*2.2.3 TiN/HfOx/HfOx/Pt inorganic memristor*

In the previous work on Pt/HfOx/ZnOx/TiN and TiN/HfOx/AlOx/Pt memristors, the asymmetric memristive functional layers of A and B are different materials. Next, we will focus on Pt/HfO2/HfOx/TiN bilayer-structured memristor, as illustrated in **Figure 6a**. 4 nm-thick non-stoichiometric HfOx films were prepared by

#### **Figure 6.**

*(a) Schematic of the Pt/HfO2/HfOx/TiN memristor. (b) Cross sectional HAADF-STEM image of the device. (c) EDS elemental mapping of Pt, Hf, O, N,Ti and Si. (d) STDP-like curves. The different synaptic weights (ΔW) versus the different spike times (Δt). The inset shows a pair of pre-synaptic and post-synaptic spikes, and the spike pair is designed to implement STDP [20].*

PEALD using the H2 plasma and 2 nm-thick stoichiometric HfO2 films by TALD using the H2O precursor, in basically consistent with the measured result by the cross-sectional high angle annular dark field (HAADF)-scanning transmission microscopy (STEM) in **Figure 6b**. The energy dispersive x-ray spectroscopy (EDS) elemental mapping images of Pt/HfO2/HfOx/TiN are shown in **Figure 6c**, revealing the stacking structure. In addition, XPS composition analyses show that the atomic ratio of Hf:O in the HfO2 and HfOx layers is 1:2.04 and 1:1.84, respectively, indicating that stoichiometric HfO2 and nonstoichiometric HfOx bilayer-structured memristors have been obtained [20]. Hence A and B herein represent HfO2 and HfOx with various oxygen contents, respectively. This device unit based on TiN/ HfOx/HfO2/Pt memristor can also simulate the biological synapse learning rule of STDP, as indicated in **Figure 6d**. When the shortest spike timing of 10 ms is applied to the memristor device, the pulse train responses give rise to the largest Δ*W* value of 83% for potentiation and � 65% for depression, respectively [20]. These Δ*W* values for STDP are similar for Pt/HfOx/ZnOx/TiN, TiN/HfOx/AlOx/Pt and TiN/ HfOx/HfO2/Pt memristors.

The paired-pulse facilitation (PPF) is a phenomenon wherein the post-synaptic response induced by the spike increases when the time interval of the two spikes is very close [20]. PPF index can be defined as follows:

$$\text{PPF} = (G\_2 - G\_1)/G\_1 - 100\text{\%} = C\_1 \cdot \exp\left(-\Delta t/\tau\_1\right) + C\_2 \cdot \exp\left(-\Delta t/\tau\_2\right) \tag{3}$$

*G*<sup>1</sup> and *G*<sup>2</sup> are the conductance values after the first and the second pulse, respectively. The time constants of *τ*<sup>1</sup> and *τ*<sup>2</sup> can be assigned to the fast and slow decaying terms, respectively.

Evidently Pt/HfO2/HfOx/TiN memristor displays the marked dependence of synaptic weight on pulse interval Δ*t* by applying the pulse of �1.5 V and 2.5 V, respectively, as seen in **Figure 7a** and **b**. For shortest Δ*t* of 400 ns, the PPF index increases to 135% under positive pulse and becomes �62% under negative pulse. For the negative pulse signals, the calculated *τ*<sup>1</sup> and *τ*<sup>2</sup> values are 357 ns and 2.47 ms, respectively; for the positive pulses, *τ*<sup>1</sup> and *τ*<sup>2</sup> are 1.48 ms and 6.79 ms, respectively. When the Δ*t* decreases, the memory effect will be improved, which is ascribed to the fact that the smaller Δ*t* between pulses produces less oxygen vacancies to drift back with more effective accumulation of the oxygen vacancies.

#### **Figure 7.**

*PPF index as the function of the time interval (Δt) of the Pt/HfO2/HfOx/TiN memristor under negative voltage pulse (a) and positive voltage pulse (b). Black points represent the measurement data, and the red lines represent the fitting data by using Eq. (3). The insets in (a) and (b) record the applied pulse waveforms [20].*

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

Pt/HfO2/HfOx/TiN also mimics a classical conditioning under different pulse stimuli, as illustrated in **Figure 8**. In the famous experiment [21], a dog salivates (unconditioned response, UR) when watching the food (unconditioned stimulus, US) (**Figure 8a**), when it does not salivate (conditioned response, CR) on hearing the ring (conditioned stimulus, CS) alone (**Figure 8b**). Nonetheless, after some rehearsals, *i.e.* feeding the dog when ringing the bell (**Figure 8c**), the dog salivates even in only hearing the ring (**Figure 8d**). This elucidates that the dog has correlated the food with the ring. Furthermore, when taking away the food, the correlation between the food and ring gradually reduces and even disappears under only the conditioned stimuli (**Figure 8e**). The whole procedure can be emulated in the Pt/HfO2/HfOx/TiN by using +4 V and 1.3 V stimuli with a single pulse duration of 5 ms.

Before rehearsing, the memristor has a low resistance state of 5 kΩ. The +4 V stimulus (US) causes a high resistance state of 3 MΩ (UR) (**Figure 8f**), when the 1.3 V stimulus (CS) only results in a low resistance state of 5 kΩ before training (**Figure 8g**). In Pavlov's experiments, the food and the ring exist simultaneously to reinforce the correlation between US and CS. In our experiments, the +2.7 V stimulus was exerted to the memristor, the same as the simultaneous stimuli of 1.3 V and + 4 V pulse signals. When two rehearsing sequences with +2.7 V pulse, the device becomes the high resistance output of 2 MΩ (CR) (**Figure 8h**). When removing the +4 V signal, the memristor continues to keep in a high resistance state under a series of 1.3 V stimuli alone and then returns to a low resistance state (**Figure 8i**), implying the setup and vanish of the classical conditional reflex.

The energy consumption is one important indicator for a practical electronic synaptic device in neuromorphic network. Pt/HfO2/HfOx/TiN memristor can be set in less than 100 ns and reset in less than 10 ns, indicating the rapid switching speed, as recorded in **Figure 9a**.

The current response curves versus the time after the applied programming signal during the set or reset operation are plotted in **Figure 9b** and **c**, respectively. The current rises after a waiting time of about 260 ns when a 2 V/1 ms stimulus is applied, indicating the beginning of the set process (**Figure 9b**). The memristor resistance decreases from the initial high resistance state (1 MΩ) to low resistance state (800 Ω). Similarly, the current reduces after a waiting time of about 70 ns when a +3 V/1 ms signal is exerted, showing the occurrence of the reset process. The energy consumption per operation can be calculated to be 520 pJ for the set process and 1.05 nJ for the reset process by considering the pulse waveforms (time, response current, and pulse voltage), corresponding to the maximum energy consumption in one set or reset operation, as the memristor has been set in the lowest resistance state with the highest response current. Nevertheless, the actual operation of the electronic synapse is generally in the mediate resistance states (80 kΩ). The response current of the memristor is inversely proportional to the resistance value of the synaptic device with a first-order approximation. So, the evaluated actual energy consumption per operation will decline in the range of around ten picojoules.

Finally, the impact of oxygen vacancy concentration in non-stoichiometric HfOx layers on resistive switching properties of Pt/HfO2/HfOx/TiN bilayer ultrathin memristor has been investigated. The memristor with 12.1% oxygen vacancy concentration in the HfOx layer exhibits comprehensively better performances such as the optimal pulse energy consumption, reset switching speed, and DC endurance and retention characteristics [20].
