Au-doped ZnO sol-gel thin films: An experimental investigation on physical and photoluminescence properties

Undoped and Au-doped ZnO thin films were prepared on glass substrates by sol-gel process and dip-coating technique. The effects of Au concentration (10-30 at%) on structural, morphological and optical properties of the ZnO films were studied using X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), UV-Vis-NIR spectrometry, Hall-effect measurements and photoluminescence (PL) spectroscopy. XRD results show that all films crystallized in a hexagonal wurtzite structure and are highly c-axis oriented. The crystal quality of the films is found to decrease with the increase in Au doping. A weak diffraction peak related to Au cubic phase is also observed at Au concentration higher than 10 at%. The Raman studies confirmed that the film's crystallinity degraded with increasing Au content. FE-SEM micrographs and AFM images show that both morphology and surface roughness are affected by the Au doping concentration. UV-Vis-NIR optical spectra reveal that increasing Au doping into ZnO films enhances their transparency. Photoluminescence measurements at room temperature put into evidence the presence of two intense bands at 373.6 and 385 nm in the UV spectra and four very weak emission bands in the visible spectral range. All the visible emissions are found to increase with Au content. The emission at 373.6 is maximal for the film doped at 20 at%, whereas the one at 385 nm reaches its maximum intensity in the film doped at 10 at%.