Non-equilibrium stabilization of sulfur-rich zinc blende Cu1+δI1−xSx alloys with ultra-degenerate p-type conductivity

High-performance p-type transparent conductors are limited by low conductivity and poor stability. Here, we report the synthesis of Cu1+δI1−xSx alloy films via non-equilibrium magnetron co-sputtering, enabling sulfur incorporation beyond the thermodynamic solubility limits. X-ray diffraction reveals stabilization of the zinc blende-dominated structure up to x ≈ 0.8, with systematic lattice contraction, consistent with substantial substitutional sulfur incorporation. Sulfur alloying induces a transition toward degenerate p-type conduction, with hole concentrations increasing from ∼1018 to 1022 cm−3, while the optical bandgap narrows from ∼3.1 to ∼2.2 eV. X-ray photoelectron spectroscopy and first-principles calculations show that sulfur incorporation preserves the Cu+ valence state and modifies the electronic structure through valence band elevation and conduction band lowering. Defect calculations indicate that sulfur alloying enhances p-type conductivity primarily by tuning the formation energetics and electronic character of copper vacancies, which dominate hole generation. These results demonstrate a non-equilibrium route for engineering CuI-based alloys with tunable electronic structure and high p-type conductivity, offering a promising platform for transparent electronic applications.