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.
Lablali et al. (Tue,) studied this question.