The properties of free carriers in amorphous silicon

• Published in: Journal of non-crystalline solids 1992, Vol.141 (1-3), p.76-87
• Main Authors: Fauchet, P.M., Hulin, D., Vanderhaghen, R., Mourchid, A., Nighan, W.L.
• Format: Article
• Language: English
• Subjects: Charge carriers: generation, recombination, lifetime, and trapping; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Conductivity phenomena in semiconductors and insulators; Electronic transport in condensed matter; Exact sciences and technology; Physics
• ISSN: 0022-3093; 1873-4812
• DOI: 10.1016/S0022-3093(05)80521-6

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 cm 2/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 ≤ 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.