Effect of concentration, aging, and annealing on sol gel ZnO and Al-doped ZnO thin films

Presented are experimental results on the effect of concentration, aging, and annealing time on the optical and structural properties of sol gel zinc oxide (ZnO) and Al-doped ZnO thin films. ZnO and ZnO:Al thin films were fabricated on glass substrates using spin coating followed by annealing. XRD confirmed that the films are polycrystalline wurtzite. For low concentration films (0.2 and 0.4 M), grain size increased with aging time up to 72 hours. For high concentration samples (0.6 and 0.8 M), grain size increased only up to 48 hours. Additional aging resulted in a decrease in the grain size. The largest grain sizes were found for 0.4 M at 72 hours and 0.6 M at 48 hours. The band gap tended to decrease with increasing aging time for all concentrations. The smallest band gap for each aging time (24, 48, and 72 hours) was observed for 0.6 M films. These results suggest that higher concentration sol gel near 0.6 M may yield better properties with shorter aging times than 0.2 and 0.4 M films. Annealing data suggests that 350 ^oC is the minimum annealing at 1 hour to achieve high-quality films and higher concentration ZnO films have stronger diffraction peaks. ZnO:Al also exhibits stronger diffraction peaks and a larger blue shift of the band edge with increasing sol gel concentration.