*2.2.4 TaN/Ti-MA/TiO2/Pt organic: inorganic hybrid memristor*

As mentioned above, we mainly elucidated the bio-synaptic functions of three asymmetric inorganic bilayer-structured ultrathin memristors. In this part,

#### **Figure 8.**

*Emulation of the acquisition and extinction of classical conditioning demonstrated by Pavlov's dog experiment. (a) The dog salivates while watching the food (US* ! *salivating UR). (b) The dog does not salivate upon hearing the ringing alone (CS* ! *no salivating). (c) In the training process, the food and ringing together stimulates the dog, and the dog salivates (US + CS* ! *salivating). (d) After sufficient training, classical conditioning is formed and the dog salivates upon hearing the ringing alone (CS* ! *salivating, CR). (e) Extinction of classical conditioning after removing the food for some time. (f) Positive +4 V pulse (US) can lead to a high resistance output, similar to the UR in (a). (g) The negative* �*1.3 V pulse (CS) cannot lead to the high resistance output before training, similar to the CS in (b). (h) After applying several training sequences of +2.7 V pulse voltage, equal to the simultaneous stimuli of* �*1.3 V and +4 V pulses, the device reaches a high resistance state of 2 MΩ (CR), analogous to the phenomenon in (c). (i) After only applying some* �*1.3 V pulse alone, the memristor remains in a high resistance state, similar to CR in (d). Then it returns to the low resistance state, consistent with the extinction of classical conditioning in (e) [20].*

*Artificial Synapses Based on Atomic/Molecular Layer Deposited Bilayer-Structured… DOI: http://dx.doi.org/10.5772/intechopen.97753*

#### **Figure 9.**

*(a) Switching speed test of the synaptic device. The voltage for the set and reset operation in the memristor is about 2 V and +3 V, respectively. The device can be switched in less than 100 ns for a set operation and less than 10 ns for a reset operation. (b) Transient current response on the applied voltage pulse for a set operation from the high resistance state to the low resistance state. The set pulse amplitude, width, rising time, and falling time are set to be 2 V, 1000 ns, 20 ns, and 20 ns, respectively. (c) Transient current response on the applied voltage pulse for a reset operation from the low resistance state to the high resistance state. The reset pulse amplitude, width, rising time, and falling time are set to be 3 V, 1000 ns, 20 ns, and 20 ns, respectively [20].*

organic–inorganic hybrid bilayer memristors of TaN/Ti-MA/TiO2/Pt were prepared by low temperature MLD/ALD at 160°C. The synaptic plasticity has been explored deeply. Some superb synaptic functions, such as nonlinear transmission characteristics, STP/LTP, PPF, and STDP have been achieved in the hybrid memristors [22].

First the narrow-scan XPS and Fourier transform infrared (FTIR) spectroscopy were used to detect the chemical composition and organic group of Ti-based maleic acid (Ti-MA) hybrid film, as shown in **Figure 10a–d**. The C 1 s XPS peaks at 284.6 eV and 288.4 eV (**Figure 10a**) result from the C-C (backbone chain carbon) bond and the O-C=O bond from carboxyl, respectively, suggesting the occurrence of organic component in Ti-MA films. The doublet at 458.7 eV and 464.5 eV with the spin orbit splitting energy of 5.8 eV can be assigned to the Ti 2p1*/*<sup>2</sup> and Ti 2p3*/*<sup>2</sup> ones from the Ti-O bond of TiO2 [13, 23] (**Figure 10b**, which indicates the inorganic component in hybrid films. Moreover, the O 1 s spectrum can be deconvoluted into two peaks at 530.0 eV and 531.6 eV, corresponding the O-Ti and O-C bonds, respectively (**Figure 10c**). The FTIR spectrum of Ti-MA hybrid film (**Figure 10d**) displays the asymmetric and symmetric stretch of carboxylate groups at 1575 cm<sup>1</sup> and 1447 cm<sup>1</sup> . The splitting of 128 cm<sup>1</sup> indicates the bidentate bond mode between the Ti ion and carboxyl. As a result, Ti-MA inorganic–organic hybrid films have been fabricated successfully.

