The indirect forbidden fundamental absorption edge in single-crystal germanium sulphide

The optical energy gap of single-crystal GeS was calculated from the absorption coefficient near the fundamental absorption edge. Measurements were carried out from room temperature to near liquid-nitrogen temperature using plane-polarised light with the plane of polarisation being parallel to either the a or the b crystallographic axes. The fundamental absorption edge in single-crystal GeS was found to be due to an indirect forbidden transition. The two-dimensional optical energy gap at room temperature was found to be 1.544 eV and 1.572 eV for the a and b axes, respectively, with phonon energies 22 meV and 26 meV and temperature coefficients of -0.54 meV K -1 and -0.63 meV K -1 for the same axes, respectively. The three-dimensional optical energy gap at room temperature was found to be 1.540 eV and 1.550 eV for the a and b axes, respectively, with phonon energies of 20 meV and 22 meV and temperature coefficients of -0.52 meV K -1 and -0.60 meV K -1 , respectively. The temperature dependence was found to be linear between 92 and 300 K.