**2. Digital-to-analog switching transformation**

In some cases, a memristor device can show both abrupt and digital behaviors in the same switching cycle, such as digital set and analog reset or vice-versa. This behavior may induce better multibit performance, but it may not be sufficient to induce satisfactory synaptic performance. For example, if the set or reset process exhibits digital behavior then we can assume that the device will not exhibit good potentiation or depression, respectively. Henceforth, it is important to have analog switching for both set and reset processes. We discuss several important techniques to induce analog behavior in memristor devices. These techniques include transforming the switching conduction from filamentary to homogeneous, adding an interfacial layer below the top electrode, and electrode engineering.

#### **2.1 Filamentary to homogeneous switching transformation**

Despite the filamentary switching is more scalable friendly, as the localized filaments (approximately 20 nm) [13] take part in the switching process than the homogeneous switching, which relies on the conduction changes of the entire bulk structure. However, the homogeneous switching tends to be easier to exhibit analog behavior. The co-existence of both filamentary and homogeneous resistive switching in a single device can be observed by adjusting the electrical operation (biasing condition).

Huang C-H et al. [14] reported that a digital unipolar in Pt/ZnO/Pt device could be controlled to show analog bipolar switching mode after reversing the bias condition at higher current compliance (CC), as depicted in **Figure 3(a)**. The reversed bias made the switching layer consists of two regions, the oxygenrich region (below the top electrode) and the oxygen-deficient region (above the electrode), which transformed the filamentary into homogeneous switching. The reversed-biased technique can also be useful to transform filamentary bipolar to homogeneous bipolar, as reported by Ryu H. and Kim S. shown in **Figure 3(b)** [15]. They observed that applying appropriate stop voltage sweep prior to the LRS, at the negative differential region (NDR) voltage regime, can induce homogeneous switching in the Pt/Al2O3/TiN devices. Even though the *I-V* hysteresis of homogeneous switching is less obvious than the filamentary, it was reported that the synaptic behavior of the homogeneous switching is significantly improved, confirming the analog nature of the device [15]. A similar result was also observed in Pt/WOx/W device [16].

The homogeneous switching tends to dominate when the device operates at a lower current regime. Li Y. et al. [17] reported that the homogeneous switching in Ag/NiO/Pt device can be observed prior to the electroformed process, as depicted *Practical Approach to Induce Analog Switching Behavior in Memristive Devices: Digital… DOI: http://dx.doi.org/10.5772/intechopen.98607*

#### **Figure 3.**

*Several strategies to induce homogeneous switching by controlling the biasing condition. The employment of (a) reversed bias and higher CC in Pt/ZnO/Pt device [14], (b) reversed bias at the NDR region in Pt/Al2O3/TiN device, [15] (c) successive voltage sweeps prior to the electroformed in the Ag/NiO/Pt device [17], and (d) opposite polarity in Fe-doped SrTiO3 device [19]. Reprinted and adapted from [14, 15, 17, 19].*

in **Figure 3(c)**. The successive voltage sweeps during positive and negative voltage incessantly decrease and increases the memristor conductance, respectively. Operating the device at a lower current regime unable to form the filament, but it is sufficient to control the movement of the intrinsic defects within the bulk and, thus, modulate the Schottky barrier height (SBH). A variation of SBH was observed in this device at different scans, due to the widened of the Ag/p-NiO interface depletion width. A possible conduction mechanism of filament formation was also conferred there. Interestingly, as discussed there, the low temperature treated NiO device is not showing analog switching. A similar analog switching result was shown in PT/BiFeO3/Pt device [14]. The Ag/CuAlO2/TiO2/p++-Si structure was also shows similar analog switching due to the Ag ions and oxygen migration under the electric field [18]. In some cases, homogeneous and filamentary switching can also co-exist at the same current regime as well. Muenstermann R. et al. reported that the Pt/SrTiO3(Fe)/Nb:SrTiO3 device exhibited non-polar behavior, as shown in **Figure 3(d)** [19]. Intriguingly, the counter eightwise and eightwise switching are controlled by different switching mechanism which is filamentary and homogeneous switching, respectively.