The resistive switching characteristics of the hybrid bilayer memristor of TaN/Ti-MA/TiO2/Pt have been examined for 100 times, as seen in in **Figure 11a**. The typical bipolar resistive switching behavior has been confirmed with narrow distribution of set voltage of 1.6 0.2 V (red line) or reset voltage of 1 0.1 V

**Figure 10.** *Narrow-scan XPS spectra of (a) C 1 s, (b) Ti 2p and (c) O 1 s and (d) FTIR spectrum from the Ti-MA hybrid films on Si [22].*

(black line). The double-logarithmic *I-V* curves and linear fits to the set process are shown in **Figure 11b**. At the low voltage stage, the I-V is dominated by the Ohm's law with the approximately linear relationship (region 1, R2 = 0.9996). When the voltage increases, the current is dependent of near square of the voltage, obeying the Child conductive law (region 2, R<sup>2</sup> = 0.9995). At critical voltage of around 1.2 V, the current is proportional to the *n*th power of the voltage with a sharp current rise

#### **Figure 11.**

*(a) I-V curve of the TaN/Ti-MA/TiO2/Pt hybrid memristor for 100 times DC ramp voltages tests. Bottom inset is the schematic of the memristor. (b) Double-logarithmic I-V curves and linear fits to the set process [22].*

*Artificial Synapses Based on Atomic/Molecular Layer Deposited Bilayer-Structured… DOI: http://dx.doi.org/10.5772/intechopen.97753*

(region 3, R2 = 0.9904). All these prove the space charge limited current (SCLC) model in hybrid bilayer memristor [24], revealing the filament model of oxygen vacancy migration.

The PPF and STDP functions have also been characterized in hybrid memristor, as shown in **Figures 12** and **13**, respectively. A pair of pulses (1 V, 400 ns) with different Δ*t* were applied to the hybrid memristor (**Figure 12a**). The measured data can be well fit exponentially (**Figure 12b**). The PPF index has reached to 361% with the 400 ns pulse interval in hybrid memristor. When the pulse interval increases to 2400 ns, the PPF index dramatically tends to 3% [22]. Compared to inorganic bilayer memristor, the organic–inorganic hybrid bilayer device has much larger PPF index in the same pulse interval of 400 ns.

**Figure 12.**

*(a) PPF function in memristors generated by two pulse spikes and the real-time response current. (b) PPF curves with different pulse interval time [22].*

#### **Figure 13.**

*STDP curves obtained in hybrid memristor. The spot is the measured data and the red line is the fitting results. The insets are the spike pulse signals designed by a pair of 0.8 V and 0.8 V pulses with pulse width of 120 μs [22].*

The STDP rule was emulated in hybrid memristor by using a pair of 0.8 V and �0.8 V pulses with 120 *μ*s pulse width. The ΔW has a strong time correlation with maximum 35% increment at the Δt of �20 *μ*s and �20% reduction at the Δt of 20 *μ*s. These values are relatively smaller than the ΔW maximum value of 60–90% of inorganic memristors. Finally, the ΔW in hybrid device obeys the exponential association with the Δt, namely

$$
\Delta \mathbf{W} = \mathbf{A} \cdot \exp\left(-\Delta t/\tau\right) \tag{4}
$$

The measured data can be fitted well.

In addition, the conditioned reflex has been mimicked in hybrid film memristor, similar to the results of Pt/HfO2/HfOx/TiN memristor in **Figure 8**.

By comparison with inorganic bilayer memristors, it can be found that the organic–inorganic hybrid bilayer memristor has similar bio-synaptic functions with comparable switching speed and energy consumption. Moreover organic–inorganic hybrid materials may possess both the advantages of organic and inorganic components with excellent flexibility and tunability. Inorganic compounds have better electrical characteristics and thermal stability. Organic compounds own various functional groups, larger stretchability and low processing temperature. By means of the synergetic and complementary effects between organic and inorganic components, the comprehensive properties of hybrid memristive materials could be expected for significant improvement. The hybrid bilayer ultrathin memristor derived by low temperature MLD/ALD is one competitive candidate for flexible neuroscience applications.

