Sputtering–Sulfurization Synthesis of Wide-Gap Barium Copper Sulfide Thin Films

Abstract

Transparent conductive materials (TCMs) that combine high optical transparency with electrical conductivity are essential as electrodes and window layers in photovoltaics, displays, and smart windows, yet the development of p-type materials comparable to well-established n-type oxides has remained limited. Here, we describe a sputtering–sulfurization approach that produces predominantly wide-gap α-BaCu4S3 thin films as a promising p-type chalcogenide transparent conductive material. Metallic Ba + Cu precursors were cosputtered and annealed between 400 and 550 °C under high sulfur vapor pressure generated from a sulfur source held at 180 °C. Grazing-incidence X-ray diffraction (GI-XRD) identifies a Cu-rich hexagonal BaCu5.65S4.5 polymorph as the kinetically favored phase at 400–450 °C, while above 500 °C, the orthorhombic α-BaCu4S3 phase dominates. Dark-field scanning transmission electron microscopy (DF-STEM) with energy-dispersive X-ray spectroscopy (EDS) confirms near-ideal Ba/Cu/S stoichiometry, with Cu/(Ba + Cu) ≈ 0.80 ± 0.03 and S/(Ba + Cu + S) ≈ 0.375. Combined transmittance–reflectance spectra show ∼80% transparency between 600 and 1000 nm for a film with a representative thickness of approximately 290 nm and a direct optical band gap of ∼2.6 eV. Hall effect measurements verify robust p-type conduction with mobility up to ∼2.4 cm2 V–1 s–1 and conductivities of 25–65 S cm–1 depending on sulfurization temperature and duration. To mitigate interfacial reactivity, a sequential sputtering strategy using 20 nm Cu as both interlayer and capping layers was implemented to stabilize the air-sensitive metallic precursors. While minor Cu2–xS secondary phases persist, this capping strategy effectively suppresses precursor oxidation and provides a pathway toward phase-pure α-BaCu4S3.