Annealing induced transformation and enhancement in the electronic defect states of Al doped ZnO films and their correlation with the optoelectronic properties

In this study, we report the transformation and/or enhancement of the electronic defect states by performing several annealing experiments on the same samples. To this end, Al-doped ZnO (AZO) films on the soda lime glass substrates were deposited by using sol-gel followed by spin coating process and then subjected to various annealing conditions at different pressure, like, air, low vacuum (1 × 10⁻² mbar) and/or high vacuum (1 × 10⁻⁶ mbar); each at 500 °C for 1 h. A wide range of experimental techniques was used to characterize the overall microstructure and optoelectronic properties of these films after each annealing stage. Though the overall microstructure of all these films are found to change only marginally, the electronic defect states are found to significantly alter because of these repetitive annealing, leading substantial change in their electrical properties and also affecting these optically. Moreover, the effectiveness of Al doping is found to increase significantly because of the second annealing step, leading an increase in carrier concentration in all these films for a wider range of initial dopant concentration (0.0–3.0 at. %).