Thickness-dependent optical and structural properties of polytetrafluoroethylene/zinc oxide films by radio frequency magnetron sputtering

PTFE/ZnO films with varying ZnO thickness were prepared by RF magnetron sputtering under high vacuum (100W RF power). The PTFE/ZnO films show slight enhancement in transmission in the visible range as compared to bare ZnO films. There is a clear UV light cutoff (rapid fall below 380 nm with negligible transmission below similar to 300 nm) after 10-min deposition of ZnO, which remains unaffected by further depositing PTFE. Transmission data reveal that absorption edge falls in the range of 300-380 nm, below which, the transmission reduced to only similar to 0.02% giving an optical density of 4 (for the film deposited for 20 min [total thickness similar to 415 nm]) in the 200-250 nm range. As the ZnO layer thickness increases, the polycrystallinity increases and simultaneously, roughness increases. It leads to a change in crystallite orientations and also in the distribution of grains on film surface. Corresponding AFM image shows island-like growth, leading to an intermixed ZnO/PTFE layer. The roughness of surface in lower as well as higher thickness range is found to be high (similar to 8-9 nm) except the film in which continuous layer seems to be grown. Based on this, a layer model was developed that consists of glass substrate, native oxide SiO2 layer on this substrate, then a dense ZnO layer followed by mixed PTFE/ZnO layer which is then covered by top PTFE as evident from the ellipsometry fitting. Overall results suggest that by choosing proper thicknesses and number of layers, such thin films might be used in antireflection coatings with UV protection below 400 nm. Based on these experimental results, an optimized simulated layer model consisting of suitable layer thicknesses of ZnO and PTFE has been suggested to achieve high transmission in the visible wavelength region.