Phase transition in ZnS thin film phosphor

The effect of an original non-vacuum annealing of thin ZnS films according to the annealing conditions and type of substrate on the film's crystalline structure and surface morphology in relation with photoluminescent (PL) properties was investigated. ZnS thin films were deposited by electron-beam evaporation (EBE) on ceramic (BaTiO₃) and glass substrates heated to 150-200 °C. Three types of the targets such as ZnS, ZnS:Cu and ZnS:Cu, Al were used. The film thickness varied from 0.6 to 1 μm. As-deposited films were annealed at the atmospheric pressure in S₂-rich ambient atmosphere at 600-950 °C for 1 h. The ZnS:Cu films were Ga co-doped by annealing in the same atmosphere and temperature with additional Ga vapor. The ZnS films were doped with Cu, Cl using the thermal diffusion method by embedding the samples in ZnS:Cu, Cl powder. X-ray diffraction (XRD) technique, atomic force microscopy (AFM) and the measurements of PL parameters were used for investigation. The temperature of the ZnS phase transition from the sphalerite to wurtzite structure depends on the presence, type and ratio of additional impurities. It was revealed that Ga and Cl act not only as co-dopant to improve the luminescent properties, but also as activators of recrystallization processes. The transition of ZnS film's sphalerite lattice to wurtzite leads to the displacement of the blue emission band position towards the short-wavelength range by 10 nm.