Variation in electrical and structural properties of Ga-doped ZnO films caused by oxygen out-diffusion

Abstract

The effect of oxidation and the subsequent vacuum annealing on the electrical and structural properties of Ga-doped ZnO (GZO) films prepared by sputtering was investigated with respect to annealing temperature and time. The annealing of the GZO films in air caused severe reduction in carrier density, but subsequent vacuum annealing partially recovered the carrier density. This change in carrier density indicates that oxygen atoms are easily incorporated into and released from the GZO film. As the vacuum annealing time increased at an annealing temperature of 500 °C, the recovery of the carrier density peaked and then decreased. This annealing-time dependence is attributed to the co-desorption of oxygen and zinc and indicates that oxygen diffusion is faster than that of zinc in Ga-doped ZnO films. Air annealing reduces the carrier density and mobility of films. However, subsequent vacuum annealing results in the recovery of the carrier density and the enhancement of mobility, depending on the annealing temperature. The enhancement of mobility suggests that an adequate arrangement of incorporated oxygen atoms at the grain boundary reduces carrier scattering in GZO films.