*2.2.2 TiN/HfOx/AlOx/Pt inorganic memristor*

The memristor device based on TiN/HfOx/AlOx/Pt can also emulate the biological synapse. Usually, the synapse operates under pulse signals rather than DC bias sweep voltage. The LTPo and LTD phenomena can be observed in Pt/AlO*x*/HfO*x*/ TiN under 180 potentiating pulses (0.5 V, 10 ms) and 180 depressing pulses (1 V, 10 ms), as shown in **Figure 3a**. The connection strength can be dynamically modulated by the consecutive external signals, determining the transfer efficiency between the electronic neurons.

#### **Figure 3.**

*(a) Change in the response current under the influence of consecutive potentiating or depressing pulses. After pulse stimulation, a 0.1 V 100 ns reading pulse was applied to read the response current. After 0.5 V potentiating pulses, the response current gradually increases (long-term potentiation), while the response current gradually decreases after 1 V depressing pulses (long-term depression). (b) Response of a memristor device to different pulse programs [17].*

Further experiments have demonstrated that a pulse signal from amplitude of 1.0–1.5 V and pulse width of 50–100 ms leads to various current responses in **Figure 3b**. That is to say, the larger pulse amplitude, the longer pulse width, and the more pulse number will produce more significant response current change, which is analogous to long-term potentiation/depression of the human brain.

Synaptic plasticity can be divided into STP and LTP according to the timelines of enhanced synaptic connections. The repeated stimulation induced STP to LTP transition is illustrated in **Figure 4**. With increasing the rehearsal pulse number (*N* = 10, 40, 70, 100, 120), the resistance remaining becomes larger (**Figure 4a**). This procedure is similar to the Ebbinghaus forgetting curve related to human memory [18, 19].

An exponential decay equation was employed to depict the relaxation process:

$$M(t) = M\_\text{e} + (M\_0 - M\_\text{e}) \cdot \exp\ (-t/\tau) \tag{2}$$

where *M*(*t*), *M*0, and *M*<sup>e</sup> are the memory level at time *t*, *t* = 0, and at steady state after a long time, and *τ* of the relaxation time constant. The experimental and simulation results after 70 identical pulses are shown in **Figure 4b**, containing the dependence of *τ* on *N* in the inset. The decay rate is faster in the beginning and then becomes slower. The *τ* value increases from several seconds to 50 seconds with the training pulse number from 10 to 120, revealing a declining forgetting rate from �57% for *N* = 10 to �5% for *N* = 120. This confirms the transition from STP to LTP through repeated rehearsal and learning.

The STDP rule has been mimicked in TiN/HfOx/AlOx/Pt memristor, as indicated in **Figure 5**. The schematic of another training pulse signal with various amplitudes is shown in **Figure 5a**, different from the pulse design in **Figure 2**. A set of pulses (1 V, �0.5 V, �0.45 V, �0.4 V, �0.35 V, �0.3 V, �0.25 V)/(0.5 V, �1 V, �0.9 V, �0.8 V, �0.7 V, �0.6 V, �0.5 V) were used as pre-synaptic/post-synaptic stimulation signals, respectively. Some different pulse signals designed at various spike timings (Δ*t*) are designed and illustrated in **Figure 5b**. When the shortest Δ*t* (10 ms) is inserted to the device, the largest Δ*W* of 50% for potentiation and � 80% for depression are realized.

The memristive mechanism of asymmetric TiN/HfOx/AlOx/Pt memristor has been deeply investigated with the aid of x-ray photoelectron spectroscopy (XPS) depth analyses, which will be discussed in the following Section 2.3.

#### **Figure 4.**

*Repeated stimulation induced STP to LTP transition. (a) Resistance remaining decay curve recorded after 10, 40, 70, 100, 120 identical pulses (1.6 V, 10 ms). A 0.1 V voltage was used to read the device current. (b) Resistance remaining decay curve recorded after 70 identical pulses and the fitted curve according to Eq. (1). The inset plots the dependence of relaxation time* τ *on the pulse number.* τ *is obtained by the fitting curve [17].*

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

#### **Figure 5.**

*(a) Training pulse signal with different amplitudes design schematic loaded on the Pt/AlO*x*/HfO*x*/TiN memristor (assuming Δ*t *= 10 ms). Each pulse width and pulse interval are 10 ms, respectively. (b) Some different pulse signals designed at various spike timings (Δ*t*). (c) STDP-like curves. The relative change of the memristor synaptic weight (Δ*W*) versus the relative spike timing (Δ*t*) [17].*
