Effect of Substrate Temperature on Structural and Optical Properties of Flexible ZnS Thin Films

Transparent zinc sulfide (ZnS) thin films were deposited on flexible polyimide(PI), In-doped SnO₂ (ITO) coated glass, and quartz glass substrates by radio frequency (RF) magnetron sputtering at different substrate temperatures. The effects of substrate temperatures on the crystal structure, optical transmittance, refractive index, and surface morphology properties of ZnS films have been systematically studied. The films showed preferential growth of ZnS (111) crystal plane with increasing substrate temperature. The average optical transmittance of the flexible and rigid ZnS films is above 80% and 85% in the visible and infrared spectral range, respectively. The refractive index of the flexible ZnS films is 2.21−2.56 in the 400 nm−890 nm wavelength range. In addition, the refractive index of the rigid ZnS films deposited at 300℃ is 2.26 at 890 nm wavelength. The surface roughness and film thickness of the flexible ZnS films decreased with increasing substrate temperature, reaching a minimum of 2.99 nm at 300℃. The optimum substrate temperature for the flexible ZnS films is 200℃−300℃ to obtain the optimized performance of ZnS-based flexible optoelectronic devices.