Microstructural, optical and magnetic properties of cobalt-doped zinc oxysulfide thin films

In this paper, we investigated the microstructural, optical and magnetic properties of Co-doped zinc oxysulfide films. Undoped zinc oxysulfide films were deposited by a spray pyrolysis method on glass. A thin Co layer evaporated on these films served as the source for the diffusion doping. Doping was accomplished by annealing the stack layers at the temperature range from 573 K to 723 K in steps of 50 K for 45 min under vacuum. The X-ray diffraction pattern of undoped films revealed the presence of two wurtzite phases corresponding to ZnS and ZnO with a strong preferred orientation along the ZnS (0 0 2) hexagonal plane direction. Results from scanning electron microscopy showed a similar surface morphology for the undoped and Co-doped films, displaying regular arrays of hexagonal micro-rods perpendicular to the substrate. The quantitative energy dispersive X-ray spectroscopy results revealed a sulfur-rich (∼39 at.% S) and oxygen-poor (∼8 at.% O) chemical composition with about 3 at.% Co doping in the sample prepared by annealing at 673 K. X-ray photoelectron spectroscopy results confirmed that Co2+ ions were substituted for Zn2+ ions in this film. The optical transmission measurements showed that both undoped and Co diffusion-doped films had a low average transmittance in the range of 8–18%. A decrease in the band gap was observed with Co-doping and increasing diffusion-doping temperatures. The magnetization of films as a function of magnetic field and temperature were measured. Clear ferromagnetic loops were observed for the Co-doped zinc oxysulfide film prepared at 673 K.