Indirect forbidden fundamental absorption edge in germanium selenide single crystals

The absorption coefficient of GeSe is determined from the measured values of transmissivity and reflectivity. The measurements are carried out with plane polarized light with the plane of polarization parallel to the a‐ and c‐axes which lie in the cleavage plane in the temperature range from room temperature to near liquid nitrogen temperature. Near the fundamental absorption edge the results are fitted to an equation corresponding to an indirect forbidden transition with a single type of phonon engaged in the interband absorption process. The optical energy gap at room temperature is found to be (1.075 ± 0.017) and (1.080 ± 0.016) eV for the a‐ and c‐axes, respectively. The dependence of the energy gap on temperature is linear in the temperature range from 96 to 250 K with a negative temperature coefficient dEg/dT equal to −0.43 and −0.49 meV/K for the a‐ and c‐axis, respectively.