Optical characteristics of iron oxide thin films prepared by spray pyrolysis technique at different substrate temperatures

The authors devoted this paper to study the influence of the substrate temperature upon optical characteristics of iron oxide thin films. Spray pyrolysis technique, SPT, was used to deposit Fe₂O₃ thin films onto preheated glass substrates (450 °C$\ge$ T_sub $\ge$ 250 °C). X-ray diffraction (XRD) revealed that the formed thin-film sample of T_sub < 300 °C has a non-crystalline nature. While it confirmed that deposited samples of T_sub > 300 °C are hematite samples of polycrystalline nature and they have the rhombohedral structure. The optical properties of the present thin films were studied at room temperature in the wavelength range 300–2500 nm. Transmittance and reflectance measurements were utilized to determine and study the optical properties and optical constants. The absorption coefficient, optical density, the optical energy gap, Urbach energy, steepness parameter, and the electron–phonon interaction of films have been evaluated and discussed. The optical energy gap, E_g, was the result of direct and indirect allowed transitions. E_g increased from 2.10 eV to 2.39 eV when the transition is direct and from 1.87 eV to 1.99 eV for indirect transition. While Urbach's energy decreased from 223 meV to 143 meV and the electron–phonon interaction from 4.089 to 1.778 when the substrate temperature was increased from 250 °C to 450 °C, respectively. The extinction coefficient and refractive index have been also estimated and studied. All optical properties of Fe₂O₃ thin films discussed in this research work have been strongly dependent upon the substrate temperature. Graphical abstract:

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