Effect of Annealing on the Structural and Optical Properties of ZnO/ITO and AZO/ITO Thin Films Prepared by Sol-Gel Spin Coating

This paper aims to investigate the effect of annealing on the structural and optical properties of ZnO/ITO and AZO/ITO thin films. In the preparation of ZnO and AZO films, zinc acetate dehydrate (Zn(CH3COO)2.2H2O), ethanol, diethanolamine (DEA), and AlCl3 were used as a starting material, solvent, stabilizer, and dopant sources, respectively. Both ZnO and AZO films were fabricated on ITO (indium tin oxide) substrates using the spin coating technique at room temperature with a rotating speed of 3,000 rpm in 30 s. The films were heated at various temperatures in the temperature range of 400 - 600 °C for 60 min. The crystallite size of the film is calculated using Debye-Scherrer and Williamson-Hall Methods. Based on the UDM results, the crystallite size of ZnO/ITO and AZO/ITO films increases after annealing in comparison with the films before annealing.  From the optical UV-Vis measurements, there was an increase in the transmittance value of the samples after annealing. The transmittance value of ZnO/ITO and AZO/ITO films increases from 40 % before annealing to approximately 80 and 90 %, respectively after annealing. The increase in the transmittance valued in both ZnO/ITO and AZO/ITO after annealing is mainly due to an improvement in the crystalline phase of these films. The band gap energy of ZnO and AZO films is reduced with increasing annealing temperatures, from 3.26 eV before annealing to 3.19 eV for ZnO and 3.23 eV for AZO films after annealing at 600 °C. 
HIGHLIGHTS 
 
The sol-gel spin coating method was used to study the effect of annealing on the structural and optical properties of ZnO/ITO and AZO/ITO films 
The transmittance valued in both ZnO/ITO and AZO/ITO after annealing increase as a result of an improvement in the crystalline phase of these films 
Reducing the amorphous phase and the increase of the crystallite size of the films are the main reason for narrowing the band gap energy of the ZnO/ITO and AZO/ITO films 
