Photoconductivity in hydrogenated amorphous silicon. I. 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 using 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 follows its own generation rate dependence. A model has been established in which the above three processes add up to give the observed steady-state photoconductivity. The relative contributions of these processes have 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 important process at 90 K, secondly that the relative contributions of D− and tail states change drastically at about 165 K, that is when the electron quasi-Fermi level is located at about 0.3 eV below the mobility edge and thirdly that at low temperatures the above three processes cannot account for the whole photoconductivity and hopping transport of electrons has to be taken into account, the contribution from which is about 50% at 90 K.