Composition-dependent electronic structure and excitonic optical response in GaX bilayers and heterobilayers.

We investigate the electronic and optical properties of GaX (X = S, Se, Te) homobilayers and heterobilayers within a common DFT-GW-BSE framework. The calculations employ optimized AA'-stacked, coherently matched commensurate models in which composition, interlayer hybridization, and lattice matching jointly determine the band-edge trends. Charge-density and projected density-of-states analyses show that the states near the band edges originate mainly from Ga and chalcogen orbitals, with their relative contributions evolving across the S-Se-Te series. Quasiparticle corrections and electron-hole interactions substantially shift the optical features and redistribute the spectral weight, together with pronounced polarization dependence. The low-energy optical response progressively moves toward lower photon energies from S-rich to Te-rich bilayers. For GaS/GaTe, the lowest bright BSE state is dominated by a direct interband transition involving GaTe-majority atom-centered projections. Within this structural family, chalcogen composition provides an effective route for tuning the band-edge properties and polarization-dependent excitonic responses.