Effects of withdrawal speed on the structural and optical properties of sol–gel derived ZnO thin films

• Nanostructured ZnO films were deposited at various WS by sol–gel process.
• Influences of WS on the structural and optical properties of the ZnO films were investigated.
• The surface roughness values of the films were in the range from 6 to 19 nm.
• The maximum transmittance of 80% was obtained for ZnO film prepared at the WS of 20 mm/min.

Nanostructured zinc oxide (ZnO) of hexagonal wurtzite crystal structure was successfully prepared on corning glass substrates by sol–gel method using dip coating technique. The effects of withdrawal speed (WS) on the crystalline structure, morphology and optical properties of the pure ZnO thin films were investigated using x-ray diffraction (XRD), atomic force microscopy (AFM) and optical transmittance measurements. It was found that, the WS significantly affects the crystalline structure, morphology and optical properties of the films. All ZnO films were observed to grow along the c-axis with a preferential orientation of (0 0 2). The lower the withdrawal speed of the substrate the higher the tendency toward preferred orientation along (0 0 2) plane. The surface roughness mean square (RMS) of the films as estimated from AFM measurements rises from 6 to 19 nm, when the withdrawal speed increased from 20 to 80 mm/min, respectively. The optical transmittance of the films within the visible and near infrared region was found to be about 80%. The optical constants of the films such as refractive index (n) and extinction coefficient (k) were determined using spectroscopic ellipsometer (SE) technique. The optical constants (n and k) were fitted according to Cauchy model.