Effect of doping on the structural, optical and electrical properties of La-doped ZnO thin films

In the current work, we studied the optical and electrical properties of pure and lanthanum-doped zinc oxide thin films prepared using the spray pyrolysis technique. All films deposited at the same deposition conditions but the La-concentration changes from 0 to 7 wt% in the starting solution. X-ray diffraction (XRD) reveals the formation of crystalline ZnO thin films with (002) as preferred orientation with no sign of impurity phases after La-doping. The crystallite size estimated from Scherrer's equation was found to decrease from 52.8 to 38.2 nm upon doping. Raman spectra confirmed the deposition of single ZnO phase without any other impurity phases such as La2O3. Scanning electron microscope (SEM) images shows the formation of agglomerates larger than the crystallite size obtained from the XRD analysis. The elemental composition of all films obtained from energy dispersive X-ray spectroscopy where an increase in the La-content is recorded by increasing its ratio in the starting solution. The optical bandgap was found to increase upon doping from 3.21 eV for pure film to 3.32 eV for 3 wt%. Further increase leads to a reduction in to 3.26 eV at 7 wt%. The refractive index were shows an anomalous dispersion in all doped films. Finally the room temperature conductivity were measured and found to be maximum at La-3 wt%.