Transition from diffusive to ballistic capture related to hydrogen incorporation in amorphous silicon

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Abstract
We have shown in previous work that, in triode d.c. sputtered amorphous silicon, the value of the capture cross-section σ of deep gap states depends strongly on the hydrogen content of the material. In this Letter, we analyse the influence of such an effect on the transport properties of a set of samples whose hydrogen content C H varies between 8 and 20%. For this purpose, we have measured at room temperature the mobility–lifetime product for electrons, μnτn, the density of states at the Fermi level N(EF) and the defect density N s derived from photothermal deflection spectroscopy experiments. Log–log plots of the μnτn N(E F) and μnτn N s products against σ show two regimes with a transition corresponding to a hydrogen content of about 12%. These two regimes are related to different modes of capture controlled either by a diffusive process (low C H) or by a ballistic process (high C H). From the transition between the two regimes, we calculáte that the electron scattering length is of the order of 11 Å.