A Comparative Study Of The Thermal Stability Of Aluminum-and Gallium-Doped ZnO Thin Films Prepared By Sol-Gel Method

Transparent conductive oxides (TCOs) have been extensively developed for the applications in optoelectronics devices because of their excellent electrical and optical properties. Indium-tin-oxide (ITO) currently used in industry is a relatively expensive material because indium is not abundant. Therefore, much attention has been attracted for the development of substitute materials for ITO.Doped ZnO is the most widely studied material for ITO substitution. It has been reported that Al-and Ga-doped ZnO (AZO and GZO) films have a low resistivity and a high transparency that are comparable to those of ITO films. For the practical use of AZO and GZO films as transparent electrodes in a wide range of applications, there are some critical issues to be resolved; one of them is the thermal stability. In the fabrication process of optoelectronics devices, such as liquid crystal display and thin film solar cells, the electrodes are generally exposed to a high temperature even higher than the conventional deposition temperature. Under such environments, the electrical and optical properties of the films must be maintained. However, it has been reported that the resistivity of AZO or GZO films changes by post-deposition annealing. Moreover, the preparation methods can directly affect the quality of the films and then result in different thermal stability. Sol-gel method has attracted much attention for preparation of AZO and GZO films because this method has advantages of simple deposition equipments, inexpensive price, easier adjustment of doping concentration at molecular level and possibility of fabricating large-area films at room temperature. However, the thermal stability of the AZO and GZO films prepared by sol-gel method has not been reported.To address these issues, the thermal stability of AZO and GZO films prepared by sol-gel method is studied and compared. Firstly, the effect of process parameters, such as sol-gel concentration, doping concentration, annealing temperature and annealing ambient on the electrical properties of these films is examined. And according to the optimal process parameters, the resistivity of the AZO and GZO prepared is about0.07Ωcm and the transmittance is above85%. Secondly, the effect of doping element, doping concentration and prepared method on the thermal stability of the AZO and GZO films in air is studied; it has been found out that the AZO and GZO films have similar thermal stability, and the doping concentration has no prominent effect on the thermal stability. The resistivity and transmittance of these films maintained unchanged when the annealing temperature is lower than250℃and then increase rapidly. Thirdly, we studied the thermal stability of AZO and GZO films in air, nitrogen and vaccum. It found out that the thermal stability of the AZO and GZO films exists big difference in the three different ambient. The films have best thermal stability in vaccum. The properties of the films keep unchanged in vaccum when the annealing temperature is500℃. And the films have worst thermal stability in air.