Defect-Engineered Electroforming-Free Analog HfO 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.