Investigation of nitrogen doped ZnO thin films: Effects on their structural and optical properties

Abstract

Un-doped and nitrogen-doped ZnO thin films were grown by using radio frequency (RF) magnetron sputtering method changing the nitrogen flow rate between 0% −12.5% and the thickness dependence of films was determined. The effect of nitrogen doping concentration on the structural, morphological, and optical properties of zinc oxide thin films was studied. X-ray diffraction (XRD) analysis confirmed that the nitrogen-doped ZnO films belong to the hexagonal crystal structure. The optical properties of the grown samples were examined by optical spectrophotometer and spectroscopic ellipsometer. Transmittance spectra were obtained by spectrophotometer measurements and the effect of nitrogen ratio was investigated. It has been observed that as the nitrogen ratio increases, the transmittance decreases up to ∼500 nm and then increases. By using the transmittance curve, the energy band gap was calculated. Further detailed optical analysis was made by the spectroscopic ellipsometry technique. Fitting was performed to ensure the agreement between the experimentally obtained Ψ values and theoretically determined Ψ values using the Cauchy model. As a result, the refractive index was found for each film and it was observed that the refractive index decreased as the nitrogen ratio increased. The scanning electron microscopy (SEM) measurements showed that the surface morphology of the films changes with N doping. Nitrogen was observed in Fourier transform infrared spectra analysis. 5% nitrogen-doped ZnO films were grown on the glass substrate using RF magnetron sputtering at room temperature. Samples are prepared by varying thicknesses during the deposition process. XRD and SEM measurements of the samples show the variation in the crystal structure and surface morphology of the film with varying thicknesses. All the samples are tested for the transmittance and band gap. The increase of film thickness increases the grain size. The transmittance is influenced by the film thickness. • Un-doped and nitrogen doped ZnO thin films were grown by using RF magnetron sputtering method. • The band gap values were changed by changing N doping. • The refractive index values of ZnO films decrease with the introduction of nitrogen into the structure. • The band gaps decreased from 3.25 eV to 3.20 eV as the thickness increased. • Zn–N bonds were attributed at ∼1380 and ∼1502 cm −1 .