PHOTOCONDUCTIVITY IN HYDROGENATED AMORPHOUS-SILICON .1. THERMAL EMISSION AND HOPPING OF TRAPPED CHARGES

The photoconductivity of hydrogenated amorphous silicon is found to be the result of three different processes that have been identified usi ng the dual-beam-modulated photoconductivity technique: the release of electrons trapped either in D- states or in the conduction-band tail, and a quenching process related to recombination. Each process follow s its own generation rate dependence. A model has been established in which the above three processes add up to give the observed steady-sta te photoconductivity. The relative contributions of these processes ha ve been determined over large temperature and light flux ranges. This allowed us to show firstly that the thermal release of electrons from gap states, either from D- or from the band tail, is still an importan t process at 90 K, secondly that the relative contributions of D- and tail states change drastically at about 165 K, that is when the electr on quasi-Fermi level is located at about 0.3 eV below the mobility edg e and thirdly that at low temperatures the above three processes canno t account for the whole photoconductivity and hopping transport of ele ctrons has to be taken into account, the contribution from which is ab out 50% at 90 K.