Oxygen-Induced Phase Separation in Sputtered Cu–Sn–I–O Thin Films

Amorphous Cu–Sn–I is a promising p-type transparent semiconductor. Therefore, herein, composition and structure of sputtered Cu–Sn–I thin films with varying thickness and Sn content are investigated by different electron microscopy techniques, energy-dispersive X-ray spectroscopy, and X-ray absorption spectroscopy at the Cu, Sn, and I K-edge. After exposure to air, the sputtered films are found to contain significant amounts of oxygen, leading to a complete phase separation of the resulting Cu–Sn–I–O films. Spatially-resolved compositional analysis and high-resolution transmission electron microscopy reveal that one phase consists of crystalline γ-CuI while the other phase is composed of amorphous Cu–Sn–O, most likely a mixture of SnO₂ and Cu₂O/CuO. X-ray absorption spectroscopy confirms that the local structural environment of the Sn and I atoms is similar to that in amorphous SnO₂ and crystalline CuI, respectively. In contrast, the X-ray absorption near edge structure and the extended X-ray absorption fine structure of the Cu K-edge both demonstrate that Cu atoms are not only bonded to I but also to O atoms. The incorporation of oxygen into the sputtered films thus completely alters the material and therefore clearly needs to be inhibited.