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 treatmentand post-selenization. Here, we studied the radio-frequency magnetron sputtering film depositionfrom a CuInSe₂ target. Substrate temperature varied between 50 and 400°C. X-ray photoelectronspectroscopy (XPS) confirmed the persistent presence of a Cu-deficient surface layer across alldeposition temperatures. Films grown below 200°C were amorphous and Cu-deficient, while at200°C and above, the films exhibited a transition to a polycrystalline chalcopyrite structure, asevidenced by X-ray diffraction (XRD). The crystallite size for the polycrystalline films depositedat 200°C and 400°C was approximately 10–12 nm. Energy dispersive spectroscopy (EDS) revealeda progressive decrease in copper content from ~20 at.% at 50–200°C to ~16 at.% at 400°C, whileindium remained nearly constant and selenium increased slightly from 50 at.% to 56 at.% withincreasing temperature. UV-Vis spectroscopy showed that the optical bandgap (Eg) decreased from1.2 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 resultsprovide valuable insights into the effect of substrate temperature on CuInSe2 thin films, contributingto the ongoing efforts to optimize their properties for photovoltaic and optoelectronic applications.