TIME-OF-FLIGHT STUDY OF THE COMPENSATION MECHANISM IN A-SE ALLOYS

Arsenic is added to amorphous selenium to retard crystallization and t o improve its mechanical properties. Arsenic in sufficient quantities acts as a weak hole trap that results in buildup of residual potential when subjected to charge-expose-erase cyclic conditions employed in e lectrophotography. A halogen, chiefly chlorine, that by itself acts as an electron trap in amorphous selenium is added to arsenic containing selenium to eliminate residual potential when operated in a positive charging mode (hole transport). Some selenium alloys contain 0.33-atom % arsenic and 30-ppm chlorine. Chlorine in excess of that required to compensate arsenic in bulk does not affect hole transport. However, e xcess chlorine decreases the electron range. When these alloys are emp loyed as photosensitive elements for xeroradiography purposes in which x-rays are bulk absorbed and both hole and electron transports contri bute to the sensitivity, the excess chlorine will have the effect of r educing the sensitivity. Tn this article, compensation of arsenic by c hlorine has been explained in terms of structural considerations. Thes e considerations and heat of formation data suggest that approximately 30-ppm chlorine is required to compensate 0.33-atom % arsenic. A tech nique has been developed to map the excess chlorine profiles. This is based on the principle that if a small electron current is injected in to the film from a biased substrate in a time-of-flight setup, the ele ctrons are selectively trapped at these excess chlorine sites. The res ulting electric field profiles have been measured and related to the e xcess chlorine profiles. Experimentally, a film containing 0.33-atom % arsenic and approximately 40-ppm chlorine is ideally compensated, and excess chlorine is observed in films containing more than 40-ppm chlo rine.