Low-voltage resistance switching of solution-processed ZnO nanoparticle films

Abstract

Resistive random access memory (ReRAM) has been considered as one of the most promising next-generation nonvolatile memory devices. ZnO is one kind of direct-band-gap semiconductors exhibiting good thermal stability, abundant raw materials for synthesis and low preparation cost. It shows extensive application potential in light-emitting diodes, photocatalysis and perovskite solar cells. Compared to ZnO bulk, nano-sized ZnO exhibits many special characteristics, such as non-toxic, non-migratory, fluorescence, piezoelectricity and conductivity. Moreover, ZnO is also one kind of transition metal oxides exhibiting resistive switching properties. There are many methods to synthesize nano-ZnO, such as chemical vapor deposition, sol-gel method and hydrothermal synthesis method. The common method to synthesize ZnO nanoparticles is the solution method using methanol as the reaction solvent. This method needs to heat methanol to 80°C, while the boiling point of methanol is only 64.7°C. Thus, this method is not safe. In this paper, ZnO nanoparticles were prepared by solution method using ethanol as the reaction solvent, which make the reaction temperature decrease to 40°C. Then, ZnO nanoparticles were dissolved in anhydrous ethanol to prepare a smooth, transparent and dense ZnO nanoparticle film by means of spin-coating under optimized preparation conditions. X-ray diffraction spectrum and scanning electron microscope image demonstrate polycrystalline ZnO nanoparticles with an average particle size of 7–10 nm were obtained. Ultraviolet-visible absorption spectra reveal the optical band gap of the n-type ZnO nanoparticle film is about 3.34 eV. Current-voltage curves of the ITO/ZnO/Al capacitor structure exhibit excellent bipolar resistive switching characteristics: the Set/Reset voltage is as low as ± 0.2 V, and the high/low resistance ratio is more than 100 at the reading voltage of 0.18 V, which is expected to be applied in the next generation of low-power-consumption nonvolatile memory devices. The formation/rupture of conductive Zn-atom filaments due to the electrochemical redox reaction of ZnO molecules induced by the external electric field is responsible for the resistive switching.