PHOTOCONDUCTIVITY IN SINGLE-CRYSTAL GERMANIUM SULFIDE

The photoconductivity spectral response was measured for germanium sulphide (GeS) single crystals at different temperatures from room temperature to near liquid-nitrogen temperature. Measurements were carried out with plane-polarised light with the plane of polarisation parallel to the a and b crystallographic axes, which lie in the plane of cleavage. The photon energy range of measurements extended from 1.0 to 3.5 eV. The photoconductivity energy gap was determined from the spectral response by the Moss rule. The room-temperature photoconductivity energy gap was found to be 1.613+or-0.010 and 1.642+or-0.028 eV for the a and b axes, respectively. The temperature dependence of the photoconductivity energy gap was also deduced. It was found that it is linear between 92 and 300 K with a negative temperature coefficient equal to -0.43+or-0.01 meV K-1 for the a axis. The results for the b axis showed a large discrepancy, which was attributed to crystal lattice bending introduced during the cleavage process. The minority-carrier lifetime was determined from the photoconductivity frequency response, which was found to be characterised by two different lifetimes.