Arsenic is added to amorphous selenium to retard crystallization and to 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 electrophotography. 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, excess chlorine decreases the electron range. When these alloys are employed as photosensitive elements for xeroradiography purposes in which x-rays are bulk absorbed and both hole and electron transports contribute to the sensitivity, the excess chlorine will have the effect of reducing the sensitivity. In this article, compensation of arsenic by chlorine has been explained in terms of structural considerations. These considerations and heat of formation data suggest that approximately 30-ppm chlorine is required to compensate 0.33-atom % arsenic. A technique has been developed to map the excess chlorine profiles. This is based on the principle that if a small electron current is injected into the film from a biased substrate in a time-of-flight setup, the electrons are selectively trapped at these excess chlorine sites. The resulting electric field profiles have been measured and related to the excess 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 chlorine.
D. M. Pai, "Time-of-Flight Study of the Compensation Mechanism in a-Se Alloys" in Journal of Imaging Science and Technology, 1997, pp 135 - 142, https://doi.org/10.2352/J.ImagingSci.Technol.1997.41.2.art00007