Field dependence of the hole transit-time dispersion in As-Cl stabilized amorphous selenium X-ray photoconductors

The tail region of the photocurrent transient observed in the conventional time-of-flight measurements has been experimentally studied as a function o f the applied field and sample thickness for amorphous selenium stabilized with 0.2-0.3% As alloying and similar to 10 ppm Cl doping (stabilized a-Se) . The stabilized a-Se films used in the present study were typical x-ray ph otoconductars for use in x-ray imaging with a detector thickness in the ran ge of 133-425 mum. Dispersions arising from the mutual Coulombic repulsion of charge carriers was found to give a noticable contribution to the total spread of the carrier packet even at relatively low injection level and ext rapolating this function to zero injected charge. The hole transit time dis persion, Deltat(TOF) reveals a power law electric field (F) dependence of t he form Deltat(TOF) similar to F-n where n approximate to 1. It is shown th at the observed dispersion cannot be interpreted on the basis of the multip le trapping model nor conventional diffusion while the concept of charge ca rrier transport within a random potential landscape allows one to explain t he observed ed field dependence of the hole photocurrent transients in term s of a Gaussian distribution of the effective carrier drift mobilities in w hich the normalized mobility spread Delta mu/mu is 0.083 - 0.042.