Lateral memristive structures based on TiO_2-x with an inert Pt and an active Ag electrode are investigated regarding their electrical DC and impedance behaviour. The TiO_2-x film is deposited with reactive pulsed DC magnetron sputtering onto p-type (100) Si with a 50 nm SiO₂ electrically insulating diffusion barrier. The electrodes on top are deposited by electron beam evaporation and structured with photo lithography and a lift-off process.
Electrical impedance spectroscopy (EIS) is conducted to reveal the frequency dependence of capacity and resistance of the device. In a subset of samples, the appearance of a so called “inductive hook“ was observed, which could indicate ion drift inside the dielectric memristive layer. That AC behaviour is compared between dark conditions and with illumination (385 nm) to reveal potential effects of additional charge carrier generation in the TiO_2-x layer and in the substrate. Additionally, varied substrates (fused silica instead of Si, thicker SiO₂ barrier of 90 nm) were used to evaluate the influence of a conductive substrate and capacitive coupling between the electrodes and substrate. Further, TiO_2-x films with different stoichiometry, as determined by X-ray photoelectron spectroscopy (XPS), to vary the concentration of O-vacancies, and consequently the electric conductivity and ion mobility, were investigated.
In an approach to model the EIS data, first it is attempted to separate the contribution of electrode and contact pad geometry and their interaction with the substrate to then describe the behaviour of the actual Pt/TiO_2-x/Ag memristive device.
DC measurements are carried out additionally to verify the time dependent (voltage ramp rate) and hysteretic behaviour and to extract memristive characteristics.
Three general types of conduction occur: i) percolation paths of Ag particles on the surface in the gap remaining from the lithography processes with short retention times and random resistance changes; ii) discharge by “micro arcing“ leading to damage of electrodes, and iii) actual memristive behaviour with reproducible loops and a low frequency “inductive hook“.