Resistive switching (RS) phenomenon of TiO2-based resistive random access memory (ReRAM) can be explained with the formation and rupture of a conductive filament (CF) in the TiO2 layer. However, the origin of a CF is still unknown and switching mechanism has not been fully clarified yet. In this work, a transition to a resistance state different from low resistance state (LRS) or high resistance state (HRS) at specific forming processes in Pt/TiO2/Pt stack structures was discovered.
TiO2 layers were deposited on Pt(70 nm)/Ti(5 nm)/SiO2/Si substrates by a reactive radio-frequency sputtering method. The thickness of the TiO2 layer was varied from 10 nm to 40 nm. Pt top electrodes with a 100 µm diameter were deposited by electron beam evaporation on the TiO2 layer. The transitions at forming processes were investigated by DC voltage or current sweeping to the cells.
Cells with TiO2 thickness of 40 nm showed a transition to LRS (less than 100 Ω) by voltage sweeping under 10 mA compliance current, followed by typical RS operation. On the other hand, the cells exhibited a transition to resistance state with large resistance (about 1 kΩ to 1 MΩ) compared to LRS by either current sweeping or voltage sweeping under 0.1 mA or 1 mA compliance current, followed by the transition to LRS by voltage reapplication and subsequent RS performance. This characteristic resistance state with a large resistance compared to LRS is designated as “semi-HRS: SHRS”. We clearly observed that the thicker TiO2 thickness became, the more frequently SHRS tended to appear after current sweeping.
As another feature of SHRS, the cell resistance in SHRS after current sweeping gradually increased as the applied voltage increased, which is different from conventional HRS before resistance state changes to LRS. To clarify the main driving force of the resistance increase, the maximum (stop) voltage to the cells of positive or negative voltage sweeping increased. The cell resistance was confirmed to increase after each voltage sweeping by a certain voltage in both (positive and negative) cases. This result indicates that the resistance increase in SHRS originates from current flow through the cells instead of the polarity of electric field applied to the cells. In other words, Joule heat caused by the current flow is heavily involved in the state change of a CF in a TiO2 layer.
Here, a CF is considered to be a row of oxygen vacancies (Vo). Resistance increase were observed owing to possible reduction in a CF width because Joule heat enhanced concentration diffusion of Vo to the horizontal direction. Furthermore, inhomogeneous distribution of Vo density around a CF in a TiO2 layer along the thickness direction may cause the difference of SHRS appearance after the forming processes to suppress overcurrent.