Influence of growth time and substrate type on the microstructure and luminescence properties of ZnO thin films deposited by RF sputtering

We report the preparation by the RF magnetron sputtering technique of ZnO thin films with deposition times of 50 and 100 min on glass, amorphous quartz, and silica-on-silicon (SiO2/Si) substrates. The effects of thickness and substrate type on structural, morphological, optical and luminescence properties are investigated by X-ray diffraction (XRD), scanning electronic microscopy (SEM), atomic force microscopy (AFM), and ultraviolet-visible (UV-Vis) and photoluminescence (PL) spectroscopies. XRD patterns show that all the films crystallize in the hexagonal wurtzite structure with a preferential orientation along the (002) plane. The crystallinity, peak intensity, and crystallite growth are found to be thickness and substrate type dependent. SEM and AFM images reveal that such parameters have a significant influence on the morphology, grain size and surface roughness of the films. According to UV-Vis measurements, ZnO thin films deposited on quartz substrate demonstrate a better transparency in the visible region than those on glass, regardless of the thickness. However, for both substrates, the optical band gap energy (E-g) decreases by the increasing of film thickness. Room temperature PL spectra of the films deposited on glass and quartz substrates show a sharp and intense UV emission and two main weak visible bands centered at blue and red regions. However, the emissions of those prepared on SiO2/Si substrate are very weak consisting of UV and several visible bands. Moreover, increasing the thickness leads to a drastic decrease of the UV emission, and the emergence of a broad and intense violet band for films on transparent substrates, as well as the suppression of the violet, and the blue emissions for the film deposited on SiO2/Si.