The optical transmission spectra of the ZnO thin films are shown in
Fig. 7. All the films show the transmission well above 80% in the visible wavelength range.Due to the fact that the transmittance is measured using a spectrometer equipped with an integrating sphere, the transmission is not greatly influenced by the surface roughness. The absorption edge shows a slight shift with the change in the condition. The optical band gap is estimated by Tauc equation [49]: