The Observation of “Conduction Spot” on NiO Resistance Random Access Memory

Abstract

We succeeded to observe the "conduction spot" (CS) in the capacitor structure ReRAM, which includes a conductive filament. In this study, we used NiO prepared by thermal oxidation at high temperature as 800°C. It requires a forming process by extra high voltage, which partly removes the top electrode from the resistance switched area.These experiments enabled us to observe the conductive filament directly in CS on NiO ReRAM by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). From SEM images, CSs seem to be produced by some kind of breakdown, but we confirmed the reproducible resistance switching at least 50 cycles after the CS generation. By means of energy dispersive X-ray spectroscopy (EDX) with TEM observations, drastic oxygen reduction was recognized in local area within CS of NiO films. Moreover, the CS area depended on the injection power for forming. These experimental data suggest that miniaturization of ReRAM will be achieved by reducing the injection power for forming. *

In-situ transmission electron microscopy of conductive filaments in NiO resistance random access memory and its analysis

“…This oxidization condition may provide sufficient resistance switching properties as proved in Pt/NiO/Pt patterned devices (100-nm-thick and oxidized at 500-800 C) on SiO 2 /Si wafers. 16,24 The in-situ TEM observation system was composed of a custom-made TEM holder attached with a piezo actuator. [26][27][28][29] The TEM instrument we used was mainly a JEM-2010 microscope (200 kV, C s ¼ 0.5 mm, 10 À5 Pa) with a CCD video camera to record the TEM images.…”

“…This phenomenon corresponds to the result from flat Pt/NiO/Pt devices prepared using thermal oxidation. 24 The degree of oxidation is expected to be with the increase in temperature. Therefore, the averaged forming voltage increased with the oxidation temperature.…”

“…Thus, controlling the forming power and the resulting bridge size are important factors for ReRAM switching. Kondo et al 24 observed crater-shaped "conduction spots" (CS) during the forming process at the edges of which switchable filaments existed. They reported that the CS area was almost proportional to the injection power (they used the maximum value just before a resistance change).…”

“…2(b). In this figure, the CS data by Kondo et al 24 are superposed as open circles. The data points of the bridge sizes are on the extrapolation line of the CS size.…”

“…[22][23][24][25] They were mainly ex-situ experiments after the resistive switching, and exploration of the tiny filaments seems hard because the filament formation occurs randomly in the device area. 24 More direct information about the filaments can be accumulated by dynamically observing the switching processes. One method is in-situ transmission electron microscopy (TEM), where electric measurements and geometric and crystallographic observations can be simultaneously performed.…”