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. The band gaps (E_g) for the single-layer ZnO and TiO₂ were found to be 3.21 eV and 3.32 eV, respectively. However, the E_g of the bilayer films increased from 3.27 eV to 3.36 eV as the TiO₂ layer thickness increased.