Morphological and optical properties of ZnO thin films grown on Si and ITO glass substrates

Abstract

In this study, ZnO thin films have been electrodeposited from zinc nitrate solution without using any catalyst, additive or seed layer on two kinds of substrates (Si and ITO glass).Using cyclic voltammetry and chronoamperometry, it was shown that the mechanism of ZnO deposition strongly depends on the substrate used and its overpotential for nitrate reduction. On Si, the nitrate reduction into nitrite occurs before that of Zn2+. This reaction induces an increase of the local pH leading to ZnO precipitation. In contrast on ITO, the Zn2+ reduction brings first metallic Zn deposition, which is then chemically oxidized by nitrate into ZnO phase. The effect of deposition time on morphology, structure and photoluminescence properties was studied using X-ray diffraction (XRD), scanning electron microscopy (SEM) and photoluminescence (PL) measurements.The chemical nature of the substrate has no influence on the orientation of nanorods, but really impacts their morphology and the optical emission properties. X-ray diffraction analysis always revealed ZnO wurtzite phase with a (002) preferential orientation enhanced with increasing deposition time. A well-defined ZnO morphology was generated under -1.4 V for 60 min on Si and ITO glass. ZnO nanorods that composed the nanoflowers grown on ITO glass tend to be shorter, wider, with higher aspect ratio than on Si. ZnO nanostructures prepared on ITO showed an intense UV emission without spreading in the visible region, thus demonstrating the formation of a defect free structure.