Ultrathin HfO₂/Al₂O₃ bilayer based reliable 1T1R RRAM electronic synapses with low power consumption for neuromorphic computing

Neuromorphic computing requires highly reliable and low power consumption electronic synapses. Complementary-metal-oxide-semiconductor (CMOS) compatible HfO₂ based memristors are a strong candidate despite of challenges like non-optimized material engineering and device structures. We report here CMOS integrated 1-transistor-1-resistor (1T1R) electronic synapses with ultrathin HfO₂/Al₂O₃ bilayer stacks (<5.5 nm) with high-performances. The layer thicknesses were optimized using statistically extensive electrical studies and the optimized HfO₂(3 nm)/ Al₂O₃(1.5 nm) sample shows the high reliability of 600 DC cycles, the low Set voltage of ∼0.15 V and the low operation current of ∼6 µA. Electron transport mechanisms under cycling operation of single-layer HfO₂ and bilayer HfO₂/Al₂O₃ samples were compared, and it turned out that the inserted thin Al₂O₃ layer results in stable ionic conduction. Compared to the single layer HfO₂ stack with almost the same thickness, the superiorities of HfO₂/Al₂O₃ 1T1R resistive random access memory (RRAM) devices in electronic synapse were thoroughly clarified, such as better DC analog switching and continuous conductance distribution in a larger regulated range (0-700 µS). Using the proposed bilayer HfO₂/Al₂O₃ devices, a recognition accuracy of 95.6% of MNIST dataset was achieved. These results highlight the promising role of the ultrathin HfO₂/Al₂O₃ bilayer RRAM devices in the application of high-performance neuromorphic computing.