Hopping Transport of Electrons and Holes at Localized Band Tail States in Amorphous Hydrogenated Silicon and Amorphous Heavy-Hydrogenated Silicon

The electric field dependence of the transit time in a time-of-flight experiment on amorphous hydrogenated silicon (a-Si:H) and amorphous heavy-hydrogenated silicon (a-Si:D) samples with device quality, prepared under different conditions, has been measured at different temperatures. It has been shown that the dependences can be explained by hopping at localized band tail states rather than multiple trapping. The hopping distance is in the range of 15 to 26 A for both electrons and holes. The thermal activation energy for hopping is in the range of 0.10 to 0.24 eV for electrons and in the range of 0.32 to 0.38 eV for holes. The attempt-to-escape frequency obtained is larger than the phonon frequency, which has been explained by the Eyring theory. The electron mobility in a-Si:H and a-Si:D is in the range of 10-1 to 10 cm2/(V s), while the hole mobility is in the range of 10-3 to 10-2 cm2/(V s). The difference in the mobility between electrons and holes has been explained from the difference in thermal activation energy between the two types of carrier.