Composition–structure–property relationships in amorphous sulfide thin films prepared by RF magnetron co-sputtering

Abstract

The local atomic structure of amorphous chalcogenides decisively governs their optical properties. In this contribution, we present a systematic composition–structure–property study of amorphous sulfide thin films prepared by radio-frequency magnetron co-sputtering across the GeS₂–Sb₂S₃, Ga₂S₃–Sb₂S₃, and GeS₂–Ga₂S₃ pseudo-binary systems, as well as the pseudo-ternary GeS₂–Ga₂S₃–Sb₂S₃ compositional range. RF magnetron co-sputtering enables the preparation of high-quality amorphous films over broad compositional ranges, including regions prone to crystallization in bulk form. Spectroscopic ellipsometry reveals distinct composition-dependent trends in refractive index and optical bandgap energy. In the GeS₂–Sb₂S₃ and Ga₂S₃–Sb₂S₃ systems, the refractive index increases systematically with increasing Sb₂S₃ content, accompanied by a corresponding decrease in Eg. In contrast, the GeS₂–Ga₂S₃ system exhibits a nearly constant refractive index across a wide compositional range despite variations in bandgap energy. The films exhibit low absorption within the measured infrared spectral range, supporting their relevance for mid-wave and long-wave infrared applications. To clarify the microscopic origin of the observed optical trends, cluster-based density functional theory (DFT) modeling was employed to analyze Raman-active modes of representative structural motifs in Ge–S networks. The calculated spectra for corner- and edge-sharing tetrahedra, homopolar bonds, and ring- and chain-like fragments reproduce the principal experimental features and enable assignment of characteristic vibrational bands to specific local configurations.