Abstract:
Amorphous selenium (a-Se) is an important photoconductive material used as a lightsensitive layer in X-ray digital imaging detectors. The performance of these detectors is determined by the electronic transport properties of the a-Se.
The problem is that pure amorphous selenium is not stable and tends to crystallize at
temperatures near room temperature (~ 40 ° C). The addition of arsenic in small amounts to selenium, less than 1%, on the one hand, hardened material and slowed down crystallization process, but on the other hand affects the transport properties by decreasing the time of life of the holes. This fact is compensated by the addition of a few ppm of chlorine, which ensures the removal of negative charged defects, which present traps for the holes. Our investigation focused on the experimental measurement of photocurrents in chlorine-doped pure a-Se, and on the changes in the density of states (DOS) made by the addition of chlorine. Samples of pure Se doped with 12.5 ppm and 67 ppm of chlorine were prepared in thin films of 10 to 20 μm thick. The transient photocurrent measurements of the carriers were made on the doped samples using current and effective techniques (TPC and TOF), at different temperatures and under different electric fields, and compared by those of pure Se. The addition of a considerable amount of chlorine, in ppm (67 ppm) radically changes the density of localized electronic states of pure Se. The defect at 0.2 eV above the edge of the valence band of pure Se, connected to the change of alternation of the dihedral angle, disappears completely and the defect at 0.45 eV above the edge of the valence band, connected to the D- centres, (negative correlation energy defect) decreases to levels below the detection limit of the technique used. The experimental results have been interpreted in the framework of the multiple trapping conduction model. The TOF traces clearly show that a transient electron photocurrent cannot be measured, this is interpreted by the creation of new D+ centres (electron trap) in the mobility gap, next to the conduction band. On the other side of the valence band, Cl atoms eliminate
