Charge transport and electron-hole-pair creation energy in stabilized a-Se x-ray photoconductors

The suitability of stabilized amorphous selenium (a-Se:0.2-0.5% As, 10-20 ppm Cl) as an x-ray imaging photoconductor is largely determined by its charge-generation, transport and trapping properties. The product of the charge-carrier drift mobility, , deep-trapping lifetime, , and applied electrical field, F, known as the Schubweg, represents the average distance that a charge carrier travels in the transport band before being trapped in deep localized states within the mobility gap. Time-of-flight and interrupted-field time-of-flight measurements on stabilized a-Se layers suitable for use in x-ray image detectors show that the hole and electron lifetimes are about 500 s and about 750 s respectively which are much longer than typical transit times in these photoconductors. The observed field dependence of the x-ray sensitivity is therefore not due to any Schubweg limitations. The energy required to create a free pair of electrons and holes, , was measured by integrating the x-ray-induced photocurrent to find the number of free charge carriers and dividing that by the energy absorbed in the selenium layer. evinced a strong field dependence which was extrapolated at the highest fields to obtain the intrinsic electron-hole-pair creation energy which was found to be about 6 eV. was shown to be temperature independent over the range 263-300 K. This result is in accord with the columnar recombination theory for the origin of the field dependence of for a-Se proposed by Hirsch and Jahankhani.