We report on direct evidence of ultrafast carrier dynamics displaying featu
res on the picosecond time scale in microcrystalline silicon (muc-Si:H). Th
e dynamics of photogenerated carriers is studied by using above-band-gap op
tical excitation and probing the instantaneous carrier mobility and density
with a THz pulse. Within the first picoseconds after excitation, the THz t
ransmission transients show a fast initial decay of the photoinduced absorp
tion followed by a slower decrease due to carrier recombination. We propose
that the initial fast decay in the THz transients is due to carrier captur
e in the trapping states. (C) 2001 American Institute of Physics.