Effects of strain and surface modification on stability, electronic and optical properties of GaN monolayer

Employing density-functional theory, G₀W₀ approach, and solving Bethe–Salpter equation (BSE), we investigate the effects of strain and surface modification (hydrogenation or fluorination) on stability, electronic and optical properties of monolayer GaN. Monolayer GaN is predicted to be an indirect semiconductor with wide gap of 4.44 eV, whose bandgap can be effectively modulated by biaxial strain, ranging from 5.49 eV (δ = −6.5%) to 2.27 eV (δ = +8%). In particular, the compressive strain arouses the structural instability of GaN monolayer. Hydrogenation or fluorination not only eliminates its instability, but also tunes the bandgap from indirect to direct. More attractively, tensile strain can significantly redshift the optical spectra of GaN monolayer into the visible-light region, broadening light harvesting. Hydrogenation or fluorination shifts optical activity to the ultra-violet region. The possibility of tuning the optoelectronic properties via strain and surface modification opens doors to novel application of the promising material.