Gap States of ZnO Thin Films by New Methods: Optical Spectroscopy, Optical Conductivity and Optical Dispersion Energy

The optical reflectance and transmittance spectra in the wavelength range of 300–2500 nm are used to compute the absorption coefficient of zinc oxide films annealed at different post-annealing temperatures 400, 500 and 600^\circ\!C. The values of the cross point between the curves of the real and imaginary parts of the optical conductivity \sigma_1 and \sigma_1 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/d\lambda and dT/d\lambda 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^\circ\!C have the minimum indirect optical band gap at about 3.15 eV. The films annealed at 600^\circ\!C have Urbach's energy minimum of 1.38 eV and hence have minimum disorder. The dispersion energy E_\rm d of films annealed at 500^\circ\!C has the minimum value of 43 eV.