Fabrication And Resistive Switching Characteristics Of ZnO Based Thin Films

With the rapid development of semiconductor technology, the current mainstream memory, Flash, will confront serious technical bottlenecks. The RRAM(resistive random access memory) has been considered as a possible competitive candidate to replace Flash due to its simple structure, high response speed, low power consumption and strong compatibility. The present studies mainly focus on the investigation on the mechanism of resistance switching(RS), the selection of RS materials, the optimization of fabrication process and modification by doping.In this paper, single layer structure devices, Ag/NixZn1-xO/FTO and Ag/CuxZn1-xO/FTO, and bilayer structure devices, Ag/Ni O/Zn O/FTO,were fabricated by a sol-gel method. RS mechanism and the effect of Ni2+, Cu2+ doping and composite structure on RS behavior were investigated. In addition, Zn O array films were fabricated by a hydrothermal method. The structure, morphology and PL spectrum of hydrothermally derived array films were characterized, and the field emission and I-V curves were measured. The main contents are as follows:(1) Single layer structure devices, Ag/NixZn1-xO/FTO, were synthesized by a sol-gel method, and the effect of Ni2+ doping on RS behavior of the devices was investigated. The results indicate that the doping of Ni2+ effectively compensates the intrinsic defects, thus enhances the RS behavior with a Roff/Ron ratio of 104~105. The dispersivity of VSET is improved by the increase of Ni2+ doping content. RS behavior is mainly attributed to the formation and fracture of oxygen vacancy conductive filaments, and the leakage current conduction mechanisms accord with Space Charge Limited Current(SCLC) model.(2) Single layer structure devices, Ag/CuxZn1-xO/FTO, were fabricated by a sol-gel method, and the crystalline structure and I-V characteristics were investigated. The conduction mechanism is controlled by SCLC model, via the relationship between the concentration of thermally generated carriers and injected ones form the electrode. Cu2+ doping could improve the dispersivity of HRS resistance and VSET but had no significant effect on LRS resistance and VRESET, considering that the formation of a filament should be more random than the destruction of an existing one.(3) Bilayer structure devices, Ag/Ni O/Zn O/FTO, were fabricated by a sol-gel method, and the effect of compliance current(ICC) on RS behavior was investigated. The results indicate that Ni O/Zn O interface states play an important role in RS characteristics via the capture and release of injected electrons at the interface, leading to the variation of depletion layer width and resistance states. An appropriate value of ICC would not only protect the devices from permanent breakdown, but also contribute to a more stable RS characteristic. Trapped charges in different energy levels can be released by specific VRESET, therefore several distinct HRS resistances are achieved.(4) Zn O array films were fabricated by a hydrothermal method, and the effect of Cu2+ and ammonia on the structure, morphology, optical and I-V properties of Zn O films was investigated. The co-doping of copper and ammonia enhances the oriented growth of nanorods along c axis, and results in the bending of nanorods. A star-like film composing of the bundling of nanorods is formed with a high field enhancing factor. The dispersibility of VSET and the stability of HRS and LRS are improved.