Structural and electrical properties of radio frequency magnetron sputtered Cu(In_xGa_{1 - X})Se₂ thin films with additional post-heat treatment

Cu(In_{1 - x}Ga_x)(Se,S)₂ (CIGSS) thin films were deposited using a single quaternary target of Cu(In_0.75Ga _0.25)Se₂(CIGS) by rf magnetron sputtering, followed by sulfurization. The effects of substrate temperature and post-sulfurization on the properties of CIGSS films were investigated. As the substrate temperature increased, the crystallinity of the films increased significantly and the grain size also increased. Energy dispersive X-ray spectroscopy of CIGS films showed that Cu, Ga and Se contents approached the stoichiometry of CIGS films with increasing substrate temperature. Post-sulfurization of as-deposited CIGS films was carried out to improve their properties. The resultant CIGSS thin films revealed a noticeable increase in (112) peak for films deposited under 200 C, while the intensities of the (220)/(204) and (312)/(116) peaks increased as well. Grains with a size of approximately 100 nm were grown after sulfurization. The results of EDX of CIGSS thin films after sulfurization indicated that the Cu content increased slightly and the Ga and (Se + S) contents decreased significantly. The band gaps of the films were also noticeably reduced after sulfurization. The carrier concentrations of the films after sulfurization decreased greatly at substrate temperatures below 400 C, while they showed little change at temperatures above 400 C. The resistivity of the films after sulfurization increased with increasing substrate temperature and showed little change at temperatures above 400 C. As a result, CIGS films were transformed to the chalcopyrite phase with well crystallized grains through sulfurization.