Nanoscale Redox Reaction at MetalOxide Interface by Takahiro Nagata

Author:Takahiro Nagata
Language: eng
Format: epub
ISBN: 9784431548508
Publisher: Springer Japan

4.2 Nanoionic-Type ReRAM Structure

4.2.1 Sample Structure

Figure 4.1a shows the structure of a sample device used in our investigation. Insulating c-plane sapphire (Al2O3) was used as a substrate to eliminate additional factors responsible for electrical conductivity. A HfO2 layer was deposited by pulsed laser deposition with a KrF excimer laser (λ = 248 nm) on a 100-nm-thick Pt bottom electrode deposited on the substrate by DC sputtering. During the deposition, the substrate temperature was maintained at 300 °C under an O2 partial pressure of 1 × 10−3 Torr. The crystal structure of the HfO2 thin film was identified by XRD. Figure 4.1b shows the XRD pattern of a HfO2 thin film deposited on Pt/Al2O3. The diffraction peaks of the HfO2 thin film exhibit a ring pattern derived from the polycrystalline monoclinic structure. The strong intensity of the (111) peak means that the HfO2 grains are preferentially oriented in the [111] direction. The surface morphology of the HfO2 film was characterized by AFM, as shown in Fig. 4.1c. The obtained root-mean-square (RMS) roughness was 0.6 nm. The surface morphology also indicated that the HfO2 thin film consisted of fine 20 nm grains. For the HX-PES measurement under bias application, gold wires fixed to the top and bottom electrodes with Pt paste were connected to electrodes on the sample stage. The thickness of the top and bottom electrodes (Cu or Pt) was 10 nm. A bias voltage was applied to the sample with a DC voltage current source/monitor. HX-PES measurements were performed at take-off angles (TOAs) of 85°, 40°, and 20° to obtain rough depth profiles; the TOA of 20° was surface/interface-sensitive, whereas the TOA of 85° was bulk-sensitive.

Fig. 4.1a Schematic illustration of measurement setup for HX-PES under bias application. b 2D-XRD and c AFM images of HfO2/Pt/Al2O3 structure

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