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

TL;DR

This study uses density functional theory to analyze the electronic and optical properties of 2D Ga$_2$X$_2$ (X=S, Se, Te), revealing their stability, indirect band gaps, and Lifshitz transitions at specific hole concentrations.

Contribution

It provides the first detailed DFT-based predictions of band structures, optical spectra, and Lifshitz transitions in 2D Ga$_2$X$_2$ materials.

Findings

All three materials are dynamically stable indirect-gap semiconductors.

They exhibit a Mexican-hat dispersion near the valence band maximum.

Lifshitz transitions occur at specific hole doping levels.

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$^{-2}$.