溶液法制备ZnO多晶薄膜电阻开关耐受性衰退物理机制

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

利用溶液法制备出平整致密的ZnO多晶薄膜,微结构观测分析表明ZnO晶粒为六方纤锌矿结构,平均粒径约为23.7 nm,由薄膜样品的紫外-可见光吸收谱计算出其光学带隙宽度约为3.3 eV.Ag/ZnO/ITO三明治结构单元的电流-电压曲线呈现出稳定的双极性电阻开关特性:置/复位电压小于±0.4 V,在-0.1 V的读取电压下可获得10~3–10~4的高/低电阻值比,明显优于类似溶液法制备ZnO薄膜的电阻开关性能.然而,在周期性电场力作用下,ZnO多晶薄膜内定向漂移的自由氧离子逐渐被晶格氧空位捕获成为不可移动的晶格氧原子.膜内氧空位缺陷浓度的逐渐降低导致膜内氧空位导电细丝通道越来越细,器件无法长时间维持稳定的低电阻态.因此,随着循环周期数的增加,器件的低电阻态逐渐向高电阻态衰退,直至电阻开关窗口消失.

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 Zn O 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/Zn O/ITO memory cells exhibit reversible low-voltage bipolar resistive switching characteristics.Their switching voltages(VSet/VReset)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 Zn O 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 Zn O film before and after I-V measurement demonstrate that as-grown film is oxygen-defective ZnO_x<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 asgrown non-stoichiometric Zn Ox<1 polycrystalline films.Reduce of oxygen-vacancy concentration results oxygenvacancy 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.