Micro-structural characterization of annealed cadmium–zinc oxide thin films obtained by spray pyrolysis

The structural properties of annealed (ZnO)x(CdO)1−x thin films are studied by x-ray diffraction methods. The films were obtained by spray pyrolysis using different values for the nominal composition (x), and then they were annealed at 450 °C from 0 to 120 min. The structural analysis confirms previous results on the formation of a homogeneously mixed oxide semiconductor, but in which crystalline and amorphous phases co-exist for both CdO and ZnO. In this work, we show that for annealed films there is a strong interaction between the amorphous, the hexagonal ZnO and the cubic CdO phases regarding the lattice constants and the crystallite growth rate. In the annealed films, for x≤0.5 the optical behavior is mainly controlled by the CdO phase, so that there is a reduction of the effective band-gap of the material when the CdO crystallite size is increased, possibly due to grain size effects. On the other hand, for x>0.5 the band-gap behavior is mainly determined by the variation of the ZnO crystallite lattice parameters and the relative volumetric concentration of amorphous and crystalline ZnO in the film.