*2.2.1 TiN/ZnOx/HfOx/Pt inorganic memristor*

The *I–V* curves of Pt/HfOx/ZnOx/TiN inorganic memristor are plotted in **Figure 1b** under a modified DC double sweep. To mimic the functions of a nerve synapse, one multiple-state resistances should be obtained in bilayer memristor. A continuous set or reset process was performed by successive increasing the compliance from 0.1 to 1.0 mA at an interval of 0.1 mA or altering the reset voltage from 1.0 to 1.7 V at an interval of 0.05 V. 8 low resistance states (LRS) and 11 distinguishable high resistance states (HRS) are observed during consecutive set and reset process, respectively. Moreover the resistance can be continuously reduced or raised between multiple intermediate states without going back to the original state, which is key for electronic synapse [14]. The device conductivity decreases continuously with six easily recognized states after exerting sweep positive bias voltage from 0 to 1.4 V six times and the elevated conductivity with difficultly distinguishable ones after sweep negative pulse voltage from 0 to �0.6 V (**Figure 1c** and **d**), indicating the conductance change caused by consecutive potentiating or depressing signals. It can be attributed to the dynamic change of oxygen vacancy concentration and distribution in asymmetric bilayer structure of HfOx/ZnOx under various electrical signals [15].

A series of pulse signals were designed and applied to the memristor to test the important STDP rule in the Hebbian learning theory, as seen in the insets of I and III of **Figure 2**, including the V�/V+ = �1.0 V/1.0 V pulse pair signal as a presynaptic and postsynaptic spike with the 3 s interval time. Such design can prevent from the disturbance of excitatory postsynaptic current [16]. The time interval between the final presynaptic spike and the initial postsynaptic spike is defined as the relative time of Δt. The relative change of the synaptic weights (ΔW) is defined as:

$$
\Delta W = (\mathbf{I}\_2 \mathbf{-I}\_1) / \mathbf{I}\_1 \times \mathbf{100} \mathbf{@} \tag{1}
$$

The initial postsynaptic or presynaptic current I1 was used as the control value. After the spike pair was applied and over for 5 min, the measured presynaptic or postsynaptic current was I2.

The dependence of Δ*W on* Δ*t of* Pt/HfOx/ZnOx/TiN in **Figure 2** II and IV follows the STDP learning rule. While the presynaptic spike happens before the postsynaptic spike (Δ*t* < 0), synaptic weights enhance, indicating long-term potentiation (LTPo); while the presynaptic spike appears after the postsynaptic spike, synaptic weights become small (Δ*t* > 0), implying long-term depression (LTD). And the shorter the Δ*t* between the two spikes, the larger the Δ*W*. The STDP data points of memristor in **Figure 2** show evident statistical scatter, similar to the biological synapse.

In addition, Pt/HfOx/ZnOx/TiN device also exhibits the nonlinear transmission efficiency, and the transition from STP to LTP (not shown here) [15].

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

#### **Figure 2.**

*Emulation of STDP learning rule in Pt/HfOx/ZnOx/TiN memristive device—The relative change of the memristor synaptic weight (ΔW) versus the relative spike timing (Δt). And the solid line is the fitting exponential curve to the experimental data. The insets illustrate various spike schemes. The pulse pair comprises a positive and a negative voltage pulse with amplitude of 1.0 V and width of 50 ms. The interval between the two pulses is Δt ms (t = 10n, n = 1, 2, … , 10). The current compliance is not set in the whole emulation process. The current values are read at 0.1 V after 5 min of the spikes [15].*
