Construction And Electrical Properties Of ZnO Thin Film Varistors With Planar Potential Barriers

Varistor material is an electronic component whose resistance is affected by the value of the voltage applied to it,and which drops sharply when the voltage rises to a certain value.Varistor ceramic components with operating voltages of hundreds and thousands of volts have been relatively well developed.With the rapid development of microelectronic circuits and the trend toward miniaturization of electronic components,varistor elements that operate at low voltages are widely demanded.However,thin film varistors operating at lower voltages still have some problems to be solved,such as poor sample performance consistency and low current withstanding capability.In this paper,ZnO-based thin film varistor with planar barrier structure is prepared.The ZnO layer,interface layer and ZnO layer are prepared sequentially by layered preparation in this study,and the relationship between the composition structure and electrical properties of thin film varistor is systematically investigated.In the third,fourth and fifth chapters of this paper,multilayer thin film structures with a single-component interfacial layer are first investigated.Bi₂O₃,MnO₂ and Co₃O₄ are used as the interface layers,respectively,and the roles of these three elements in the formation of Schottky barriers are also investigated.In order to further improve the current tolerance of thin film varistors,planar barrier structures with multi-component interface layers are prepared in Chapters 6 and 7,and the effect of multi-component interface layers on the improvement of current tolerance is investigated.1.The ZnO/Bi₂O₃/ZnO multilayer thin film structure was prepared by magnetron sputtering on a silicon substrate,and the effects of Bi₂O₃ layer thickness,annealing temperature,and annealing atmosphere on the electrical properties of the multilayer thin film structure were systematically investigated.As the thickness of Bi₂O₃ layer increases,the nonlinear coefficient of the multilayer thin film structure increases first and then decreases.Under the premise of suitable Bi₂O₃ layer thickness,the nonlinear characteristics of the multilayer thin film structure are obviously affected by the annealing temperature.The nonlinear coefficient of the multilayer thin film structure first increases gradually with the increase of the annealing temperature,and when the annealing temperature reaches 600°C,the nonlinear coefficient of the thin film decreases due to the aggregation and shrinkage of the Bi₂O₃ layer.At a suitable annealing temperature,the oxidizing atmosphere can improve the nonlinear coefficient of the multilayer film structure.The nonlinear coefficient of the film can be increased to about 15 by MnO₂doping,but the nonlinearity is not stable due to the inhomogeneous mixing of Bi₂O₃ and MnO₂layers.2.The ZnO/MnO₂/ZnO（ZMZ）multilayer film structure was obtained by sequentially sputtering ZnO,MnO₂ and ZnO layers on the substrate,and the effects of annealing temperatures on the microscopic composition and structure as well as the electrical properties of the multilayer film structure were also investigated.During the annealing process,the surface morphology of the ZnO layer changes and the MnO₂ layer located between the two ZnO layers is gradually absorbed into the ZnO layer and reacts with ZnO to generate a large number of zinc vacancies at the interface.The best nonlinear coefficient of 30.1 is obtained at an optimum annealing temperature of 400°C,and the minimum leakage current reaches 6.3μA.3.The ZnO/Co₃O₄/ZnO（ZCZ）multilayer film structure was prepared by sputtering using Co₃O₄ alone as the potential barrier forming layer,and the effects of annealing temperature on the structural composition,microstructure and electrical properties of the multilayer film were systematically investigated.During the annealing process,Co elements penetrate into the ZnO layer surface due to the change of the ZnO layer surface morphology and react with defects between the ZnO grains to generate zinc vacancies and form Schottky barriers at the interface.The maximum nonlinearity value of 26.4 and the minimum leakage current of 8.0μA were obtained at an annealing temperature of 400°C.4.To further improve the electrical properties of thin-film varistors,multi-component potential barrier layers need to be prepared,and the sol-gel method is more advantageous than the sputtering method in controlling the film composition.The ZnO/Bi₂O₃/ZnO multilayer film structure was prepared by the sol-gel method,and the effect of annealing temperature on the nonlinearity of the multilayer film structure was investigated.On the basis of the optimum temperature obtained from the above study,the Bi₂O₃ grain boundary layer doped with Co and Mn elements using the sol-gel method significantly improved the nonlinear properties of the multilayer film structure.However,because the ZnO layer prepared by sputtering is not dense enough,the multilayer film structure can only be maintained stable at lower temperatures,and at this temperature,the content of Zn vacancies generated by defect reactions is low,so the film nonlinearity is still not relatively low only about 10.7.5.In order to obtain higher nonlinear coefficient,the ZnO/Bi₂O₃-Co₃O₄-MnO₂/ZnO（ZBCMZ）multilayer film structure was constructed by using the sputtering method to prepare the ZnO layer and the sol-gel method to prepare a homogeneous composition containing Bi₂O₃-Co₃O₄-MnO₂ interfacial layer（BCM）,and the effect of annealing temperature on the nonlinear characteristics of the multilayer film structure was systematically investigated.The layered structure can remain stable at higher temperatures because the ZnO layer prepared by sputtering is dense enough.After annealing at 500°C,the maximum nonlinear coefficient of this planar barrier structure is 25.6 and the minimum leakage current is 2.4μA.Also,the maximum test current reaches 100 m A due to the uniform planar structure of the film.6.The potential barrier heights of three planar structures:ZMZ,ZCZ,and ZBCMZ were measured using the C-V test method.The measured barrier heights at 10 k Hz are 0.83 e V,0.80e V,and 0.94 e V,respectively.