The optical reflectance and transmittance spectra in the wavelength range of 300–2500nm are used to compute the absorption coefficient of zinc oxide films annealed at different post-annealing temperatures 400, 500 and 600∘C. The values of the cross point between the curves of the real and imaginary parts of the optical conductivity 𝜎1 and 𝜎1 with energy axis of films exhibit values that correspond to optical gaps and are about 3.25–3.3 eV. The maxima of peaks in plots 𝑑𝑅/𝑑𝜆 and 𝑑𝑇/𝑑𝜆 versus wavelength of films exhibit optical gaps at about 3.12–3.25 eV. The values of the fundamental indirect band gap obtained from the Tauc model are at about 3.14–3.2 eV. It can be seen that films annealed at 600∘C have the minimum indirect optical band gap at about 3.15 eV. The films annealed at 600∘C have Urbach’s energy minimum of 1.38 eV and hence have minimum disorder. The dispersion energy 𝐸d of films annealed at 500∘C has the minimum value of 43 eV.
