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 the microwave electron-cyclotron-resonance PE CVD and by the glow discharge decomposition. Pump and probe techniques were used to measure transient absorption on a picosecond time scale. TWO components are resolved in the dynamics: one component (similar to 10(-10) s) is interpreted as a bimolecular recombination (bimolecular coefficient B approximate 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 the recombination of carriers trapped in the localized states. The similarity 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 picosecond absorption dynamics is dominated by relaxation and recombination 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 potential localization sites for photocarriers in wide gap a-Si:H is obviously larger. (C) 1998 Elsevier Science B.V. All rights reserved.