Huge Second-Harmonic-Generation Difference for Phase Identification for h- and m-GaTe

Two-dimensional (2D) GaTe, an emerging III–VI semiconductor, can exist in hexagonal (h) or monoclinic (m) phases with fascinating phase-dependent properties, e.g., isotropic or anisotropic electrical transport. Fast, nondestructive, easy, and low-cost characterization of these two phases on a large scale is essential for controlling production and assembling processes. In this study, the h-phase and m-phase are found to have very close values of band gaps using first-principles methods (at the HSE06 level). Besides, the linear optical spectrum is not easy to use to identify these two phases. However, second harmonic generation (SHG) spectra under the independent particle approximation are quite different. The SHG coefficient of h-GaTe monolayer is up to 520.53 pm/V at 557 nm, higher than that of the MoS2 monolayer. In contrast, there is no SHG in the m-phase. The high SHG mainly stems from interband transitions near the energy bandgap, and it has excellent localization within the Brillouin zone. The angle-resolved SHG intensity from the h-GaTe monolayer exhibits highly anisotropic behavior. Overall, our theoretical calculations demonstrate that SHG offers a rapid, simple, and nondestructive approach to distinguish h-phase and m-phase monolayers, underscoring its potential as a practical tool for phase identification in related research fields.