Post-annealing effects on the physical and optical waveguiding properties of RF sputtered ZnO thin films

Abstract

ZnO thin films were deposited at room temperature onto glass substrate by RF sputtering technique. Effects of the post-annealing at 300–500°C on the structural, morphological, optical and waveguiding properties were investigated using different characterization techniques. X-ray diffraction (XRD) analyses have shown that all thin films have a hexagonal wurtzite structure with higher c-axis preferred orientation (002), better crystallinity and larger crystallite size as post-annealing temperature increases. Fourier transform infrared (FTIR) spectra of annealed samples confirmed that the ZnO stretching vibration bond was found to be stable at 419 cm−1. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) images have revealed that film morphology and surface roughness were influenced by heat treatment temperatures. The UV-Vis-NIR spectrophotometry characterisations have indicated that all the films were highly transparent with average transmittance exceeding 81% within the visible region, and the bandgap energy of the as-deposited film was increased with increasing of the annealing temperature.The obtained results from m-lines spectroscopy (MLS) measurements at 632.8 nm wavelength have demonstrated that all ZnO thin film optical waveguides were single mode and the ordinary and extraordinary refractive index values of the film annealed at 500°C were very close to the corresponding ZnO single-crystal values.