Drift Mobilities of Electrons and Holes and Space-Charge-Limited Currents in Amorphous Selenium Films

The band model has been applied to semiconducting amorphous selenium. Drift mobilities in pure films agreed well with previous results of Spear, and some evidence was obtained that the shallow electron and hole traps were caused by imperfections, with further considerations suggesting that the traps were distributed continuously in energy. Electron drift mobilities were reduced when arsenic was added to the films but their temperature dependence was unchanged, suggesting that arsenic increased the concentrations of imperfections which produce shallow electron traps. Hole ranges were (2-4) 10^-8 cm^2/v and exhibited no observable temperature dependence, while electron ranges were (1-2) 10^-7 cm^2/v at 300^ and decreased with decreasing temperature. Gold and tellurium electrodes were used to inject holes into amorphous selenium, producing space-charge-limited currents at high fields. The observed currents indicated hole capture centers distributed uniformly in energy in a region extending downwards from 1 ev above the valence band edge. The densities of these levels were 10^15 cm^-3 ev^-1. Electrical conductivities of 10^-14 to 10^-16 ohm^-1 cm^-1 were obtained at low fields. The lowest value of conductivity agreed well with values extrapolated from liquid selenium. The drift of photoinjected electrons in the presence of space-charge-limited currents was used to probe the electric field. The general spatial dependence was that expected of space-charge-limiting by holes, but could not be explained quantitatively.