### **2.3 Memristive mechanism**

In Section 2.2, we focused on the electrical Performance and synaptic functions of several bilayer ultrathin memristors. In this section, the asymmetric memristive mechanism of the bilayer-structured memristors on TiN or TaN will be studied carefully. Taking Pt/AlO*x*/HfO*x*/TiN memristor as an example, the XPS depth profiles of asymmetric bilayer device units were obtained under various resistance states of the initial state, low resistance state (LRS), high resistance state (HRS), and medium resistance state [17]. XPS is a powerful surface analytical tool to determine the chemical valence and the oxygen vacancy contents in multilayer-structured metal oxide thin films [23, 25].

**Figure 14a–d** records the high-resolution Al 2p, Hf 4f and O 1 s peaks in AlO*<sup>x</sup>* and HfO*<sup>x</sup>* layers for as-deposited Pt/AlO*x*/HfO*x*/TiN in the initial state. The Hf 4f spectra from the HfO*<sup>x</sup>* layer can be deconvoluted into four peaks (**Figure 14b**). The stronger peaks at �16.7 eV and 18.6 eV originate from the Hf4+ in the HfO*<sup>x</sup>* layer, whereas the weaker ones with slightly lower energies of 15.6 eV and 17.9 eV come from the Hf(4�*x*)+ in the low valence Hf sub-oxide. The content percentage of two Hf valence states in the HfO*<sup>x</sup>* layer can be roughly evaluated by calculating the area proportion of each peak, as shown in the inset of **Figure 14b** [26–28]. The percentage of Hf4+ and Hf(4�*x*)+ in the HfO*<sup>x</sup>* layer is around 89.7% and 9.4%, respectively. A similar analysis can be also carried out for the Al 2p spectra from AlO*<sup>x</sup>* layer (**Figure 14a**). Meanwhile the O 1 s spectra from the AlO*<sup>x</sup>* and HfO*<sup>x</sup>* layers can also be deconvoluted into two peaks. The stronger peaks at around 531.5 and 531.0 eV result from Al-O and Hf-O bonding in the AlO*<sup>x</sup>* and HfO*<sup>x</sup>* layers, respectively, whereas the weaker ones with a slightly higher energy of 532.1 eV in the O 1 s spectra are ascribed to the oxygen vacancies in the AlO*<sup>x</sup>* and HfO*<sup>x</sup>* layers according to the literature reports [26–29]. The calculated percentage of oxygen vacancies in the AlO*<sup>x</sup>* and HfO*<sup>x</sup>* layers is around 0.7% and 8.1%, respectively (**Figure 14c** and **d**). *Artificial Synapses Based on Atomic/Molecular Layer Deposited Bilayer-Structured… DOI: http://dx.doi.org/10.5772/intechopen.97753*

#### **Figure 14.**

*Narrow-scan (a) Al 2p, (b) Hf 4f and O 1 s peaks of (c) AlO*<sup>x</sup> *and (d) HfO*<sup>x</sup> *in as-prepared Pt/AlO*x*/HfO*x*/ TiN in the initial state. (e) XPS depth profile of Pt/AlO*x*/HfO*x*/TiN in the initial state. (f) The depth distribution of the average oxygen vacancy concentration in the initial state, LRS, HRS, and medium resistance state after 40 pulses (1.5 V, 0.5 ms). The gray region in figure (e) and (f) is the interfacial layer [17].*

Significantly, the oxygen vacancy content in the HfO*<sup>x</sup>* layer is much higher than that in the AlO*<sup>x</sup>* layer.

