First-principles prediction of the missed Pmmn phase for a GaTe monolayer as a new two-dimensional semiconductor

In this study, we predicted a new semiconducting phase for a GaTe monolayer with remarkable stability using first-principles calculations and the RG2 code, which implements the random sampling strategy combined with space group and graph theory. The new phase (Pmmn) possesses a very similar atomic configuration to the ground state C2/m phase. The calculated total energy of the GaTe monolayer in the Pmmn symmetry is just 4 meV/atom higher than that of the experimentally realized C2/m phase, and exfoliation energy of Pmmn GaTe is lower than that of C2/m phase, which indicates a high probability of synthesizing GaTe in this novel Pmmn configuration in the future. The calculated vibrational spectrum and elastic constants also suggest that the Pmmn configuration of the GaTe monolayer is dynamically and mechanically stable. It is a direct band gap semiconductor with a gap of 2.15 eV, which is very similar to that of the C2/m phase (2.08 eV). The simulated mechanical properties show that the GaTe monolayers in both the Pmmn and C2/m symmetries exhibit anisotropy. Our present work provides a new structural candidate for GaTe monolayers synthesized using the bottom-to-top method. Moreover, such a new structure with Pmmn symmetry can also be considered as a fundamental configuration for predicting other III–VI monolayer semiconductors.