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Finally, we should discuss the other transport mechanisms which have been proposed so far. The temperature-dependent dc conductivity below room temperature, on one hand, has been represented by the following expression in undoped and doped ^c-Si:H films:

where T0 is a constant and x = 1 [12, 13]. This type behavior can be understood either by variable-range hopping in the Coulomb gap [32] or by nearest-neighbor hopping between granular (conducting) islands [33]. Ram et al. [13] suggest that the unrealistically large Coulomb gap is estimated from the parameter T0 and hopping of electrons between granular islands dominates the transport. It should be noted that the pre-factor o0 in Eq. (4) has a serious problem: It is well known that there is the discrepancy, by many orders of magnitude, between experimental and theoretical values of the constant o0 [34]. This means that the experimentally observed conductivity cannot be replicated by Eq. (4) quantitatively. To the present author's view, therefore, a careful treatment using the concept of variable-range hopping may be required for reaching proper conclusion.

The model of thermionic emission over potential barriers cannot be applied to the carrier transport at lower temperatures below 20 K. Instead, nearest-neighbor hopping of electrons between donor sites has been suggested, in which carriers move through potential barriers (hopping sites also exist in barrier regions). Hopping of carriers is a special issue and hence the details of hopping conductivity at lower temperatures are discussed elsewhere [14].

There are also many interesting features of these inho-mogeneous materials, for example, the growth mechanisms and related ac transport [2] and the photoconductivity [19]. A general behavior of photoconductivity in nc-Si:H is very similar to that found in a-Si:H [19]. A recent work of ac and dc photoconductivities in a-Si:H [35, 36], in which "inho-mogeneity" is taken into consideration, may be useful to understand the photoconductivity in the present materials as well. These were not discussed in the present brief review.

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