Resonant exciton scattering reveals Raman forbidden phonon modes in layered GeS

Abstract Semiconducting layered group-IV monochalcogenides such as black phosphorous and germanium monosulfide with an anisotropic puckered crystalline structure in each layer have recently attracted much attention due to their unique optical and electronic properties. However, exciton-phonon interactions were only superficially elucidated, although they tremendously affect the opto-electronic operation principles and performance. We study the resonant Raman scattering and the photoluminescence of the optically active Γ-exciton in layered GeS flakes and evaluate the exciton and phonon responses on variations in the excitation energy, laser-light and emission polarizations, temperature, and laser power. The resonant Raman scattering leads to the observation of dark first- and second-order optical phonon modes whose symmetries and energies are calculated by means of a density functional perturbation theory. We reveal a double-resonance mechanism activating the Raman forbidden (dark) longitudinal-optical scattering processes: For (quasi)-resonantly exciting excitons in the GeS flakes the selection rules become relaxed so that a fourth-order Fröhlich intraband process is mediated by the scattering of the electron with a longitudinal-optical and an acoustic phonon. Our experiments demonstrate considerable coupling between phonons and photogenerated carriers in GeS flakes and the high efficiency of multi-order scattering in optical processes, and outline that layered GeS as direct band-gap semiconductor provides a promising material system for opto-electronic applications.