The XPS depth data of Pt/AlO*x*/HfO*x*/TiN memristor by Ar ion etching under various resistance states may provide some valuable information on the valence states and defects of metal oxide layers [24], for the initial state sample recorded in **Figure 14e**. The AlO*x*/HfO*<sup>x</sup>* bilayer structure could be recognized with an evident interfacial diffusion between AlO*x*/HfO*<sup>x</sup>* and HfO*x*/TiN (gray region). The depth distribution of the average oxygen vacancy concentration in the asymmetric Pt/AlO*x*/HfO*x*/TiN memristors under various resistance states of the initial state, LRS, HRS, and medium resistance state is illustrated in **Figure 14f**. **Table 3** lists the average oxygen vacancy concentration values of Pt/AlO*x*/HfO*x*/TiN for four resistance states at different positions of A, B, C, D, and E, corresponding to an etch time of 0 s, 90 s, 210 s, 390 s, and 510 s. Herein A, B, C, D, and E locate in the interface of the Pt/AlO*x*, AlO*<sup>x</sup>* layer, the interface of the AlO*x*/HfO*x*, HfO*<sup>x</sup>* layer, and the interface of HfO*x*/TiN, respectively. The oxygen vacancy distribution is inhomogeneous in the Pt/AlO*x*/HfO*x*/TiN memristor, and the oxygen vacancy concentration of the interfaces between AlO*x*/Pt (A), AlO*x*/HfO*<sup>x</sup>* (C), HfO*x*/TiN (E) is markedly higher than that of the adjacent AlO*<sup>x</sup>* (B) and HfO*<sup>x</sup>* (D) layers. Furthermore, the oxygen vacancy concentration in HfO*<sup>x</sup>* (D) is much higher than that in AlO*<sup>x</sup>* (B) layer.

In general, the resistive switching mechanism of metal oxide memristors is related to the connection and rupture of conductive filaments of oxygen vacancies. But the simple increase of oxygen vacancy concentration is not always effective. The non-uniform distribution of oxygen vacancies in memristors is the critical factor affecting the resistive switching behavior of memristive devices [30].

Based on the oxygen vacancy concentration and distribution in the Pt/AlO*x*/ HfO*x*/TiN memristors under various resistance states in **Figure 14f**, we proposed a memristive mechanism of an asymmetric bilayer metal oxide synaptic device to explain synaptic plasticity, as illustrated in **Figure 15**.

There are much more random oxygen vacancies in the HfO*<sup>x</sup>* layer than in the AlO*<sup>x</sup>* layer for as-deposited Pt/AlO*x*/HfO*x*/TiN device. Meanwhile, the oxygen vacancy concentration in the interfaces of AlO*x*/HfO*<sup>x</sup>* and HfO*x*/TiN is evidently higher than the HfO*<sup>x</sup>* layer (**Figure 15a**). During the forming process, the disorderly distributed oxygen vacancies in the bilayer oxide layers and interfacial layers form conductive filaments under the external electrical field, similar to the soft breakdown of the capacitor. So the connection and disconnection of the conductive filaments lead to resistive switching. When inserting a 3 V forming voltage, the device turns from the initial state to LRS with suddenly resistance drop from 10 MΩ to 600 Ω, suggesting that the oxygen vacancies with positive charges (VO 2+) in the AlO*x*/HfO*<sup>x</sup>* interface, HfO*<sup>x</sup>* layer, and HfO*x*/TiN interface move to the AlO*<sup>x</sup>* layer and AlO*x*/Pt interface. Simultaneously, the oxygen vacancy concentration gradient help to the migration of the oxygen vacancies, forming localized conductive filaments of oxygen vacancies in the bilayer structured AlO*x*/HfO*<sup>x</sup>* device (**Figure 15b**).

