Effects of Gibbs free energy difference and oxygen vacancies distribution in a bilayer ZnO/ZrO2 structure for applications to bipolar resistive switching

We investigated the effects of the Gibbs free energy difference (Delta G(o)) and distribution of oxygen vacancies in a bilayer ZnO/ZrO2 structure. The device exhibited high endurance characteristics of up to 1000 DC repetitive resistive cycles, long retention (10(4)), and a low coefficient of variation of SET and RESET-voltages (6% and 5%, respectively). In addition, X-ray photoelectron spectroscopy (XPS) analysis of the ZnO and ZrO2 layers indicated that the oxygen vacancies/defects in the ZnO layer (44.13%) were larger than in the ZrO2 layer (34.11%), and that the oxygen ions of O-Zr (65.89%) were larger than those of O-Zn (55.87). This XPS analysis confirmed that the differing oxygen vacancy distributions in the ZnO/ZrO2 layer are responsible for improving the switching performance. The switching behavior, endurance, and retention time strongly depend on the types of electrode and switching materials used. We obtained qualitative and quantitative evidence that Delta G(o) of the oxide materials plays a significant role in determining the resistive switching characteristics. Bipolar switching mechanisms are explained by considering Delta G(o) in the ZnO and ZrO2 layers, where the formation and rupture of conductive filaments are caused by oxygen vacancies. Our findings suggest that a bilayer ZnO/ZrO2 structure is promising for application to non-volatile memory.