Hopping Recombination in Trigonal Selenium Single Crystals

The decay of the photocurrent in trigonal selenium single crystals is investigated as a function of temperature, applied voltage, and simultaneous illumination with quenching light. At 90°K the photocurrent decays logarithmically with time. This behaviour is explained by the assumption that the recombination process is due to transitions of minority carriers, i. e. electrons, from traps to recombination centres. The decay rate is considerably enhanced when trapped minority carriers are raised to the conduction band: at higher temperature by thermal activation or at low temperature both by optical activation and by a strong electric field. Two kinds of electron traps are found, the optical activation energies of which exceed by far the thermal ones.