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 SnO 2 and Cu 2 O/CuO. X‐ray absorption spectroscopy confirms that the local structural environment of the Sn and I atoms is similar to that in amorphous SnO 2 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.