| Zinc Oxide is an important wide band-gap oxide semiconductor, which has potential application in a wide range of optoelectronic devices, eg. solid-state short-wavelength light sources. Due to this reason, in recent years ZnO-related topics, including the realization of p-type doping in ZnO thin films, have attracted much attention. At the same time, Zinc Oxide and its alloys have been frequently utilized as building blocks for varistors. With the IC technology advancing, development of compact low-voltage varistors is in increasingly urgent demand. In this paper, Ag-doped ZnO thin films were prepared by reactive pulsed DC magnetron sputtering technique and then underwent thermal treatment to achieve p-type conduction. On the other hand, using thermal evaporation and oxidation methods, Bismuth Oxide-Zinc Oxide bilayers and multilayers were prepared and treated in order to realize the bismuth-zinc transition junctions, so as to study the mechanism and performance of the ZnO-based varistor devicesFirstly, the ZnO and ZnO:Ag films were grown on glass substrates by means of reactive sputtering. Morphological inspection by SEM showed that the average grain size and the density of the films depend on the sputtering parameters used, which among others include Ar/02 flow ratio and post-growth annealing temperature. ZnO and ZnO:Ag films grown with Ar/02 flow ratio of 30:8 were found to be the densest, and the average grain size of the films increased with increasing annealing temperature. Post-growth annealing performed at temperatures around or higher than 600℃resulted in abnormal growth of the grains. XRD scans on all the ZnO:Ag samples showed only (002) diffraction peaks, indicating that the films were c-axis oriented. The observed diffraction peaks shifted to higher angle after the annealing process, corresponding to decreasing lattice constant along the c-axis. Hall measurements revealed that thermal annealing in oxygen atmosphere at 600℃for 60min turned the as-grown n-type ZnO:Ag films into p-type conduction, with lower resistivity. Secondly, metallic zinc films prepared by thermal evaporation were annealed in air and pure argon atmosphere. Treatment in air produced small spherical grains of zinc oxide, but the density of the films was not sufficient. On the other hand, XRD analysis showed that treatment in argon at 400℃for 180min turned the samples into zinc oxide films, which were dense and smooth, and the average grain size was determined to be 24.8nm. The oxidation took place because of the penetration of small amount of oxygen into the furnace and its slow chemical reaction with the zinc films, thus facilitating the formation of large grains and the improvement of film quality.Measurement of the breakdown characteristics on the Bi2O3-ZnO bilayers revealed the maximal nonlinear coefficient to be 8 and the breakdown voltage to be 2.3V, which matches the single-crystal breakdown voltage value. The number of the layers can be adjusted to tune the breakdown voltage. Using the semiconductor theory, by analyzing the temperature-dependence measurement results, we obtained the Schottky barrier height of the bilayers. It's believed that the observed Schottky barrier is the result of the formation of the bismuth-zinc transition layer and its junction effect, and the device's performance relates closely to its microstructure that's in turn the result of the thermal treatment.
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