Band structure and optical transitions in atomic layers of hexagonal gallium chalcogenides

We report density-functional-theory calculations of the electronic band structures and optical absorption spectra of two-dimensional crystals of Ga₂X₂ ($X=$ S, Se, and Te). Our calculations show that all three two-dimensional materials are dynamically stable indirect-band-gap semiconductors with a sombrero dispersion of holes near the top of the valence band. We predict the existence of Lifshitz transitions---changes in the Fermi-surface topology of hole-doped Ga₂X₂---at hole concentrations nS=7.96×10¹³ cm^-2, nSe=6.13×10¹³ cm^-2, and nTe=3.54×10¹³ cm^-2.