Positive and negative photoconductivity coexist in two-dimensional ternary SnSSe alloy

Van der Waals two-dimensional materials are drawing increasing interest for nanoelectronics and optoelectronic applications. Their atomically thin channels and clean interfaces help overcome several scaling constraints faced by silicon technologies at the nanoscale, positioning them as leading candidates for next-generation electronics. To be truly competitive, however, they must also match silicon’s key strengths such as high processing quality, high carrier mobility, effective broadband optical response, and tunable carrier density and type through doping. While some of the required features have been widely demonstrated, controlling electronic properties through conventional doping techniques remains challenging. Against this backdrop, ternary alloys of transition- and post-transition-metal dichalcogenides are emerging as a practical route: they retain the advantages of their binary parents while offering compositional tunability for bandgap and electronic properties. Here, we report field-effect transistors based on single SnSSe-alloy flakes (atomic ratio Sn:S:Se = 1:1:1) and present a comprehensive assessment of their electrical and optoelectronic properties in the dark and under illumination, at both ambient pressure and under vacuum. We obtain a maximum carrier mobility of 2.84 cm² V⁻¹ s⁻¹ and a remarkable peak responsivity of more than 100 A/W at 460 nm. In addition to positive photoconduction, which decreases with increasing wavelength, we observe negative photoconductivity across the entire investigated range from 460 to 1000 nm, with a significant infrared responsivity of 2.85 A/W at 860 nm. The mechanisms underlying both positive and negative photoconductivities are analyzed in detail, and a phenomenological model is proposed to explain their origin, emphasizing the role of SnSSe-alloy-substrate coupling.