Thickness, annealing and substrate effects on structural, morphological, optical and waveguiding properties of RF sputtered ZnO thin films

In this work, nanostructured ZnO thin films were grown at room temperature by RF magnetron sputtering technique on glass and silica-on-silicon (SiO₂/Si(100)) substrates for two deposition times (60 and 120 min). The as-grown ZnO thin films were then annealed in air for 60 min at various temperatures, ranging from 300 to 500 °C. Effects of film thickness, thermal annealing and substrate type on the structural, morphological, optical and waveguiding properties were investigated using several characterization techniques. X-ray diffraction study indicated that all ZnO thin films have a hexagonal wurtzite structure and the films deposited on SiO₂/Si substrates exhibit a better crystalline quality, higher c-axis orientation, larger crystallite size and lower compressive stress than those grown on glass substrates. Furthermore, it is found that the structural properties are enhanced with increasing film thickness and annealing temperature. Scanning electron microscopy micrographs and atomic force microscopy images reveal that the morphology and surface roughness of the ZnO samples depend on the film thickness and heat treatment temperature. From the analysis of the UV–Visible spectroscopy results, the as-grown films prepared on glass substrates show an average transmittance ranging from 63 to 74% in the visible region. In addition, the optical transmission and the band gap are found to increase with increasing annealing temperature. M-lines spectroscopy measurements at a 632.8 nm wavelength put into evidence that as-grown ZnO planar waveguides support single and multi-well confined transverse electric (TE) and transverse magnetic (TM) guided modes. The measured refractive indices of these films are found to be independent of film thickness and substrate type for both TE and TM polarizations. Moreover, moderate propagation losses of 1.3 ± 0.2 dB/cm were observed in the ZnO planar waveguide grown for 60 min on SiO₂/Si substrate.