, respectively. Ideality factors and series resistance exceeded 6.8 and 40 kΩ, respectively, suggesting additional transport mechanisms. In addition, magnetization as a function of the applied field was generated in the TiO₂:Co nanotubes, evidencing their ferromagnetic-like behavior.