Effect of electrode roughness on electroforming in HfO2 and defect-induced moderation of electric-field enhancement

The roughness of Pt electrodes is shown to have a direct impact on the electroforming characteristics of Pt/Ti/HfO2/Pt device structures. Specifically, an increase in roughness leads to a reduction in the electroforming voltage of HfO2, an increase in the failure rate of devices, and a corresponding reduction in resistive switching reliability. A finite-element model is used to investigate the significance of local electric-field enhancement on the breakdown process. This simulation shows that high-aspect-ratio asperities can produce field enhancements of more than an order of magnitude but that the generation and redistribution of defects moderate this effect prior to dielectric breakdown. As a consequence, the effect of field enhancement is less than anticipated from the initial electric-field distribution alone. Finally, it is argued that the increase in the device failure rate with increasing electrode roughness derives partly from an increase in the film defect density and effective device area and that these effects contribute to the reduction in breakdown voltage.