*2.2.2 Cation drift barrier layer*

The conductive-bridge type memristor utilizes the electrochemical metallization process where the atoms from the active electrode (Cu, Ag, or Ni) drift to the switching layer and form the conduction bridge (filament) [22, 23]. A diffusion barrier is usually employed in this type of memristor device to control the atomic drift [24, 25]. Aftab S. et al. used an oxidizable metal TiW as a diffusion barrier layer inserted between the switching layer and active metal top electrode to transform TaOx-based memritor from digital set into analog [26]. **Figure 7** shows the I-V curves and synaptic behavior of Cu/TaOx/TiN (device A) and Cu/TiW/TaOx/ TiN (device B) memristors. It was observed that the device made without the TiW barrier layer exhibits digital set with a poor synaptic behavior.

In **Figure 7(a)** and **(b)** I-V curves for both devices is shown. The forming compliance current for both devices is 500 μA and set/reset cycle compliance current is 1 mA. The device with 20 nm TiW barrier layer (Device B) insertion clearly shows gradual switching for both set and reset cycles as compared to without barrier layer (Device A) device. The gradual behavior superiority is further confirmed by synaptic plasticity as shown in **Figure 7(c)** and **(d)**. By using an optimized pulse scheme with pulse width of 10 μs they observed abrupt conductance change when positive pulses are applied for potentiation process in Device A. However, Device B shows gradual change in conductance states when positive pulses are applied for

#### **Figure 6.**

*Cross-sectional TEM image of (a) 0Ti and (b) 20Ti device stacks. (c) Intensity of N1s core level and (d) deconvolution of O1s core level spectra at various depth within the 20Ti stack. (e) Concentration of oxygen and titanium elements, and lattice-oxygen in the respective depth. (f) Conduction mechanism of the devices. Reprinted from [21].*

*Practical Approach to Induce Analog Switching Behavior in Memristive Devices: Digital… DOI: http://dx.doi.org/10.5772/intechopen.98607*

#### **Figure 7.**

*Typical I-V curves and synaptic behavior of devices made without (device A) and with (device B) TiW barrier layer. Reprinted from [26].*

potentiation. Note that they also observed a significant improvement in data retention of the device made with a barrier layer.

The inserted barrier layer restricts excessive metal ions diffusion into the TaOx layer and forms an oxygen vacancy-rich TiWOx layer at the interface, as depicted in **Figure 8(a**–**d)**. After the insertion of barrier layer, Cu diffusion into

#### **Figure 8.**

*Cross-sectional TEM image of (a) 0Ti and (b) 20Ti device stacks. (c) Intensity of N1s core level and (d) deconvolution of O1s core level spectra at various depth within the 20Ti stack. (e) Concentration of oxygen and titanium elements, and lattice-oxygen in the respective depth. (f) Conduction mechanism of the devices. Reprinted from [21].*

the switching layer is limited to a great extent as can be seen by XPS depth spectra in **Figure 8(c)** and **(d)**. These results indicate towards the role of barrier layer TiW and interfacial layer TiWOx. The TiW insertion layer also promote confined filament which was not the case with device A having abundant Cu diffusion (**Figure 8e**). They suggest that the TiWOx interfacial layer promotes the formation of hybrid filament. Thus, device B shows superior device stability and performance compared to device A (pure metallic filament), as depicted in **Figure 8(e**–**f )**.

Nevertheless, Wan T. et al. [27] suggest that the digital to analog switching in Ag/ SrTiO3/FTO device can also be achieved with a pure metallic filament by controlling the size of the filament during the switching operation. They inserted a reduced graphene oxide (RGO) layer on top of the FTO bottom electrode. The RGO has high interfacial resistance and help to dissipate the Joule heat through the RGO. Hence, the size of the metallic filament can be easily tuned to perform good analog behavior.

#### **2.3 Electrode engineering**

Jang J.T. et al. [28] observed that a careful choice of top electrode material is crucial in achieving analog behavior. They compared Mo/IGZO/Pd (sample 1) and Pd/IGZO/Pd (sample 2) stacks. The devices made with the Pd top electrode exhibits digital switching, an abrupt transition in the resistance state during the set and reset operations are depicted in **Figure 9(a)**. On the other hand, the Mo/IGZO/Pd stack exhibits gradual transformation of resistance state for the set and reset operation, as shown in **Figure 9(b)**.

Energy band diagram analysis for sample 1 and sample 2 is depicted in **Figure 10(a)**–**(d)**. The abrupt switching in the Pd/IGZO/Pd stack is predominately due to a more significant barrier height of about 1 eV. A larger barrier height between the metal and semiconductor interface induces the formation of Schottky junction near the top and bottom electrode. However, the Mo/IGZO/ Pd stack is observed to have a minimal barrier height of 0.3 eV, which significantly results in the formation of an ohmic junction near the top electrode and a Schottky junction near the bottom electrode. Thus, in the presence of an ohmic junction near the top electrode for sample 2, they observed a gradual switching behavior on engineering the top electrode. A similar phenomenon was also observed by Tang M.H. et al. [29] in Pt/ZnO/Pt and Ag/ZnO/Pt stacks devices. Digital unipolar switching behavior is observed in Pt/ZnO/Pt device and, conversely, the Ag/ZnO/Pt device exhibits analog bipolar switching. Although the paper does not discuss the detailed physics of such phenomenon, we assume that the contribution of Ag cations should play a role in achieving the analog behavior.

**Figure 9.** *Typical I-V curves of (a) Pd/IGZO/Pd (sample 1) and (b) Mo/IGZO/Pd (sample 2) reprinted from [28].*

*Practical Approach to Induce Analog Switching Behavior in Memristive Devices: Digital… DOI: http://dx.doi.org/10.5772/intechopen.98607*

**Figure 10.**

*The schematic of (a) flat band diagram, (b) equilibrium, (c) positive bias, and (d) negative bias energy band diagram of sample 1 and sample 2. Reprinted from [28].*

Li X. et al. [30] investigated the role of inert and oxidizable top electrode materials in trilayer AlOx/TaOx/TaOy devices. The digital switching behavior is observed for the device with Pt as top electrode. The device made with Al top electrode tends to exhibit analog switching behavior. The Al top electrode interacts with the AlOx layer leading to the formation of oxygen deficient interfacial layer between AlOx and Al (top electrode). The oxygen ion migration and accumulation occur in a continuous manner and the device with Al top electrode is exhibiting gradual switching behavior during the continuous set and reset operation.

The method that Li X. et al. [30] proposed was further explored by C. Sun. et al. [31]. They suggest that an alloy of inert-oxidizable metal, FePt electrode, can induce analog characteristics on SiO2 based devices. The **Figure 11(a)** and **(c)** depict the schematic representation of Device A (TiN/SiO2/TiN) and Device B (TiN/SiO2/FePt/TiN). The FePt electrode act as the oxygen reservoir layer which assisting in trapping of oxygen ions during the resistive switching transformation. The barrier height of the electrode is also engineered on introducing FePt electrode which influence the switching transformation from abrupt to gradual as depicted in **Figure 11(b)** and **(d)**.
