Phase Identification of Layered GaS by Polarization-Dependent Angle-Resolved Oblique Incident Second Harmonic Generation

Layered gallium sulfide (GaS), a material that has attracted much attention in the field of micro-nano optoelectronics recently, is predicted to have four stable stacking orders (β-, γ-, ε-, and δ-GaS) with close formation energies; β-GaS is the most considered, and other phases are seldom discussed. However, considering the ease of the phase transition in few-layer materials, the lack of accurate crystal phase identification prevents a full understanding of this material for specific applications requiring other crystal phases. Here, we report a novel in situ and nondestructive method to identify the phase of layered GaS by polarization-dependent angle-resolved oblique incident second harmonic generation (SHG). Through this method, we discovered the presence of γ-GaS with a portion of approximately one-sixth of the total samples. Our work has laid a foundation for the application of GaS, and our approach has established a technical guarantee for structural analysis of van der Waals layered materials.