After applying the +2.5 V reset voltage to the LRS device, the memristor transfers from LRS (600 Ω) to HRS (1 MΩ) (**Figure 15c**). During the reset process, the oxygen vacancies migrate from the AlO*x*/Pt interface and AlO*<sup>x</sup>* layer to the AlO*x*/ HfO*<sup>x</sup>* interface and HfO*<sup>x</sup>* layer, leading to the rupture of oxygen vacancy conductive filaments in the AlO*<sup>x</sup>* layer. Besides, considering the thermophoresis/diffusiondriven oxygen migration [31, 32], the middle position of the conductive filament in the AlO*<sup>x</sup>* layer first breaks up, causing a spatial gap, as indicated by the red arrow in **Figure 15***c. electron* tunneling happens through the physical gap with the enhanced resistance. During the reset process, the oxygen vacancy concentration declines at the AlO*x*/Pt interface and AlO*<sup>x</sup>* layer and rises at the AlO*x*/HfO*<sup>x</sup>* interface and HfO*<sup>x</sup>*


#### **Table 3.**

*Average oxygen vacancy concentration of Pt/AlO*x*/HfO*x*/TiN in various positions for different resistance states [17].*

*Artificial Synapses Based on Atomic/Molecular Layer Deposited Bilayer-Structured… DOI: http://dx.doi.org/10.5772/intechopen.97753*

#### **Figure 15.**

*Model of the formation and rupture of a conductive filament consisting of oxygen vacancies. After 3 V forming voltage, the device transfers from an initial resistance state (a) to a low resistance state (LRS) (b); after 2.5 V reset voltage, the device transfers from a low resistance state to a high resistance state (HRS) (c); after 40 continuous pulses (+1.5 V, 10 ms), the device transfers from LRS to a medium resistance state (d) [17].*

layer, as proved by the relative variation of oxygen vacancy concentrations in **Figure 14f** and **Table 3**.

Because the synapse device usually operates under pulse mode to get more intermediate resistance states, a continuous pulse experiment was exploited to alter the device from a LRS (600 Ω) to a medium resistance state (50 kΩ) by imposing 40 pulses (+1.5 V, 10 ms) (**Figure 15d**). The XPS result (**Figure 14f**) indicates that the oxygen vacancy concentration curve of medium resistance state devices lies approximately between HRS and LRS in the AlO*<sup>x</sup>* layer and HfO*<sup>x</sup>* layer. The difference in oxygen vacancy concentration at the interface layers of AlO*x*/HfO*<sup>x</sup>* and HfO*x*/TiN among the medium resistance state, LRS, and HRS is slightly little. In consequence, during regular operations of synaptic memristors, the formation/ rupture of nanoscale conductive filaments tend to appear in the low *k* AlO*<sup>x</sup>* layer with lower electric field intensity [33]. Furthermore, the device conductance can be modulated by the oxygen vacancy drift under pulse electric field, producing a change in the concentration and distribution of oxygen vacancies at the interface of the metal/oxide and the interior. In the LRS of 600 Ω, the conductive filament is thick with the conductance of 22 *G*0, corresponding to a wide conductive filament with classical metallic properties. After 40 pulse stimuli, a medium resistance state of 50 kΩ is obtained with a conductance of 0.26 *G*0, where the conductive filament behaves as a quantum wire, producing a single-defect conducting path [31, 33, 34].

The oxygen vacancy migration/diffusion model can be used to explain the transition from STP to LTP in bilayer memristive device (**Figure 4a**). When imposing the +1.6 V pulse, the oxygen vacancies move from the AlOx layer to the HfOx layer

with the reduced response current. When the voltage is removed, some oxygen vacancies may stay in a new steady position, however some oxygen vacancies may diffuse back to the old position owing to the gradient of oxygen concentration. This leads to the device conductance change with a reduced synaptic weight during the relaxation time. After applying repetitive pulse stimuli, the subsequent voltage forces the reversely diffused oxygen vacancies to move forward again so as to improve the migration efficiency until most oxygen vacancies attain new equilibrium positions. The remaining synaptic weight gradually increases with the increasing pulse number. This process is called repeated training and learning, corresponding to the transformation from STP to LTP [17].

The memristive mechanism from Pt/AlO*x*/HfO*x*/TiN device is also applicable to other bilayer-structured memristors such as Pt/HfO2/HfOx/TiN, Pt/TiO2/Ti-MA/TaN.
