Refractive index controlled by film morphology and free carrier density in undoped ZnO through sol-pH variation

Zinc oxide thin films, prepared by the sol-gel process, were deposited on glass substrate using spin coating technique. The sol-pH effect on the optical parameters was studied for alkaline sol. The surface roughness was investigated by atomic force microscopy (AFM) and varied from 20 to 40 nm. The optical transmission measurements were carried out to evaluate the behavior of the extinction coefficient and the refractive index. An exponential decay of the refractive index 'n' as a function of wavelength was observed. The refractive index increases slightly when the pH increases to pH = 9.5 where it reaches its maximum. Beyond this value, it decreases sharply. This behavior has been linked to the effects of free carrier density and the films morphology. Thus, a linear variation of the refractive index as a function of surface roughness has been demonstrated. (C) 2018 Elsevier GmbH. All rights reserved.