conductance, evaluated after the application of each input pulse, showed that the
conductance state has peaks at integer multiples of G 0 with small distributions
around each integer. This result indicates that an atomic point contact with different
integer multiples can be formed in a thin Ta 2 O 5 film. Because a limited number of
Ag
+ ions is expected to participate in the formation and dissolution of the atomic
contact, we believe that it is appropriate to definition this oxide-based MIM structure
as a “gapless-type atomic switch”.
The conductance state of the Ag/Ta 2 O 5 /Pt device was found to drastically change
depending on the interval of the input pulses. Figure 12a represents the device
conductance variation under the application of ten input pulses of V ¼ 0.4 V with
W ¼ 20 ms and T ¼ 2 s. The device exhibited an enhanced increase in conductance
up to ~2G 0 with the application of each input pulse. However, the conductance
immediately decayed after the input pulses. Each voltage pulse induces the formation of a metal filament, but because of the small input voltage, the filament is
unstable and dissolves immediately after the pulse, and the Ag ions diffuse away
from the filament position in an ion concentration gradient, as illustrated in the inset
of Fig. 12a. If T was decreased to 0.2 s (Fig. 12b), the device initially exhibited a
temporary increase for the first few inputs, similar to Fig. 12a. However, with
increased input pulses the conductance gradually increased, with accompanying
temporary increases, to finally reach ~1G 0 . This conductance state was maintained
for more than 1 min. after the tenth input pulse. The long-lived high conductance
state for a shorter time interval can be explained by the formation of a larger metal
filament before the Ag ions diffuse completely in a succession of input pulses,
resulting in the formation of a stable atomic contact, as illustrated in the inset of
Fig. 12b.
The observed conductance behavior for different interval times of input pulses is
very similar to that of gap-type Ag 2 S and Cu 2 S atomic switches [2, 3], and is
analogous to the LTP behavior of biological synapses, in which the strength of
synaptic weight (or conductance) depends upon the time interval between the
stimulating pulses [35, 36]. Our experimental observations indicate that the migration and reduction of metal ions in a thin oxide layer could mimic biological synaptic
behavior.
It was also observed that higher input pulse amplitudes result in higher integer
multiples of the quantized conductance state. Figure 12c–e show the conductance
behavior under the application of ten consecutive pulses with different amplitudes.
The conductance reached ~2G 0 , ~3G 0 , and ~4G 0 for pulses with V ¼ 0.45, and 0.5,
and 0.55 V, respectively. It can be seen that higher input pulse amplitudes result in
higher integer multiples of the quantized conductance state. This result indicates that
the atomic switch possesses a unique property in which the conductance strength
also depends upon the strength of the stimulating pulses, which is similar to
biological synapses. These synaptic behaviors of the Ta 2 O 5 -based atomic switch
are difficult to achieve by on/off bi-stable devices based on conventional CMOS
transistors.
Artificial Synapses Realized by Atomic Switch Technology
189
conductance state has peaks at integer multiples of G 0 with small distributions
around each integer. This result indicates that an atomic point contact with different
integer multiples can be formed in a thin Ta 2 O 5 film. Because a limited number of
Ag
+ ions is expected to participate in the formation and dissolution of the atomic
contact, we believe that it is appropriate to definition this oxide-based MIM structure
as a “gapless-type atomic switch”.
The conductance state of the Ag/Ta 2 O 5 /Pt device was found to drastically change
depending on the interval of the input pulses. Figure 12a represents the device
conductance variation under the application of ten input pulses of V ¼ 0.4 V with
W ¼ 20 ms and T ¼ 2 s. The device exhibited an enhanced increase in conductance
up to ~2G 0 with the application of each input pulse. However, the conductance
immediately decayed after the input pulses. Each voltage pulse induces the formation of a metal filament, but because of the small input voltage, the filament is
unstable and dissolves immediately after the pulse, and the Ag ions diffuse away
from the filament position in an ion concentration gradient, as illustrated in the inset
of Fig. 12a. If T was decreased to 0.2 s (Fig. 12b), the device initially exhibited a
temporary increase for the first few inputs, similar to Fig. 12a. However, with
increased input pulses the conductance gradually increased, with accompanying
temporary increases, to finally reach ~1G 0 . This conductance state was maintained
for more than 1 min. after the tenth input pulse. The long-lived high conductance
state for a shorter time interval can be explained by the formation of a larger metal
filament before the Ag ions diffuse completely in a succession of input pulses,
resulting in the formation of a stable atomic contact, as illustrated in the inset of
Fig. 12b.
The observed conductance behavior for different interval times of input pulses is
very similar to that of gap-type Ag 2 S and Cu 2 S atomic switches [2, 3], and is
analogous to the LTP behavior of biological synapses, in which the strength of
synaptic weight (or conductance) depends upon the time interval between the
stimulating pulses [35, 36]. Our experimental observations indicate that the migration and reduction of metal ions in a thin oxide layer could mimic biological synaptic
behavior.
It was also observed that higher input pulse amplitudes result in higher integer
multiples of the quantized conductance state. Figure 12c–e show the conductance
behavior under the application of ten consecutive pulses with different amplitudes.
The conductance reached ~2G 0 , ~3G 0 , and ~4G 0 for pulses with V ¼ 0.45, and 0.5,
and 0.55 V, respectively. It can be seen that higher input pulse amplitudes result in
higher integer multiples of the quantized conductance state. This result indicates that
the atomic switch possesses a unique property in which the conductance strength
also depends upon the strength of the stimulating pulses, which is similar to
biological synapses. These synaptic behaviors of the Ta 2 O 5 -based atomic switch
are difficult to achieve by on/off bi-stable devices based on conventional CMOS
transistors.
Artificial Synapses Realized by Atomic Switch Technology
189