Effects of oxygen/argon ratio and annealing on structural and optical properties of ZnO thin films

Zinc oxide (ZnO) thin films were prepared on quartz substrate using radio frequency magnetron sputtering at various oxygen/argon (O2/Ar) flow ratios. The structural and optical properties of ZnO films were investigated in dependence of O2/Ar ratio and annealing temperature by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Raman scattering spectroscopy, UV–visible spectroscopy and photoluminescence (PL) spectroscopy. XPS spectra showed that the peak positions of the O1s and Zn 2p shifted to opposite directions of binding energy with increasing O2/Ar flow ratio, and the as-deposited ZnO films changed from the Zn-rich to O-rich. Raman spectra revealed that ZnO films consisted of wurtzite structure exhibited high crystalline quality and c-axis preferred orientation after annealing at 420 °C. The transmittance and energy band gap (Eg) of the as-deposited ZnO films increased with increasing O2/Ar flow ratio. As the films were annealed, the Eg decreased first and then increased with annealing temperature. The PL spectra indicated that all films exhibited a violet emission peak and the ZnO films prepared at O2/Ar = 35/35 showed an additional green emission. The origin of both emission peaks and the change in the intensity with O2/Ar ratio and annealing temperature were discussed.