Hidden Defect Chemistry in Ion-Irradiated MoS$_2$ Field-Effect Transistors Revealed by Photocurrent Loss
Defect engineering in monolayer MoS$_2$ is a promising route to tune field-effect transistors (FETs), but the electronic response of defects in processed devices can be masked by contacts, substrate effects, adsorbates, and chemical passivation. Here, we irradiate MoS$_2$ FETs with low-energy 40~eV Ar$^+$ ions to preferentially create sulfur vacancies (V$_S$) in the channel while minimizing substrate damage. We compare dark and illuminated electrical characterization with surface analysis and first-principles calculations. Dark transfer characteristics show an apparent robustness against irradiation up to moderate fluences, with pronounced degradation only at the highest fluence. Under 532~nm illumination, however, the photocurrent and light-induced photodoping decrease systematically with increasing ion fluence, revealing irradiation-induced changes that are hidden in standard dark measurements. Atomic force microscopy and X-ray photoelectron spectroscopy show substantial carbon-containing residues on processed devices even after extended cleaning. We propose that such residues may provide a reservoir for hydrocarbon-mediated passivation of sulfur vacancies. Density-functional-theory calculations provide a microscopic model consistent with this scenario: unsaturated V$_S$ introduce in-gap states, H-C$_S$ configurations suppress these states, and carbon substitution without hydrogen leaves defect states in the band gap. Our results highlight carbon-containing surface contamination as a key factor in interpreting defect engineering experiments on MoS$_2$ and related TMDC devices.