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

The suitability of stabilized amorphous selenium (a-Se:0.2-0.5% As, 10-20 p pm Cl) as an x-ray imaging photoconductor is largely determined by its char ge-generation, transport and trapping properties. The product of the charge -carrier drift mobility, mu, deep-trapping lifetime, tau, and applied elect rical field, F, known as the Schubweg, represents the average distance that a charge carrier travels in the transport band before being trapped in dee p localized states within the mobility gap. Time-of-flight and interrupted- held 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 abo ut 500 mu s and about 750 mu s respectively which are much longer than typi cal transit times in these photoconductors. The observed field dependence o f the x-ray sensitivity is therefore not due to any Schubweg limitations. T he energy required to create a free pair of electrons and holes, W-EHP, 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 sel enium layer. W-EHP 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. W-EHP was shown to be temperature independent over the range 263-300 K. This result is in accord with the col umnar recombination theory for the origin of the field dependence of W-EHP for a-Se proposed by Hirsch and Jahankhani.