Band Structure and Optical Transitions in Atomic Layers of Hexagonal Gallium Chalcogenides

Abstract

We report density-functional-theory calculations of the electronic band structures and optical absorption spectra of two-dimensional crystals of Ga${}_2$X${}_2$ (X=S, Se, and Te). Our calculations show that all three two-dimensional materials are dynamically stable indirect-band-gap semiconductors with a Mexican-hat 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 Ga${}_2$X${}_2$---at hole concentrations $n_S=7.96\times 10^{13}$ cm${}^{-2}$, $n_{Se}=6.13\times 10^{13}$ cm${}^{-2}$, and $n_{Te}=3.54\times 10^{13}$ cm${}^{-2}$.

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