Stoichiometry-induced band gap opening in epitaxial degenerate copper sulfide thin films

Controlling the crystal phase and electronic structure of copper sulfide remains challenging due to its narrow stability window and sensitivity to sulfur stoichiometry, limiting reproducible optoelectronic performance. Here we report a low-temperature sulfur adsorption-corrosion strategy that converts 12-15 nm evaporated Cu into continuous copper sulfide thin films with tunable composition and electronic structure. Increasing the sulfurization time (10-60 s) reduces the Cu/S ratio from 2.4 to 1.2 and induces a phase transition from orthorhombic chalcocite-like Cu2S to hexagonal covellite CuS. Optical spectroscopy reveals band gap opening in sulfur-deficient films with indirect gaps of 1.15-1.41 eV. Ultraviolet photoelectron spectroscopy and Kelvin probe force microscopy show the Fermi level shifting toward the valence band, accompanied by a semiconductor-to-semimetal transition. Photoconductors based on lightly sulfurized films exhibit a wavelength-selective photoresponse and linear current transfer under 405 nm illumination, with a spectral cutoff consistent with the indirect band gap, highlighting the potential for integrated photodetectors and optocouplers.