3 Synaptic Behavior of the Gapless-Type Atomic Switch
As described in the previous section, the gap-type atomic switch clearly exhibits
synaptic plasticity, depending on the input pulse conditions. For practical applications, it is important to realize this property in MIM structures with thin oxide films,
because of their high compatibility with current CMOS device fabrication processes.
In this section, we describe the results for two types of MIM structures that show
synaptic plasticity, based on the transport of metal ions and oxygen ions, respectively, in a thin oxide film.
3.1 Ag/Ta 2 O 5 -Based Switch
If a thin oxide film is sandwiched between an electrochemically active metal
electrode (for example, Cu or Ag) and an inert metal electrode (such as Pt and
Au), such a structure exhibits bi-stable resistive switching under bias voltage
sweeping, in a manner similar to the gap-type atomic switch. The observed resistive
switching is attributed to the formation of a metal filament between the two electrodes due to inhomogeneous nucleation on the inert electrode, and dissolution of the
metal filament formed assisted by thermochemical reactions [21–23]. From the
similarity of its operating mechanism to that of a “gap-type atomic switch”, this
MIM structure, with an electrochemically active metal electrode, can be referred to
as a “gapless-type atomic switch” [24]. We have investigated resistive switching and
novel functions of gapless-type, oxide-based atomic switches using Ta 2 O 5 as a
model matrix, and have revealed the redox reactions at the metal/oxide interfaces
[25, 26], the effect of moisture absorption by the oxide matrix [27–29], fast-speed
switching dynamics [30], quantized conductance, and synaptic behaviors. Ta 2 O 5
films are usually prepared by RF sputtering or EB deposition. The composition and
film density of the oxide matrix was found to affect the switching characteristics,
which depend strongly on the ambient humidity conditions [28, 31].
Ta 2 O 5 -based atomic switches show learning abilities that are dependent on the
bias conditions, such as sweep cycles and sweep rates. Figure 11a, b represent I-V
curves of a Ag/Ta 2 O 5 /Pt device with a Ta 2 O 5 thickness of 15 nm, measured with
increased sweep cycles and different sweep rates, respectively [32]. No current
compliance was applied in these measurements. The device exhibits bipolar
switching with small turn-off voltages, which is a typical characteristic of
Ag-Ta 2 O 5 -based atomic switches [23]. With increasing sweep cycles, the on current
increased gradually, as seen in Fig. 11a. This is because the diameter of the formed
metal filament becomes larger as the voltage sweep was repeated, due to the
increased number of Ag ions in the Ta 2 O 5 matrix. On the other hand, as the sweep
rate was decreased, the on current increased while the turn-on voltage slightly
decreased (Fig. 11b). This behavior can be explained by a higher concentration of
Ag ions in the vicinity of the Pt electrode at lower sweep rates. Once inhomogeneous
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187
As described in the previous section, the gap-type atomic switch clearly exhibits
synaptic plasticity, depending on the input pulse conditions. For practical applications, it is important to realize this property in MIM structures with thin oxide films,
because of their high compatibility with current CMOS device fabrication processes.
In this section, we describe the results for two types of MIM structures that show
synaptic plasticity, based on the transport of metal ions and oxygen ions, respectively, in a thin oxide film.
3.1 Ag/Ta 2 O 5 -Based Switch
If a thin oxide film is sandwiched between an electrochemically active metal
electrode (for example, Cu or Ag) and an inert metal electrode (such as Pt and
Au), such a structure exhibits bi-stable resistive switching under bias voltage
sweeping, in a manner similar to the gap-type atomic switch. The observed resistive
switching is attributed to the formation of a metal filament between the two electrodes due to inhomogeneous nucleation on the inert electrode, and dissolution of the
metal filament formed assisted by thermochemical reactions [21–23]. From the
similarity of its operating mechanism to that of a “gap-type atomic switch”, this
MIM structure, with an electrochemically active metal electrode, can be referred to
as a “gapless-type atomic switch” [24]. We have investigated resistive switching and
novel functions of gapless-type, oxide-based atomic switches using Ta 2 O 5 as a
model matrix, and have revealed the redox reactions at the metal/oxide interfaces
[25, 26], the effect of moisture absorption by the oxide matrix [27–29], fast-speed
switching dynamics [30], quantized conductance, and synaptic behaviors. Ta 2 O 5
films are usually prepared by RF sputtering or EB deposition. The composition and
film density of the oxide matrix was found to affect the switching characteristics,
which depend strongly on the ambient humidity conditions [28, 31].
Ta 2 O 5 -based atomic switches show learning abilities that are dependent on the
bias conditions, such as sweep cycles and sweep rates. Figure 11a, b represent I-V
curves of a Ag/Ta 2 O 5 /Pt device with a Ta 2 O 5 thickness of 15 nm, measured with
increased sweep cycles and different sweep rates, respectively [32]. No current
compliance was applied in these measurements. The device exhibits bipolar
switching with small turn-off voltages, which is a typical characteristic of
Ag-Ta 2 O 5 -based atomic switches [23]. With increasing sweep cycles, the on current
increased gradually, as seen in Fig. 11a. This is because the diameter of the formed
metal filament becomes larger as the voltage sweep was repeated, due to the
increased number of Ag ions in the Ta 2 O 5 matrix. On the other hand, as the sweep
rate was decreased, the on current increased while the turn-on voltage slightly
decreased (Fig. 11b). This behavior can be explained by a higher concentration of
Ag ions in the vicinity of the Pt electrode at lower sweep rates. Once inhomogeneous
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187