3.2 Pt/WO 3 – x -Based Switch
As mentioned in the preceding subsections, versatile synaptic neuromorphic functions have been achieved in various atomic switch devices by precisely controlling
the local metal-ion migration and redox reactions on the nano- and atomic-scales. In
these atomic switch systems, an electrochemically active electrode, made from
material such as Ag or Cu, is used in a device. Resistive switching in the device
occurs through the formation and annihilation of a metallic atom bridge resulting
from the migration of highly mobile cations such as Ag
+ and Cu
+ ions. Actually,
resistive switching behavior can also be observed in a device without an electrochemically active electrode. In such cases, the migration of anions, usually oxygen
ions play a crucial role. Local oxygen ion migration, which is better described as the
migration of an oxygen vacancy (V O
‥ ) and a resultant change in the electronic
barrier at the interface, is associated with the resistive switching behavior. Similar
synaptic functions, described in the preceding subsections, are also demonstrated in
these anion migration devices. In this subsection, we will introduce neuromorphic
and electrical multifunction, realized in anion-migration based on a WO 3 – x
layer [37].
The devices have an MIM structure of Pt/WO 3 – x /Pt fabricated on a glass or SiO 2 /
Si substrate. The thin WO 3 – x film was prepared by RF sputtering from a polycrystalline WO 3 target in a gas mixture containing Ar and O 2 . Details of the device
fabrication and characterization methods are presented in Ref. [37]. Both volatile
and nonvolatile reconfigurable rectification and resistive switching were realized by
adjusting the electric stimuli input parameters, as shown in Fig. 13. The resistance of
the pristine device decreased nonlinearly after voltage seeping in both the positive
and negative voltage regions (Fig. 13a). However, the current boost induced by
voltage sweeping quickly fades over time, which means this resistive switching is
volatile. In contrast, after a forming process (Fig. 13c), the device exhibited normal
bipolar resistive switching, in which the resistance of the device decreased/increased
by applying positive/negative sweeping, as shown in Fig. 13b. In this case, the state
with relatively high conductance, obtained after positive voltage sweeping, automatically faded over time but did not return to the original state even over the
measurement period, which indicates nonvolatile, or at least partially nonvolatile
resistive switching. The positive (negative) forming process is triggered by applying
positive (negative) voltage sweeping or voltage pulses on the top Pt electrode to
induce a soft-breakdown, with a current compliance to prevent the device from hardbreakdown, see Fig. 13c. Note that the device possesses stable resistive switching
only in an oxygen-rich atmosphere (air or O 2 gas) after the positive electroforming
process [38].
The I-V curves of both volatile and nonvolatile resistive switching in Fig. 13 are
smooth, without abrupt change, indicating that the device performed analog resistive
switching. As shown in the bottom-right panel of Fig. 13a, the volatile resistance of
the device is gradually changed by applying consecutive voltage sweeping. The
nonvolatile resistance states can also be precisely modified by controlling the
Artificial Synapses Realized by Atomic Switch Technology
191
As mentioned in the preceding subsections, versatile synaptic neuromorphic functions have been achieved in various atomic switch devices by precisely controlling
the local metal-ion migration and redox reactions on the nano- and atomic-scales. In
these atomic switch systems, an electrochemically active electrode, made from
material such as Ag or Cu, is used in a device. Resistive switching in the device
occurs through the formation and annihilation of a metallic atom bridge resulting
from the migration of highly mobile cations such as Ag
+ and Cu
+ ions. Actually,
resistive switching behavior can also be observed in a device without an electrochemically active electrode. In such cases, the migration of anions, usually oxygen
ions play a crucial role. Local oxygen ion migration, which is better described as the
migration of an oxygen vacancy (V O
‥ ) and a resultant change in the electronic
barrier at the interface, is associated with the resistive switching behavior. Similar
synaptic functions, described in the preceding subsections, are also demonstrated in
these anion migration devices. In this subsection, we will introduce neuromorphic
and electrical multifunction, realized in anion-migration based on a WO 3 – x
layer [37].
The devices have an MIM structure of Pt/WO 3 – x /Pt fabricated on a glass or SiO 2 /
Si substrate. The thin WO 3 – x film was prepared by RF sputtering from a polycrystalline WO 3 target in a gas mixture containing Ar and O 2 . Details of the device
fabrication and characterization methods are presented in Ref. [37]. Both volatile
and nonvolatile reconfigurable rectification and resistive switching were realized by
adjusting the electric stimuli input parameters, as shown in Fig. 13. The resistance of
the pristine device decreased nonlinearly after voltage seeping in both the positive
and negative voltage regions (Fig. 13a). However, the current boost induced by
voltage sweeping quickly fades over time, which means this resistive switching is
volatile. In contrast, after a forming process (Fig. 13c), the device exhibited normal
bipolar resistive switching, in which the resistance of the device decreased/increased
by applying positive/negative sweeping, as shown in Fig. 13b. In this case, the state
with relatively high conductance, obtained after positive voltage sweeping, automatically faded over time but did not return to the original state even over the
measurement period, which indicates nonvolatile, or at least partially nonvolatile
resistive switching. The positive (negative) forming process is triggered by applying
positive (negative) voltage sweeping or voltage pulses on the top Pt electrode to
induce a soft-breakdown, with a current compliance to prevent the device from hardbreakdown, see Fig. 13c. Note that the device possesses stable resistive switching
only in an oxygen-rich atmosphere (air or O 2 gas) after the positive electroforming
process [38].
The I-V curves of both volatile and nonvolatile resistive switching in Fig. 13 are
smooth, without abrupt change, indicating that the device performed analog resistive
switching. As shown in the bottom-right panel of Fig. 13a, the volatile resistance of
the device is gradually changed by applying consecutive voltage sweeping. The
nonvolatile resistance states can also be precisely modified by controlling the
Artificial Synapses Realized by Atomic Switch Technology
191