Optical properties of orthorhombic germanium selenide: an anisotropic layered semiconductor promising for optoelectronic applications

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, and band gap ranging from 1 to 2 eV make these materials suitable for various optoelectronic applications; however more in-depth investigation of their fundamental properties is required. We present a comprehensive study of bulk GeSe by means of optical absorption and modulation spectroscopy, supported by theoretical density functional theory (DFT) calculations of the electronic band structure. Our experimental results reveal that the optical properties of GeSe are dominated by direct transitions; however the fundamental band gap might in fact be indirect and could not be observed in the experiment due to low oscillator strength. Such interpretation is in agreement with our calculations, providing the picture of the first Brillouin zone with multiple band extrema in close energy proximity. In order to investigate the anisotropy of the material, polarization-resolved measurements have been performed, revealing a strong dependence of the observed optical transitions on light polarization. Photogenerated current measurements resulted in reasonably high photoconversion efficiency and fast response time, implying that GeSe is a promising material for photoconversion applications.