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Large polaron and bipolaron formation is reviewed with particular emphasis on the screening effects which become important at relatively high carrier density (plasma frequency, ωp, larger than longitudinal optical frequency, ω1). At high density polarons are mobile particles even at very high electron-phonon coupling constant (a»l) and bipolaron binding energy tends to vanish giving evidence of the existence of metastable bipolaron states. The coexistence of metastable bipolarons (bosons) and polarons (fermions) supports the so-called boson-fermion model of superconductivity. This model can be generalized in order to take into account the dependence on the carrier density given in the polaron-bipolaron framework. The results are in good agreement with the density dependence of the critical temperature exhibited by many high-Tc superconductors.

In this paper we report on the frequency dependence of the AC conductivity of porous silicon in the range 10 Hz-100 kHz. Two types of testing devices have been fabricated on three different series of samples formed electrochemically using as a starting material ptype, n −-type and n +-type silicon substrates. For frequencies less than 20-40 kHz the conductivity is found to follow a sublinear frequency dependence. This behaviour is typical of a carrier transport mechanism determined by an anomalous diffusion process. At higher frequencies we find that surface states influence the transport mechanism. This suggests a double-channel transport mechanism: one related to porous-silicon “volume” properties and the other more connected to the “surface” itself.