Study of the structural and optical properties of CuInSe₂ thin films obtained by RF sputtering as a function of the deposition temperature

The formation of a copper-deficient surface layer is a common phenomenon in CuInSe₂ thin films, typically arising in deposition processes that involve multiple steps, such as thermal treatment and post-selenization. Here, we studied the radio-frequency magnetron sputtering film deposition from a CuInSe₂ target. Substrate temperature varied between 50 and 400°C. X-ray photoelectron spectroscopy (XPS) confirmed the persistent presence of a Cu-deficient surface layer across all deposition temperatures. Films grown below 200°C were amorphous and Cu-deficient, while at 200°C and above, the films exhibited a transition to a polycrystalline chalcopyrite structure, as evidenced by X-ray diffraction (XRD). The crystallite size for the polycrystalline films deposited at 200°C and 400°C was approximately 10–12 nm. Energy dispersive spectroscopy (EDS) revealed a progressive decrease in copper content from ~20 at.% at 50–200°C to ~16 at.% at 400°C, while indium remained nearly constant and selenium increased slightly from 50 at.% to 56 at.% with increasing temperature. UV-Vis spectroscopy showed that the optical bandgap (Eg) decreased from 1.2 eV at 50°C and 1.15 eV at 100°C to approximately 0.95–0.94 eV at 200°C and 400°C. Our results provide valuable insights into the effect of substrate temperature on CuInSe2 thin films, contributing to the ongoing efforts to optimize their properties for photovoltaic and optoelectronic applications.