Defect-Engineered Electroforming-Free Analog HfO_x Memristor and Its Application to the Neural Network

The thin-film growth conditions in a plasma-enhanced atomic layer deposition for the (3.0–4.5) nm thick HfO2 film were optimized to use the film as the resistive switching element in a neuromorphic circuit. The film was intended to be used as a feasible synapse with analog-type conductance-tuning capability. The 4.5 nm thick HfO2 films on both conventional TiN and a new RuO2 bottom electrode required the electroforming process for them to operate as a feasible resistive switching memory, which was the primary source of the undesirable characteristics as the synapse. Therefore, electroforming-free performance was necessary, which could be accomplished by thinning the HfO2 film down to 3.0 nm. However, the device with only the RuO2 bottom electrode offered the desired functionality without involving too high leakage or shorting problems, which are due to the recovery of the stoichiometric composition of the HfO2 near the RuO2 layer. In conjunction with the Ta top electrode, which provided the necessary oxygen vacancies to the HfO2 layer, and the high functionality of the RuO2 as the scavenger of excessive incorporated oxygen vacancies, which appeared to be inevitable during the repeated switching operation, the Ta/3.0 nm HfO2/RuO2 provided a highly useful synaptic device component in the neuromorphic hardware system.