Transit Time Measurements of Charge Carriers in Amorphous Selenium Films

A technique similar to that used in crystal counter experiments is applied to the study of transit times of electrons and holes in thin evaporated films of amorphous Se carrying Au electrodes. Electron-hole pairs are generated near the top electrode by short electron pulses (or light flashes) of 10 -7 sec duration. The pulses observed when electrons are drawn across the specimen show a perfectly linear rising edge. The measured transit times lead to consistent effective mobility values μ - , between 4.7 and 5.5 × 10 -3 cm 2 sec -1 v -1 . For holes, μ - ≃ 0.15 cm 2 sec -1 v -1 . The measurements are a direct confirmation of deductions made from previous experiments. An investigation of the temperature dependence of mobility shows that μ - is predominantly controlled by thermal release from trapping states lying at a depth of 0.25 eV below the conduction band; for holes the corresponding energy is about 0.16 eV. The observed discharge effects indicate the presence of a range of deeper trapping states. Their effect on the field is normally negligible, but they are likely to act as recombination centres. Recombination processes play an important part in the region of excitation; they cannot be described by a bi-molecular model.