Effects of annealing and dopant concentration on the optical characteristics of ZnO:Al thin films by sol–gel technique

ZnO:Al thin films doped with different aluminum concentrations were deposited on (0 0 0 1) sapphire substrates by the sol–gel technique. Thermal annealing of ZnO:Al films was carried out in an argon ambient at various annealing temperatures from 600 to 950 °C. The effects of thermal annealing and dopant concentration on the optical properties of ZnO:Al thin films were investigated. It is found that near band edge UV emission is enhanced by increasing the annealing temperature and dopant concentration. But defect-related deep-level emission decreases with the increasing dopant concentration and thermal annealing has little influence as the deep-level emission. The optical band gap of ZnO:Al films increases from 3.21 to 3.25 eV on increasing the dopant concentration from 0.01% to 1%. The optical transmittance decreases in the visible region, while it increases in the ultraviolet region with an increase in the annealing temperature.