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, μ, deep-trapping lifetime, τ, 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 μs and about 750 μs 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, WEHP, 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. WEHP 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. WEHP 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 WEHP for a-Se proposed by Hirsch and Jahankhani. (author)