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 (∼ 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 ≥ 1 eV/ps. In addition, the decay time of optic phonons into acoustic phonons is ≤ 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 ≤ 1021 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.