ULTRAFAST CARRIER DYNAMICS IN WIDE-GAP AMORPHOUS-SILICON

We report on study of the picosecond and femtosecond carrier dynamics in wide gap hydrogenated amorphous silicon (a-Si:H). Samples of a-Si:H with band gap >2.1 eV were prepared from silane diluted with He by th e microwave electron-cyclotron-resonance PE CVD and by the glow discha rge decomposition. Pump and probe techniques were used to measure tran sient absorption on a picosecond time scale. TWO components are resolv ed in the dynamics: one component (similar to 10(-10) s) is interprete d as a bimolecular recombination (bimolecular coefficient B approximat e to 5 x 10(-10) cm(3) s(-1)) of carriers in extended states, and the slower component (greater than or similar to 10(-9) s) originates in t he recombination of carriers trapped in the localized states. The simi larity of ultrafast carrier dynamics in wide gap a-Si:H, standard a-Si :H, and in porous silicon is discussed. We have concluded that the pic osecond absorption dynamics is dominated by relaxation and recombinati on of carriers in the delocalized and in the tail states in a similar way as in standard a-Si:H, in spite of the fact that the variety of po tential localization sites for photocarriers in wide gap a-Si:H is obv iously larger. (C) 1998 Elsevier Science B.V. All rights reserved.