Characteristics of Al-doped ZnO thin films prepared in Ar + H2 atmosphere and their vacuum annealing behavior

The microstructure and electrical–optical properties of Al-doped ZnO (AZO) films have been studied as a function of H2 flux in the magnetron sputtering process at 150 °C and postannealing temperature in vacuum. As H2 flux increases in the sputtering gas, the AZO films deposited have a (002) preferred orientation rather than the mixed (100) and (002) orientations, the grain size shows a tendency to first increase then decrease, and (002) diffraction peak position is inclined to shift to higher angles first then to lower angles. The resistivity of the films first decreases then increases with H2 flux, and the lowest resistivity of 4.02 × 10−4 Ω cm is obtained at a H2 flux of 10 sccm. The average transmittance in the visible region shows little dependence on H2 flux. As a whole, the AZO films with higher values of figure of merit are obtained when the H2 flux is in the range of 6–12 sccm. The AZO films deposited in Ar and Ar + H2 exhibit different annealing behaviors. For the AZO film deposited in Ar, the grain size gradually increases, the stresses are relaxed, the resistivity first decreases then increases, and the average transmittance in the visible region is unchanged initially then somewhat decreased as annealing temperature is increased. The optimum annealing temperature for improving properties of AZO films deposited in Ar is 300 °C. For the AZO films deposited in Ar + H2, annealing does not significantly change the microstructure but increases the resistivity of the films; the average transmittance in the visible region remains unchanged initially but greatly reduced with further increase in annealing temperature. The carrier transport in the as-deposited and annealed films appears to be controlled by a mechanism of grain boundary scattering, and the value of Eg increases with the increase in carrier concentration due to Burstein–Moss effect.