Unveiling Polymorphs and Polytypes of the 2D Layered Semiconducting Gallium Monosulfide

Polymorphism is gaining attention among van der Waals layered materials. In the case of gallium monosulfide, a hexagonal phase has predominantly been reported. Here the successful growth of rhombohedral gallium sulfide (GaS) with R̄3m space group on sapphire substrates using chemical vapor deposition (CVD) is presented. Crystallographic analysis reveals that the CVD GaS R̄3m has preferred 𝑐-axis orientation, low microstrain and moderate mosaicity. A combined approach of X-ray diffraction, Raman spectroscopy and photoluminescence measurements with first principles calculations is used to determine phononic and photonic properties of the rhombohedral GaS with direct band gap of 2.55 eV and near-blue light emission at room temperature. These results pave the way to novel applications in optoelectronics and photonics, particularly for development of efficient light-emitting devices such as LEDs or lasers, quantum dots, photodetectors, and integrated photonic circuits.