Correlation between structural, optical, and electrical properties of sol–gel-derived ZnO thin films

Abstract

To study their structural, electrical, and optical properties, samples of Zinc oxide (ZnO) thin films were deposited from zinc acetate dehydrate (ZAD) sol of different sol concentrations prepared by using sol–gel technology and spin coating process. After deposition, the deposited thin layers were cured at different sintering temperatures. The impact of changes in sol molarity and sintering temperature on the microstructure properties of the ZnO films was studied by X-ray diffractometry (XRD). The microstructure results of grain orientation, crystallite size, lattice strain, and residual stress were measured for different sol concentrations and different sintering temperatures. From the optical measurements along wavelengths from 200 to 2200 nm, all ZnO thin films exhibited a very high transmittance (~90%) in the infrared and visible light range. At wavelengths of ultraviolet range (λ ≤ 400 nm) the value of transmittance decreased sharply owing to the high value of optical gap energy of ZnO thin layers. The estimated value of optical gap energy augmented from 3.24 eV to 3.55 eV when the solution molarity raised from 0.2 to 0.8 M, respectively. For ZnO thin films of 0.4 M concentration, the band-gap energy increased with an excess in the sintering temperature. The electrical resistivity of the ZnO film decreased with an augment in sol concentration from 0.2 to 0.4 M and its value increased by increasing the sol concentration. The lowest electrical resistivity was 22.0 Ω.cm, which was achieved after sintering 0.4 M ZnO film at 550°C for 1 h followed by annealing at 500°C in forming gas for 30 min. The variation of microstructural parameters was correlated strongly with the electrical and optical properties of the ZnO thin films. The relationship between the structural, optical and electrical parameters is discussed.