Improved resistive switching characteristics of solution processed ZrO2/SnO2 bilayer RRAM via oxygen vacancy differential

In this study, a solution-processed bilayer structure ZrO₂/SnO₂ resistive switching (RS) random access memory (RRAM) is presented for the first time. The precursors of SnO₂ and ZrO₂ are Tin(Ⅱ) acetylacetonate (Sn(AcAc)₂) and zirconium acetylacetonate (Zr(C₅H₇O₂)₄), respectively. The top electrode was deposited with Ti using an E-beam evaporator, and the bottom electrode used an indium–tin–oxide glass wafer. We created three devices: SnO₂ single-layer, ZrO₂ single-layer, and ZrO₂/SnO₂ bilayer devices, to compare RS characteristics such as the I–V curve and endurance properties. The SnO₂ and ZrO₂ single-layer devices showed on/off ratios of approximately 2 and 51, respectively, along with endurance switching cycles exceeding 50 and 100 DC cycles. The bilayer device attained stable RS characteristics over 120 DC endurance switching cycles and increased on/off ratio ∼2.97 × 10². Additionally, the ZrO₂/SnO₂ bilayer bipolar switching mechanism was explained by considering the Gibbs free energy (ΔG ^o) difference in the ZrO₂ and SnO₂ layers, where the formation and rupture of conductive filaments were caused by oxygen vacancies. The disparity in the concentration of oxygen vacancies, as indicated by the Gibbs free energy difference between ZrO₂ (ΔG ^o = −1100 kJ mol⁻¹) and SnO₂ (ΔG ^o = −842.91 kJ mol⁻¹) implied that ZrO₂ exhibited a higher abundance of oxygen vacancies compared to SnO₂, resulting in improved endurance and on/off ratio. X-ray photoelectron spectroscopy analyzed oxygen vacancies in ZrO₂ and SnO₂ thin films. The resistance switching characteristics were improved due to the bilayer structure, which combines a higher oxygen vacancy concentration in one layer with a lower oxygen vacancy concentration in the switching layer. This configuration reduces the escape of oxygen vacancies to the electrode during RS.