Discrete defect levels in the amorphous selenium bandgap

Two discrete defect levels that are distinct from the levels of the standard negative-U model have been located in the a-Se bandgap. One level, situated near the equilibrium Fermi energy level, corresponds to the deep electron trap whose effects on carrier transport have been discussed in the past. It leads to a low-temperature sensitization of the photocurrent. The other defect level is a much more shallow electron trap, some 0.28 eV below the conduction band mobility edge. Its signature is clearly seen in the transient photocurrent after pulsed across-gap excitation.