PROPERTIES OF FREE-CARRIER TRANSPORT IN A-SE AND A-SI-H

The existence of the localized tail states, typical of amorphous semiconductors, makes the study of the mobility of free carriers (mu-o) very difficult. Here we first review a few ways how to exclude the effect of localized states, which have been successfully used in a-Se. It is shown how the subnanosecond TOF (Time of Flight) method can be used for the study of trapping kinetics into the tail states and for the finding of the temperature dependence of the free mobility of holes in a-Se. Deactivation of the drift mobility (mu) and the impact ionization at high electric fields allowed us to describe the properties of hot holes in a-Se and experimentally construct the mobility gap. Although the properties of drift mobilities in a-Se and a-Si:H (related with the tail states) are very similar, the free carrier transport related properties are completely different. The Onsager photogeneration, Langevin-like recombination and trapping (the proof of the existence of which is discussed in detail), and field induced impact ionization, the properties typical of a-Se, have not been observed in a-Si:H.