GaSe and GaS for 2D Optoelectronics: Opportunities and Challenges

Graphene and group-VI transition-metal dichalcogenides dominate research on two-dimensional optoelectronics, while related layered semiconductors remain comparatively underexplored. Gallium selenide (GaSe) and gallium sulfide (GaS), two group-III monochalcogenides, exhibit thickness-tunable band gaps spanning the visible and near-ultraviolet spectral regions. GaSe is particularly attractive for second-order nonlinear optics, high-gain photodetection, and strain-localized single-photon emission, whereas the wider band gap of GaS offers opportunities for UV-selective and potentially solar-blind detection. Both materials are also compatible with flexible devices and van der Waals heterostructures. This perspective paper examines these opportunities together with the principal barriers to practical implementation, including ambient degradation, immature scalable synthesis, uncontrolled doping, contact resistance, and trap-mediated transport. It also identifies key priorities, particularly clarification of ultrathin band structures, the origin of GaSe quantum emitters, and systematic GaSe–GaS comparisons under matched experimental conditions.