Characterization of niobium-doped zinc oxide thin films: Structural changes and optical properties

Zinc oxide (ZnO) thin films doped with different ratios of niobium (Nb) were successfully synthesized by the use of the direct current/ radio frequency sputtering technique. The effectiveness of Nb atoms on ZnO films was examined via different experimental methods. The ratio of the film elements was first detected by X-ray photoelectron spectroscopy (XPS) which showed that Nb had existed in the metallic state with ratio of 0.3 %, 2.4 %, and 3.7 % as the sputtering power of Nb changed in the range 30 W, 40 W and 50 W, respectively. X-ray diffraction analysis (XRD) was used to observe the nature of the films. Two characteristic peaks were observed for (002) and (103) planes, respectively. These peaks were affected with the ratio of Nb dopant, which their intensity values were decreased, and the structural disorder in the ZnO lattice was observed in 3.7 % Nb sample. The roughness and morphology of each of the films were recorded and captured via atomic force microscopy. The grain size was decreased from 29.78 nm for the undoped ZnO films to 17.5 nm for 2.4 % Nb sample. The embedding of Nb dopant to ZnO lattice head for improving the film roughness. Optical properties were considered in terms of the Nb ratio. The direct optical band gap was confirmed in all samples, and found to increase as the Nb content increase from 3.24 eV for ZnO sample to 3.62 eV for 3.7 % Nb-ZnO sample. Photoluminescence investigations were conducted to observe and examine the impact of Nb on ZnO films.