Detailed microstructure analysis of as-deposited and etched porous ZnO films

ZnO nanostructured materials in thin film forms are of particular interest for photovoltaic or photo-catalysis processes but they suffer from a lack of simple methods for optimizing their microstructure.We have demonstrated that microporous ZnO thin films with optimized inter grain accessibility canbe produce by radio frequency magnetron sputtering process and chemical etching with 2.75 mM HClsolution for different duration. The as-deposited ZnO thin films were first characterized in terms ofstructure, grain size, inter grain space, open cavity depth and total thickness of the film by XRD, AFM,SEM, profilometry and optical measurements. A specific attention was dedicated to the determinationof the surface enhancement factor (SEF) by using basic geometrical considerations and images treat-ments. In addition, the porous fraction and its distribution in the thickness have been estimated thanksto the optical simulation of the experimental UV–Visible–IR spectrums using the Bruggeman dielectricmodel and cross section SEM images analysis respectively. This study showed that the microstructureof the as-deposited films consists of a dense layer covered by a porous upper layer developing a SEF of12–13 m2m−2. This two layers architecture is not modified by the etching process. The etching processonly affects the upper porous layer in which the overall porosity and the inter-grain space increase withthe etching duration. Column diameter and total film thickness decrease at the same time when the filmsare soaked in the HCl bath. The microporous structure obtained after the etching process could gener-ate a great interest for the interfaces electronic exchanges for solar cells, photocatalysis and gas sensorsapplications