Temperature- and photo-excitation effects on the electrical properties of Tl4Se3S crystals

The extrinsic energy states and the recombination mechanism in the Tl4Se3S chain crystals are being investigated by means of electrical and photoelectrical measurements for the first time. The electrical resistivity is observed to decrease exponentially with increasing temperature. The analysis of this dependence revealed three impurity levels located at 280, 68 and 48 meV. The photocurrent is observed to increase as temperature decreases down to a minimum temperature T-m=200 K. Below this temperature the photocurrent decreases upon temperature lowering. Two photoconductivity activation energies of 10 and 100 meV were determined for the temperature ranges below and above T-m, respectively. The photocurrent (I-ph) versus illumination intensity (F) dependence follows the I-ph proportional to F-gamma law. The value of gamma decreases from similar to 1.0 at 300K to similar to 0.34 at 160K. The change in the value of gamma with temperature is attributed to the exchange of roles between the monomolecular recombination at the surface near room temperature and trapping centers in the crystal, which become dominant as temperature decreases