Crystalline and non-crystalline nickel oxide (NiO) thin films were obtained by spray pyrolysis technique (SPT) using nickel acetate tetrahydrate solutions onto glass substrates at different temperatures from 225 to 350℃. Structure of the as-deposited NiO thin films have been examined by X-ray diffraction (XRD) and atomic force microscope (AFM). The results showed that an amorphous structure of the films at low substrate temperature (T<sub>s</sub> = 225℃), while at higher T<sub>s</sub> ≥ 275℃, a cubic single phase structure of NiO film is formed. The refractive index (n) and the extinction coefficient (k) have been calculated from the corrected transmittance and reflectance measurements over the spectral range from 250 to 2400 nm. Some of the optical absorption parameters, such as optical dispersion energies, <i>E<sub>o</sub></i> and <i>E<sub>d</sub></i>, dielectric constant, <i>ε</i>, the average values of oscillator strength, <i>S<sub>o</sub></i>, wavelength of single oscillator <i>λ<sub>o</sub></i> and plasma frequency, <i>ω<sub>p</sub></i>, have been evaluated.
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