Structural and Optical Properties of Zinc - Cobalt Oxide Thin Films Prepared by Chemical Spray Pyrolysis Method: The Effect of Aqueous Solution Molarity

Abstract

Abstract In this research , Zinc-Cobalt Oxide (Zn(1-x)CoxO) thin films, where x = 0, 4, 6 and 8 % with different molarities (0.05, 0.1, 0.15 and 0.2 M) have been successfully deposited on glass substrates by chemical spray pyrolysis (CSP) technique at substrate temperature (400 ±5oC) and thickness of about 300 ±10 nm. The structural and optical properties of these films have been investigated by using XRD, AFM, and UV-Visible spectroscopy. The XRD results showed that all films are polycrystalline in nature with hexagonal structure and preferred orientation along (100),(002) and (101) plane. The crystallite size was calculated by using Scherrer formula. The crystallite size of the samples was maximum (61.20nm) for the Zn 0.92Co 0.08O thin film prepared at molarity of 0.1M, and it was minimum (12.11nm) for the Zn0.96Co0.04O thin film prepared at the 0.15M. The average grain size, average roughness and root mean square (RMS) roughness for Zinc-Cobalt Oxide were estimated from AFM. The absorbance and transmittance spectra have been recorded in the wavelength range of (300- 900) nm in order to study the optical properties. The transmittance for all thin films increases rapidly as the wavelength increases in the range (300- 390) nm, and increases slowly at higher wavelengths. The absorbance decreases rapidly at short wavelengths (high energies) corresponding to the energy gap of the film, (when the incident photon has an energy equal or more than the energy gap value). The absorption coefficient was estimated for all samples and because its value was higher (<104cm-1), it was concluded that the thin films material have direct band gap. The optical energy gap for the allowed direct electronic transition was calculated using Tauc equation and this was achieved by taking the first derivative of the absorbance. It was found that the band gap increased when the molarity increases and the band gap values ranges between 3.26 eV and 3.48 eV the change in the band gap values can be explained by Burstein-Moss effect. The optical constants including (absorption coefficient, real and imaginary parts of dielectric constant) were calculated as a function of photon energy, refractive index and extinction coefficient for all films were estimated as a function of wavelength.