| 其他摘要 | Aluminum doped zinc oxide films (ZAO) possess some unique characteristics including excellent optical and electrical properties, high cost efficiency, non-toxicity, stability in hydrogen etc., and are therefore potential substitutes for tin doped indium oxide films (ITO), gaining much attention in recent years because of their potential application. The problem exists for ZAO films as transparent conductive electrodes when it comes to the preparation of large-area and high-quality films. Sol gel method, as a cost-effective technique, has the advantage of preparing large-area homogeneous films. However, the films have bad quality, especially in electrical properties. In the present study, some fundamental aspects during the preparation of ZnO based films by sol gel method were investigated to provide improved understanding of sol gel process. It is expected that this work can provide some experimental and theoretical support for the preparation of high-quality films by sol gel method.
ZnO based films were prepared by sol gel dip coating process using 2-methoxyethanol solutions of zinc acetate stabilized by monoethanolamine, with aluminum chloride as dopant sources. The crystallization behavior was investigated by changing parameters during heat treatment. Then the suppression of crystal growth due to aluminum doping and scattering mechanism of carriers in the film were discussed by comparing the microstructure and properties of the film before and after doping. Furthermore, vacuum annealing and plasma treatment were carried out and the mechanism for the improvement in electrical properties after vacuum annealing was proposed.
The results show that, heating temperature and heating time have an obvious effect on the microstructure of ZnO films. At higher temperature, the grain size becomes larger and columnar structure predominates in the films. And the transition from granular structure to columnar structure can be observed with the extension of heating time at fixed heating temperature, which suggests that the formation of columnar structure was kind of grain growth process with an incubation time above critical temperature. While for ZAO films heated at the same temperature for different time, the situation is different. The grain size keeps almost constant and granular structure is popular in the films during the whole process. It seems that the variation of heating time makes little change in the microstructure in the films. It is believed that the segregation of aluminum atoms at the grain boundary suppresses the grain growth process, resulting from the increase of activation energy and the decrease of growth rate.
Aluminum doping not only influences the microstructure of ZnO films, but also increases the carrier concentration in the films, which means different scattering mechanism will take charge before and after doping. For undoped ZnO films with lower carrier concentration, the Hall mobility is mainly subjected to grain boundary scattering, determined by grain boundary potential and grain size. The potential can be influenced by the density of trap states at the grain boundaries and the evolution of microstructure can effectively decrease the density of trap states, resulting in the increase of carrier concentration. Ionized impurity scattering will then take the place of grain boundary scattering with the carrier concentration in the films above the critical value. In the case of ZAO films, the carrier concentration can be increased remarkably by doping and the effect of grain boundary on the Hall mobility can be neglected. Consequently, ionized impurity scattering and neutral impurity scattering predominates in the films and have much effect on Hall mobility of the carriers. The increase of dopant concentration will result in the formation of more neutral scattering centers because of low doping efficiency. Thus the effect of neutral impurity scattering will become more prominent.
The electrical properties of the films can be obviously improved by vacuum treatment. The resistivity decreases with the increase of annealing temperature and the cooling rate can also influence the electrical properties. The resistivity can be further decreased by rapid cooling. It is thought that chemisorbed oxygen on film surface and at grain boundary can be partly removed after vacuum annealing, releasing some carriers trapped by oxygen, which can decrease the resistivity of the films. higher annealing temperature is helpful to activate the out-diffusion process of oxygen chemisorbed at the grain boundary and the reverse diffusion process can be partly prevented by rapid cooling, which can effectively decrease the concentration of oxygen at the grain boundary. Plasma treatment provides some information about the effect of active particles on the improvement in electrical properties, which is favorable in decreasing the temperature of treatment process and achieving the improvement in properties by low-temperature process. |
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