Thermal annealing-induced formation of ZnO nanoparticles: Minimum strain and stress ameliorate preferred c-axis orientation and crystal-growth properties

Nanocrystalline zinc oxide (ZnO) thin films have been deposited on glass substrates using a sonicated sol-gel dip-coating technique at various thermal annealing (Ta) temperatures. The Ta temperature was varied to range from 300 to 600 °C in intervals of 50 °C in an open atmosphere. To obtain desirable piezoelectric properties, the correlations between Ta and the characteristics of ZnO thin films (crystallisation, optical and electrical behaviour) were investigated. The as-deposited films have large compressive stresses of 0.49 GPa, which relaxed to 0.27 GPa as the Ta temperature increased to 500 °C. Optical parameters, such as optical transmittance, absorption coefficient and energy band gap, have been studied and discussed with respect to Ta. All films exhibit a transmittance above 50% in the visible region. It was found that the compressive stresses in the films cause a decrease in the optical band gap, whereas the tensile stress reveals an incline pattern with the optical band gap. This result corroborated with the crystallinity along the c-axis plane. The highest crystallinity value was achieved at the lowest stress value. An identical trend was observed for the resistivity values from the I-V measurement. Moreover, an increase in the crystallite size from 10 to 39 nm as the level of Ta increased was noticed. The thickness of the films also decreased when Ta increased, and denser films were obtained as a result. A qualified ZnO thin film with good piezoelectric properties has been prepared using a sonicated sol-gel dip-coating technique with various Ta. Experimental results show that Ta has the greatest influence on the final properties of the ZnO thin films.