**3. Conclusion**

Our asymmetric bilayer-structured memristors fabricated by ALD/MLD and their main memristive features are summarized in **Table 4**, including set/reset voltage, ON/OFF ratio, and some important synaptic functions. Some similar work with asymmetric bilayer structure has also been listed in **Table 4** for comparison. It can be seen that all memristors with asymmetric bilayer structure exhibit better resistive switching performance. Our memristors have relatively thinner functional layers, relatively smaller ON/OFF ratio and emulate more artificial synaptic functions such as LTPo, LTD, the transition from STP to LTP, PPF, STDP, and conditional reflex (CR). The memristive mechanism of our bilayer-structured ultrathin device has been proposed to explain the synaptic plasticity based on oxygen vacancies migration/diffusion model. The non-uniform distribution of oxygen vacancies in asymmetric bilayer memristors plays the crucial role in affecting the linkage/ rupture of conductive filaments.

In light of these promising results and the fabrication compatibility with semiconductor industry, the ALD/MLD-derived bi-layer ultrathin memristor devices have tremendous potential as billions of electronic synapses in next-generation artificial neural network and flexible electronics.


#### **Table 4.**

*Comparison of main memristive features of our ultrathin memristors fabricated by ALD/MLD and other asymmetric bilayer-structured memristors.*

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