Endurance degradation of solution-processed ZnO polycrystalline film-based resistive switching memory

Metal oxide-based resistive switching memory cell has potential applications in the next generation of nonvolatile memory devices due to its simple structure, fast reading/writing speed, and good endurance. Zinc oxide (ZnO) is one of excellent resistive switching candidate materials owing to its low-cost, simple preparation process and significant resistive switching characteristics. In this work, homogeneous ZnO precursor solution is spin-coated on indium tin oxide (ITO)-coated glass substrate to deposit a smooth and dense film composed by ZnO nanoparticles with the hexagonal wurtzite structure. Atomic force microscopy images reveal the average grain size of ZnO polycrystalline nanoparticle is about 23.7 nm. An optical band gap of about 3.3 eV is derivate from the ultraviolet-visible light absorption spectra. Current-voltage (I-V) curves of Ag/ZnO/ITO memory cells exhibit reversible low-voltage bipolar resistive switching characteristics. Their switching voltages (V-Set/V-Reset) are lower than +/- 0.4 V, and the ON/OFF resistance ratio is 10(3)-10(4) at the read voltage of -0.1 V, which are superior to resistive switching performance of other ZnO polycrystalline films prepared via similar solution methods. However, the memory cell can only maintain 60 stable I-V cycles. After that, the low resistance state gradually degrades to the high resistance state, and the resistive switching memory window completely disappears after 100 I-V cycles. X-ray photoelectron spectra of Zn 2p and O 1s core-levels for ZnO film before and after I-V measurement demonstrate that as-grown film is oxygen-defective ZnOx<1 film, but it becomes stoichiometric ZnO film after I-V measurement. Under sustained electric field stress, some drifted oxygen ions combine with oxygen vacancies and change to localized lattice oxygen atoms, this partially remedies the oxygen-deficiency of as-grown non-stoichiometric ZnOx<1 polycrystalline films. Reduce of oxygen-vacancy concentration results oxygen-vacancy conductive filaments become slender and easy to rupture. Thus, the memory cell can't stably stay on the low resistance state. With the gradual degradation from the low resistance state to the high resistance state, the resistive switching window gradually disappears.