A comprehensive investigation of structural and optical properties of the spray coated Nd-doped ZnO

We report on the synthesis and characterization of Nd³⁺ doped Zinc oxide (ZnO) thin films fabricated using the chemical spray pyrolysis technique at 723 K. A comprehensive investigation using a combination of experimental techniques is undertaken to unveil the structural, morphological, compositional, electrical, linear, and nonlinear optical characteristics of the Nd-doped (a higher ionic radius dopant) ZnO thin films. The X-ray diffraction reveals a hexagonal wurtzite structure of the polycrystalline deposits with preferred orientation change from (101) to (002) plane at dopant concentrations above 5%. The inclusion of Nd dopants causes the morphology to change from fibrous to needle-like. The oxidation states of the dopant elements were identified by using XPS spectra. The Raman spectroscopy elucidates the deterioration of crystallinity with the increase in the dopant concentrations. The additional absorption band at higher dopant concentrations in the Fourier transform infrared spectra points to a new phase. The optical bandgap and charge carrier concentrations decrease and increase, respectively, with dopant concentrations up to 5 at%, followed by a reversal in the trend at still higher doping. The optical refractive index and extinction coefficient of the doped samples show an increase compared to pure ZnO. The defect-mediated energy transfer from the host ZnO to Nd dopants was identified from photoluminescence spectra. The sign of the third-order nonlinear refractive index changes from negative for pure ZnO to positive with Nd doping.