Oxygen migration process in the interfaces during bipolar resistance
switching behavior of WO3-x-based nanoionics devices
Rui Yang, Kazuya Terabe, Tohru Tsuruoka, Tsuyoshi Hasegawa and Masakazu Aono
Supplementary Materials
1. The positive and negative forming process applied on the Pt, Au/WO3-x/Pt device
Fig. S1 shows the positive and negative forming process applied on the Pt/WO3-x/Pt
device in air. The abrupt current increase appearing in the forming process corresponds to the
formation of a conducting filament composed of a VO‥-enriched phase of WO3-x-δ.1 During
the positive forming process, the conducting filament forms from the bottom interface and
grows toward the top interface due to electroreduction of the WO3-x.2 Concurrently, oxygen
ions migrate from the WO3-x interior to the top electrode and partially erupt as gas bubbles, as
described elsewhere.3 However, the conducting filament does not span the entire whole
WO3-x
FIG. S1 Positive (a) and negative (b) forming process applied on the Pt/WO3-x/Pt device.
1
layer due to the non-linear I–V curves in the LRS and HRS, as shown in Fig. 1a in the main
text. Thus, the subsequent biplor resistance switching (BRS) behavior mainly occurs in the
top interface. When the negative forming process is applied, the filament forms from the top
interface and grows toward the bottom interface. Then, the BRS behavior mainly occurs in
the bottom interface through modulation of the residual Schottky-like barrier.The similar
positive and negative forming process is applied on the Au/ WO3-x/Pt device.
2. Structure of Pt and Au top electode
Fig. S2 shows the x-ray diffraction (XRD) patterns of a Pt and Au electrode with a
thickness of 100 nm deposited on WO3-x thin film at room temperature. Both of them showed
high (111) orientation. The cross-section morphology of the Pt/WO3-x interface was examined
by scanning electron microscopy (SEM). As shown by the inset in Fig. S2, the Pt electrode
was dense.
FIG. S2 XRD pattern of Pt and Au top electrode with thickness of 100 nm. Inset shows
cross-section SEM image of Pt/WO3-x interface cross-section.
2
3. The I-V curves and retention characteristics observed for the Pt/WO3-x/Pt devices
with a Ta2O5 cover layer
To eliminate the effect of the atmosphere on BRS behavior occurred at the top Pt/WO3-x
interface, a dense cover layer of Ta2O5 with a thickness of 90 nm was deposited on the top Pt
electrode. Stable BRS was observed not only in air but also in vacuum and N2 gas after a
positive forming process, as shown in Fig. S3a. The resistances states observed in air,
vacuum, and N2 gas had good retention, see Fig. S3b. That is because the cover layer blocks
the evacuation of oxygen along Pt grain boundaries in vacuum or N2 gas. These results
validate the reasonableness of our scenario that the oxygen migration mainly occurred
between Pt/WO3-x interface and Pt electrode.
FIG. S3 Switching I–V curves (a), and retention characteristics (b) for Ta2O5/Pt/WO3-x/Pt
device after application of positive forming process in air, vacuum, and N2 gas. Read voltage
was 0.1 V.
Note that the resistance states observed in vacuum were much lower than those observed
in air and N2 gas. This may be because some oxygen absorbed in the top Pt electrode diffused
3
‥
into the cover layer due to pressure stress caused by the vacuum. The VO concentration in
the Pt/WO3-x interface in vacuum was thus higher than that in air and N2 gas, and the
resistance values in vacuum were lower than those in air and N2 gas.
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