  The dynamics of charge carriers in amorphous semiconductors fundamentally
differ from those in crystalline semiconductors due to the lack of long-range
order and the high defect density. Despite intensive technology-driven research
interests and the existence of well-established experimental techniques, such
as photoconductivity time-of-flight and ultrafast optical measurements, many
aspects of the dynamics of photo-excited charge carriers in amorphous
semiconductors remain poorly understood. Here we demonstrate direct imaging of
carrier dynamics in space and time after photo-excitation in hydrogenated
amorphous silicon (a-Si:H) by scanning ultrafast electron microscopy (SUEM). We
observe an unexpected regime of fast diffusion immediately after
photoexcitation along with spontaneous electron-hole separation and charge
trapping induced by the atomic disorder. Our findings demonstrate the rich
dynamics of hot carrier transport in amorphous semiconductors that can be
revealed by direct imaging based on SUEM.
