Resistive switching (RS) plays an important role of Resistive Random Access Memory (ReRAM). The formation and rupture of conductive filaments have been widely accepted as an origin of RS mechanism especially in binary Transition Metal Oxides (TMOs). “Forming” by applying electrical stress to a pristine cell is an initial process generally required to create conductive filaments in TMO layers between top and bottom metal electrodes. The forming exhibits some analogies with dielectric breakdown of thin SiO2 films . In this study, we examine Time-Dependent Forming (TDF) characteristics in Pt/NiO/Pt RS cells.
Two kinds of Pt bottom electrode (BE) on a SiO2/p-Si substrate were prepared, deposited either by Electron Beam (EB) evaporation or RF Sputtering (SP). A sample with the Pt/NiO/Pt cells using Pt BE deposited by EB or SP is referred to as EB-Pt samples or SP-Pt samples, respectively. A NiO film as a resistance change layer was deposited on the BE by rf reactive sputtering. Pt top electrodes were deposited on the NiO layer by EB evaporation. The time to forming (tform) in the cells was measured while keeping a constant applied voltage. Cell resistance remains almost unchanged before forming. After forming, all of the cells in both samples were confirmed to show repeatable RS characteristics.
Cross-sectional Transmission Electron Microscopy (TEM) uncovers that Pt BE and NiO layers in EB-Pt samples contain granules, and that those in SP-Pt samples exhibits a columnar polycrystalline structure with a grain diameter of tens of nm. Moreover, the layers in EB-Pt samples exhibit diffraction peaks by both (111) and (200) planes and columns of the layers in SP-Pt samples are preferentially oriented to the  direction as confirmed by X-ray diffraction curves. NiO crystallinity is turned out to strongly depend on the BE crystallinity .
TDF measurements reveal that the slope of Weibull distribution (Weibit) of tform is clearly different between EB-Pt samples (1.5) and SP-Pt samples (0.9). These values are independent of NiO thickness, applied voltage, initial resistance, and surface roughness of cells. Furthermore, the Weibits normalized by cell sizes according to the area scaling law overlay each other. These results indicate that formation of the filaments at forming follows a weakest-link theory, and that weakest spots are almost randomly distributed in a NiO film according to Poisson statistics, each of which can contribute conductive paths locally generated by an electrical stress.
We also confirm inverse correlation between variation of initial resistance and that of time to forming. The different values of the slope of Weibits between EB-Pt and SP-Pt samples are considered to originate from a difference of NiO crystallinity. These results suggest that distribution of grain boundaries is the key to form the conductive filaments in NiO by the forming.
 Y. Nishi, et al, J. Appl. Phys. 120, 115308 (2016).