Formation of a Ti -> TiO2-graded layer and its effect on the memristive properties of TiOx(/Ti/TiOx) structures

In this study DC reactive HiPIMS-sputtered TiOx/Ti/TiOx and TiOx-based memristors were fabricated. Metallic Ti layer was created on TiO2 next to the bottom electrode, expected to create a soft transition from metallic state to TiO2 as oxygen purged into the chamber. Each structure roughness remained well below nm. The band gap of thin-pure TiO2 was found as 3.52 eV and 3.80 eV for TiO2 with intermetallic layer from absorption measurements. Raman analysis indicates rutile and anatase phases. Ti6O11, TiO2, TiO2 (rutile), TiO2 (anatase), and Ti3O5 phases from each sample are identified from XRD measurements. The memristive characteristics of both the devices were determined by time-dependent current-voltage (I-V-t) measurements and current transport mechanisms were investigated in terms of pinched hysteresis of I-V-t loops. The devices with Ti -> TiO2 transition layer exhibited much larger hysteresis area in comparison pure TiO2-based devices. All of the devices exhibited Fowler-Nordheim tunneling, Schottky emission, Space-Charge-Limited Conduction, and Poole-Frenkel emission, which depended on the voltage scanning direction and order of loops.