When the Ag salt concentration increases, the number of free Ag
+ ion decreases
by the formation of ionic aggregates (Fig. 3e). In addition, ClO 4
À anions can form
radicals by decomposition, giving rise to the degradation of polymer molecules to
produce a passivation layer at the electrode interface. This may form higher-ordered
ionic associations. These species are immobile in the polymer matrix and contribute
to the further build-up of the passivation layer, which prevents the supply of Ag
+
ions into the polymer matrix. As a result, metal filament formation becomes increasingly difficult and higher bias voltages are needed to turn on the device. For Ag salt
concentrations higher than 5 wt%, metal filaments are no longer formed (Fig. 3f) and
resistive switching never take place. This model explains the experimental observations well and indicates the importance of the salt concentration so as to obtain the
desired characteristics.
3 Kinetic Factors Determining Filament Formation
To realize stable resistive switching, an understanding of the filament growth
kinetics under voltage biasing is very important. The structural characteristics of
SPE films are considered to play a major role in both ion transport and subsequent
filament growth behavior [15, 16]. One can expect that the morphology of thin SPE
films has a major impact on ion transport as well as on the formation of conducting
filaments in the atomic switch. We have systematically studied the filament growth
behavior of SPE-based atomic switches by means of in situ and ex situ microscopy
[17–19]. In this section, we discuss how switching behavior is controlled by kinetic
factors, which dominates the filament formation kinetics in SPE-based atomic
switches.
3.1 Direct Observation of Filament Growth Processes
Planar devices were fabricated on a SiO 2 -covered Si substrate, in which opposing Ag
(or Pt) and Pt (or Ag) electrodes were formed with different gaps of 0.5–8 μm
[17, 18]. An SPE film with a Ag salt concentration of 3 wt% was formed on the
electrodes by a spin coating method. Thickness of the coated SPE films was
~200 nm. Under voltage biasing, direct observation and video capture of the device
were performed using an optical microscope equipped with a CMOS camera.
Figure 4a shows a typical current-time (I–t) plot obtained for a Ag/Ag-PEO/Ag
planar device with a gap of ~8 μm under constant voltage application of +1 V. A low
I CC of 11 nA was preset to regulate the initial stage of filament growth. The
corresponding optical microscope images, taken at the selected times, are presented
in Fig. 4b1–b8. After biasing, the current gradually decreased with time. This initial
current originates from the oxidation of Ag at the biased electrode interface. Small
voids, appeared in the biased electrode, evidence the dissolution of Ag atoms. These
Solid-Polymer-Electrolyte-Based Atomic Switches
145
+ ion decreases
by the formation of ionic aggregates (Fig. 3e). In addition, ClO 4
À anions can form
radicals by decomposition, giving rise to the degradation of polymer molecules to
produce a passivation layer at the electrode interface. This may form higher-ordered
ionic associations. These species are immobile in the polymer matrix and contribute
to the further build-up of the passivation layer, which prevents the supply of Ag
+
ions into the polymer matrix. As a result, metal filament formation becomes increasingly difficult and higher bias voltages are needed to turn on the device. For Ag salt
concentrations higher than 5 wt%, metal filaments are no longer formed (Fig. 3f) and
resistive switching never take place. This model explains the experimental observations well and indicates the importance of the salt concentration so as to obtain the
desired characteristics.
3 Kinetic Factors Determining Filament Formation
To realize stable resistive switching, an understanding of the filament growth
kinetics under voltage biasing is very important. The structural characteristics of
SPE films are considered to play a major role in both ion transport and subsequent
filament growth behavior [15, 16]. One can expect that the morphology of thin SPE
films has a major impact on ion transport as well as on the formation of conducting
filaments in the atomic switch. We have systematically studied the filament growth
behavior of SPE-based atomic switches by means of in situ and ex situ microscopy
[17–19]. In this section, we discuss how switching behavior is controlled by kinetic
factors, which dominates the filament formation kinetics in SPE-based atomic
switches.
3.1 Direct Observation of Filament Growth Processes
Planar devices were fabricated on a SiO 2 -covered Si substrate, in which opposing Ag
(or Pt) and Pt (or Ag) electrodes were formed with different gaps of 0.5–8 μm
[17, 18]. An SPE film with a Ag salt concentration of 3 wt% was formed on the
electrodes by a spin coating method. Thickness of the coated SPE films was
~200 nm. Under voltage biasing, direct observation and video capture of the device
were performed using an optical microscope equipped with a CMOS camera.
Figure 4a shows a typical current-time (I–t) plot obtained for a Ag/Ag-PEO/Ag
planar device with a gap of ~8 μm under constant voltage application of +1 V. A low
I CC of 11 nA was preset to regulate the initial stage of filament growth. The
corresponding optical microscope images, taken at the selected times, are presented
in Fig. 4b1–b8. After biasing, the current gradually decreased with time. This initial
current originates from the oxidation of Ag at the biased electrode interface. Small
voids, appeared in the biased electrode, evidence the dissolution of Ag atoms. These
Solid-Polymer-Electrolyte-Based Atomic Switches
145