Metamorphosis of strain/stress on optical band gap energy of ZAO thin films via manipulation of thermal annealing process

We report on the growth of Al-doped ZnO (ZAO) thin films prepared by the sol-gel technique associated with dip-coating onto Corning 7740 glass substrates. The influence of varying thermal annealing (Ta) temperature on crystallisation behaviour, optical and electrical properties of ZAO films has been systematically investigated. All films are polycrystalline with a hexagonal wurtzite structure with a preferential orientation according to the direction 〈0 0 2〉. The metamorphosis of strain/stress effects in ZAO thin films has been investigated using X-ray diffraction. The as growth films have a large compressive stress of 0.55 GPa, which relaxed to 0.25 GPa as the Ta was increased to 500 °C. Optical parameters such as optical transmittance, absorption coefficient, refractive index and optical band gap energy have been studied and discussed with respect to Ta. All films exhibit a transmittance above 80-90% along the visible-NIR range up to 1500 nm and a sharp absorption onset below 400 nm corresponding to the fundamental absorption edge of ZnO. Experimental results show that the tensile stress in the films reveals an incline pattern with the optical band gap energy, while the compressive stress shows opposite relation.