Effects of Na content on structural and optical properties of Na-doped ZnO thin films prepared by sol-gel method

In this paper, Na-doped ZnO (NZO) thin films with doping contents of 3-30 at.% were synthesized on quartz glass substrates by sol-gel spin coating method. The structure, morphology, optical transmittance and photoluminescence properties of NZO films were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), atomic force microscope (AFM), UV-VIS spectrometer and fluorescence spectrophotometer, respectively. The results showed that all the NZO thin films exhibit a strong preferential c-axis orientation with a hexagonal polycrystalline structure. An apparent increase in crystallinity was observed with the increasing Na content. The surface of the heavily Na doped samples (30 at.%) exhibits nanorods morphology. The RMS roughness changed from 16.219 nm to 53.072 nm with the increase of Na concentrations. As Na doping content increases to 24 at.%, the average transmittance was still larger than 62% in the visible range. The optical band gap initially increased and then decreased nearly linearly with the increase of Na content. The refractive index of NZO films in the visible range was found to increase gradually with increasing Na dopants. Room temperature photoluminescence (PL) spectra showed sharp ultraviolet emissions centered at 388 nm and broad green-yellow emissions (450-650 nm). The violet emission centered at 424 nm appeared in the spectra of 30 at.% NZO. In addition, it was found that the low Na doping concentration contributed to the enhancement of I_NBE/I_DLE.