Photocurrent measurements in chlorine-doped amorphous selenium

Transient photocurrent (TPC) measurements have been carried out on evaporated a-Se films doped with either 12.5 or 67 ppm of Cl and compared with analogous measurements on undoped a-Se samples. The two doping levels lead to strikingly different resulting current traces. While the 12.5 ppm doping level weakens, but does not eliminate the signature of the two hole traps that are observed in the TPC signals of undoped a-Se, no indication for those levels remains in the 67 ppm Cl-doped samples. It is argued that suppression of the trap generated by the negative-U D- centre agrees with the notion that they are being supplanted by doping-induced Cl- centres. The enhanced density of D+ centres and accompanying cross-linking of Se chains by those three-fold coordinated sites leads in turn to the weakening and disappearance of the shallower hole trap ascribed to dihydral angle variations. Results from constant-photocurrent and time-of-flight (TOF) experiments support this interpretation of the TPC observations.