observed in biological systems, in which the transition from STP to LTP can be
achieved by repeatedly stimulating a synapse using a strong potential with high
frequency [5].
The LTP in a biological system can remain for long periods of time (several
minutes, days, or even years), but it still exponentially fades over time, due to the
forgetting effect, at a much slower pace than does STP. This forgetting effect can
also be emulated by our device, as shown in Fig. 15c. The current of the device
gradually increases by successively applying voltage pulses of V ¼ 2.3 V with
W ¼ 10 μs and T ¼ 100 s. After these stimuli, the current of the device decays over
time, but does not return to its initial state even after ~4000 s, clearly indicating the
nonvolatile characteristics of the switching behavior. The current decay processes
can be fitted by an exponential function, as shown in the inset of Fig. 15c. Moreover,
the decay time constant increases as the successive current decay process proceeds.
These current decay features are strikingly similar to the biological forgetting effect,
which endows our device with capabilities that make it an ideal synaptic emulator.
The mechanisms of volatile and nonvolatile resistive switching can be explained
as follows. A certain amount of V O
‥ is induced during the device fabrication process,
as has been verified by X-ray photoelectron spectroscopy (XPS) [37]. For devices
without forming processes, a Schottky-like barrier forms at both the top and bottom
Pt/WO 3 – x interfaces, see Fig. 13c. When a positive pulse is applied to the top
interface, the external electric field drops mainly at the bottom interface and induces
V O
‥ migration and accumulation at the bottom interface. This results in the lower
portion of the Schottky-barrier to be located at the bottom interface, whereas the
Schottky-like barrier at the top interface increase slightly, or at least remains
unchanged. Thus, forward rectification appears. After the electric stimuli, the subsequent relaxing of the V O
‥ driven by the electrochemical potential gradient results
in the volatile characteristics of this rectification and the subsequent resistive
switching behaviors. This oxygen migration process has been verified by hard
XPS under bias operation measurements [37].
An electroforming process with soft-breakdown is required to trigger the nonvolatile rectification and subsequent resistive switching behaviors. During the
forming process, conducting filaments with high oxygen deficient WO 3 – x – δ is
expected to form in the WO 3 – x matrix. From combined optical microscopy and
cross-sectional TEM observations, we found that the filament is composed of
conductive Magnéli phases, with more oxygen vacancies than the mother phases
[39]. The observed switching originates from a nano-gap between the top electrode
and this conductive filament. On the other hand, the electroforming process
exhibited high temperature dependence, suggesting that the Joule heating effect
plays a key role in the electroforming process [40]. It was also found that the oxygen
migration process during resistive switching occurs mainly between the Pt/WO 3 – x
interface and Pt electrode. The high catalytic activity and oxygen absorbability of Pt
electrode are indispensable for realizing stable bipolar resistive switching behavior
in a Pt/WO 3 – x interface. The partial current decay process is attributed to the
spontaneous back diffusion of oxygen vacancies after switching.
Artificial Synapses Realized by Atomic Switch Technology
195
achieved by repeatedly stimulating a synapse using a strong potential with high
frequency [5].
The LTP in a biological system can remain for long periods of time (several
minutes, days, or even years), but it still exponentially fades over time, due to the
forgetting effect, at a much slower pace than does STP. This forgetting effect can
also be emulated by our device, as shown in Fig. 15c. The current of the device
gradually increases by successively applying voltage pulses of V ¼ 2.3 V with
W ¼ 10 μs and T ¼ 100 s. After these stimuli, the current of the device decays over
time, but does not return to its initial state even after ~4000 s, clearly indicating the
nonvolatile characteristics of the switching behavior. The current decay processes
can be fitted by an exponential function, as shown in the inset of Fig. 15c. Moreover,
the decay time constant increases as the successive current decay process proceeds.
These current decay features are strikingly similar to the biological forgetting effect,
which endows our device with capabilities that make it an ideal synaptic emulator.
The mechanisms of volatile and nonvolatile resistive switching can be explained
as follows. A certain amount of V O
‥ is induced during the device fabrication process,
as has been verified by X-ray photoelectron spectroscopy (XPS) [37]. For devices
without forming processes, a Schottky-like barrier forms at both the top and bottom
Pt/WO 3 – x interfaces, see Fig. 13c. When a positive pulse is applied to the top
interface, the external electric field drops mainly at the bottom interface and induces
V O
‥ migration and accumulation at the bottom interface. This results in the lower
portion of the Schottky-barrier to be located at the bottom interface, whereas the
Schottky-like barrier at the top interface increase slightly, or at least remains
unchanged. Thus, forward rectification appears. After the electric stimuli, the subsequent relaxing of the V O
‥ driven by the electrochemical potential gradient results
in the volatile characteristics of this rectification and the subsequent resistive
switching behaviors. This oxygen migration process has been verified by hard
XPS under bias operation measurements [37].
An electroforming process with soft-breakdown is required to trigger the nonvolatile rectification and subsequent resistive switching behaviors. During the
forming process, conducting filaments with high oxygen deficient WO 3 – x – δ is
expected to form in the WO 3 – x matrix. From combined optical microscopy and
cross-sectional TEM observations, we found that the filament is composed of
conductive Magnéli phases, with more oxygen vacancies than the mother phases
[39]. The observed switching originates from a nano-gap between the top electrode
and this conductive filament. On the other hand, the electroforming process
exhibited high temperature dependence, suggesting that the Joule heating effect
plays a key role in the electroforming process [40]. It was also found that the oxygen
migration process during resistive switching occurs mainly between the Pt/WO 3 – x
interface and Pt electrode. The high catalytic activity and oxygen absorbability of Pt
electrode are indispensable for realizing stable bipolar resistive switching behavior
in a Pt/WO 3 – x interface. The partial current decay process is attributed to the
spontaneous back diffusion of oxygen vacancies after switching.
Artificial Synapses Realized by Atomic Switch Technology
195