Space-charge-limited current flow and deep trapping in vitreous selenium.

The electrical properties of evaporated vitreous selenium layers have been studied using both the injection and volume-generation of additional charge carriers. The current-voltage characteristics of specimens between 2 and 31mum thick were investigated and the results analysed in terms of the Rose-Lampert 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 indicates the existence of a volume distribution of hole traps of density 9 x 1013 cm.;-3; 0.79 eVabove the valence band. The comparative shallowness of the characteristics at higher voltages suggests that this level forms part of a distribution of states increasing in density towards the valence band. It is shown that the model of an exponential trap distribution is consistent with the experimental results and also leads to an explanation of the unusual optical properties of vitreous selenium. Measurements of the transient response of carriers generated by pulses of light or high energy electrons show the traps in the 0.79 eV level have a release time of about 50 seconds at room temperature and a capture cross-section for holes of 8 x 10.;-16 cm2. The lifetime of voltage injectedholes with respect to these centres is about 10? seconds. On the basis of the electrical results, both the absorption and photoconductive edges have been calculated as a function of the photon energy and it has been possible to account for the marked energy difference between the two. The interpretation shows that at room temperature the band gap for vitreous selenium is 2.45 eV. Band models of selenium are briefly reviewed and it is suggested that the trap distribution is most likely connected with chain ends.