Optical and structural properties of ZnO/TiO₂ bilayer thin films deposited by pulsed DC magnetron sputtering

Pulsed DC magnetron sputtering was employed to deposit ZnO/TiO2 bilayer thin films of varying thicknesses on glass substrates, with both layers being 80 nm thick. The structural and optical properties of the thin films were investigated using X-ray diffraction (XRD), atomic force microscopy (AFM), photoluminescence (PL), and optical transmittance measurements. The AFM analysis revealed a fine dispersion of spherical particles on the bilayer, with thicker ZnO layers leading to an increase in particle size. The single-layer film exhibited lower surface roughness (4.56 nm and 4.71 nm for ZnO and TiO2, respectively) compared to the ZnO/TiO2 bilayer (approximately 8 nm). The adhesion force decreased with increasing TiO2 thickness, from 50 mN (80 nm ZnO) to 10 mN (80 nm TiO2). XRD analysis indicated that the ZnO/TiO2 bilayer are amorphous, while the single ZnO layer is semi-crystalline with a hexagonal wurtzite crystal structure with an average crystallite size of 52 nm for the ZnO (100) plane. PL spectroscopy showed a strong violet emission at 420 nm, along with weaker emissions at 461 and 467 nm for all samples. The intensity of UV emission increased with TiO2 layer thickness, peaking at 20 nm ZnO/60 nm TiO2. ... mehrThe band gaps (Eg) for the single-layer ZnO and TiO2 were found to be 3.21 eV and 3.32 eV, respectively. However, the Eg of the bilayer films increased from 3.27 eV to 3.36 eV as the TiO2 layer thickness increased.