Kim S. et al. [20] reported that the analog switching can also be achieved by a partial reset scheme. They observed that the reset process of the Cu/HfAlOx/Si device consist of two stages where the first stage (partial reset) is controlled by an electric field and the second stage (full reset) is dominated by Joule heating mechanism, as depicted in **Figure 4(a)**. **Figure 4(b)** and **(c)** show the device exhibits digital or analog switching when it operates with a full reset or a partial reset, respectively. As expected, device that was operated with a partial reset performs better potentiation and depression than that of the full reset one, as shown in **Figure 4(d)** and **(e)**. It is still not clear the physics behind this phenomenon; however, we hypothesis that this may due to the filamentary to homogenous switching transformation as well. Nevertheless, the relationship between the conduction mechanism, analog switching, and synaptic behavior should be investigated further.

### **2.2 Insertion layer engineering**

Analog switching can be induced by inserting a metal film between the electrode and the storage layer. Here, we discuss insertion layer techniques that can transform

#### **Figure 4.**

*(a) Two stages of reset process in Cu/AlHfOx/Si device. Switching characteristics of devices having (b) full and (c) partial resets. Synaptic behavior of the devices that operate with (d) full and (e) partial resets. Reprinted and adapted from [20].*

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

valence change and conductive-bridge type memristors from digital into analog switching. In these techniques, a metal layer is inserted between the top electrode and storage layer to control the drift of the anions and cations defects during the switching process.

### *2.2.1 Oxygen scavenging layer*

The formation and rupture of the oxygen conducting filament in the valence change memristor are controlled by redox of oxygen, where it is mainly taken place at the electrode/oxide interfaces [6]. Hence, in order to achieve analog behavior, we need to ensure that the oxygen ions that injected to- (set process) or from (reset process) the electrode should be continuously drifted during the entire switching process.

Chang L-Y et al. suggest that the continuous drift can be done by inserting a metal layer that has similar Gibbs free energy of oxide formation ( ∆*Gf* ) value to the storage layer [21]. **Figure 5(a)** and **(b)** show the *I-V* curves of TiN/TiO2/Ti and TiN/Ti/TiO2/TiN devices, respectively; the thickness of the Ti insertion layer was 4 nm. It is observed that the device without Ti insertion layer (0Ti) exhibits digital switching; conversely, analog switching can be observed after the insertion layer was employed (4Ti). This digital-to-analog switching transformation is further confirmed by the behavior of the synaptic plasticity of the devices, as shown in **Figure 5(c)** and **(d)**. Under a given pulse scheme, the conductance change of the 0Ti device rises (potentiation) and falls (depression) abruptly; meanwhile, the synaptic plasticity in the 4Ti device is more gradual.

#### **Figure 5.**

*Typical I-V curves of devices made (a) without (0Ti) and (b) with Ti (4Ti) insertion layer. Potentiation and depression synaptic plasticity of (c) 0Ti and (d) 4Ti devices. Reprinted from [21].*

Chang L-Y et al. compared the interfacial properties between the stacks made without and with Ti layer, as depicted in **Figure 6**; for this purpose, they inserted 20 nm thick of Ti (20Ti) to obtain a more obvious reaction at the interface. Based on the depth-XPS analysis (**Figure 6(c**–**e)**), the Ti layer absorbed oxygen from the TiO2 layer and forming TiOx interfacial layer at the TiN/TiO2 interface. Note that the formation of the interfacial layer was occurred during the deposition process (pristine stack). Based on the material analysis, they proposed that TiOx interfacial layer can gradually ionize the oxygen ions during set/potentiation and reset/ depression process that promotes the occurrence of gradual switching in the device (**Figure 6(f )**).
