Space Charge Limited Current Flow and Deep Trapping in Selenium

Current-voltage characteristics of evaporated specimens of vitreous selenium have been studied. They show an ohmic part at low applied voltages and, with increasing hole injection, a V 2 region which precedes the trap-filled limit as predicted by Lampert's theory. Ohmic contacts of tellurium or platinum were formed on newly prepared specimens by the prolonged application of a moderately high electric field, but this was not found possible with a low work function electrode such as magnesium. The detailed analysis of the results from formed specimens between 2 and 31 μ thick indicates the existence of a volume distribution of hole traps of density 9×10 13 cm -3 , 0.79 ev above the valence band. These values agree with independent results from measurements of the transient photoconductivity after saturation of the centres with strong illumination. At room temperature the average trapping time is 50 seconds leading to a capture cross section of 8×10 -16 cm 2 . The presence of the 0.79 ev level offers some explanation for the pronounced difference between the absorption and photoconductive edges in vitreous selenium. It is concluded that the level lies near the upper edge of a comparatively dense distribution of states extending towards the valence band.