We present a comprehensive study of bulk GeS optical properties and their anisotropy, by investigation of the fundamental band gap of the material and the energetically higher direct transitions.

GeS and its analog compounds exhibit unique properties that combine some of the most desired features of other two-dimensional compounds, such as transition-metal dichalcogenides and graphene. These include high electron mobilities or valley physics that result in strong optical and electronic anisotropy. Here, we present an experimental and theoretical study of the electronic band structure of GeS at high hydrostatic pressures. Polarization-resolved high-pressure photoreflectance measurements allow us to extract the energies, optical dichroic ratios, and pressure coefficients of the direct optical transitions. These findings are discussed in view of first-principles calculations, which predict that nondegenerate states in different valleys can be individually selected through linearly polarized light. Based on this, an assignation of the direct optical transitions to the electronic band structure is provided. Finally, the effect of pressure on the electronic band structure is discussed in terms of orbital composition. These results provide evidence that GeS is a strong candidate for valleytronic applications in nondegenerate systems.

Group-IV monochalcogenides, such as germanium selenide (GeSe) are strongly anisotropic semiconducting van der Waals crystals isoelectronic to black phosphorus, with superior stability in air conditions. High optical absorption, good conductivity,...