Computational study on the electronic structure, phonon, dynamic, thermal, and optical properties of XTe (X = Ga, Ge and GaGe) monolayers

Abstract

We perform an extensive investigation of two-dimensional, 2D, semiconductor structures consisting of hexagonal XTe (where X is Ga, Ge, or GaGe). The $h$-XTe have naturally planar buckling which effectively influences their physical properties. The buckling values of GaTe and GaGeTe are equal, and it is smaller than the buckling of GeTe. AIMD simulations confirm that the structure with lower buckling is more thermally stable than the higher buckled one and the phonon dispersion indicates all three considered monolayers are dynamically stable. A monolayer with higher buckling has stronger covalent interactions compared to lower buckled monolayers, and it thus induces stronger localization of electronic states, which often results in a larger band gap. This difference in the band gap due to planar buckling influences thermoelectric properties such as the Seebeck coefficient, and the power factor at the low temperature regime. On the other hand in the high temperature regime, a highly buckled structure exhibits low thermal properties such as heat capacity due to degenerate phonon bands. The optical properties reveal that all three monolayers primarily absorb in the IF region around the Fermi energy, while strong peaks in the optical conductivity are seen arising from absorption in the visible light region. A higher plasmon frequency is found for lower buckled monolayers compared to the more buckled ones. The XTe monolayers are exceptional promising for advanced applications in optoelectronics, plasmonics, and thermoelectrics, particularly when incorporated into nanodevices.