THE PROPERTIES OF FREE-CARRIERS IN AMORPHOUS-SILICON

The properties of free carriers photogenerated in the extended states of hydrogenated amorphous silicon have been investigated using the techniques of femtosecond time-resolved spectroscopy. The optical susceptibility of free carriers can be described by a Drude model with a relaxation time shorter than 1 fs. This relaxation time seems to remain constant for various experimental situations. It implies a very small mobility (approximately 6 cm2/V s) in the extended states. The hot carriers thermalize quickly to the mobility edge by emission of phonons. The thermalization rate is found to be greater-than-or-equal-to 1 eV/ps. In addition, the decay time of optic phonons into acoustic phonons is less-than-or-equal-to 100 fs. The photogenerated carriers recombine non-radiatively in a time that can be as short as 1 ps at very large injected density (N less-than-or-equal-to 10(21) cm-3). Several regimes are distinguished, depending on the value of N. In general, the characteristic times for all these processes are much shorter in a-Si:H than in a typical direct-gap crystalline semiconductor such as GaAs. The difference can be traced to the lack of momentum conservation in amorphous semiconductors.