Effects of thermal annealing on analog resistive switching behavior in bilayer HfO/ZnO synaptic devices: the role of ZnO grain boundaries

The effects of thermal annealing on analog resistive switching behavior in bilayer HfO 2 /ZnO synaptic devices were investigated. The annealed active ZnO layer between the top Pd electrode and the HfO 2 layer exhibited electroforming-free resistive switching. In particular, the switching uniformity, stability, and reliability of the synaptic devices were dramatically improved via thermal annealing at 600 °C atomic force microscopy and X-ray diffraction analyses revealed that active ZnO films demonstrated increased grain size upon annealing from 400 °C to 700 °C, whereas the ZnO film thickness and the annealing of the HfO 2 layer in bilayer HfO 2 /ZnO synaptic devices did not profoundly affect the analog switching behavior. The optimized thermal annealing at 600 °C in bilayer HfO 2 /ZnO synaptic devices dramatically improved the nonlinearity of long-term potentiation/depression properties, the relative coefficient of variation of the asymmetry distribution σ / μ , and the asymmetry ratio, which approached 1. The results offer valuable insights into the implementation of highly robust synaptic devices in neural networks. Schematic illustration of the transition mechanism during the conductive filamentary formation in bilayer HfO 2 /ZnO synaptic devices (a) without and (b) with thermal annealing.