Band transport

In an attempt to combine band-Uke charge carrier motion realized in an -inevitably fragile - crystalline FET structure with structural robustness and flexibility, Sakanoue and Sirringhaus [167] prepared FETs using spin coated films of 6,13-bis(triisopropylsilylethynyl)(TIPS)-pentacene films in contact with a perfluorinated, low dielectric-constant polymer gate electrode. The (linear) hole mobility at room temperature is 0.8 cm /V s with tendency of an apparent band-like negative temperature coefficient of the mobility (d/i/dT 0). 

The authors use optical spectroscopy of gate-induced charge carriers to show that, at low temperature and small lateral electric field, charges become localized onto individual molecules in shallow trap states, but that at moderate temperatures an electric field is able to detrap them, resulting in transport that is not temperature-activated. This work demonstrates that transport in such systems can be interpreted in terms of classical semiconductor physics and there is no need to invoke onedimensional Luttinger liquid physics [168]. 

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At very low temperatures, Holstein predicted that the small polaron would move in delocalized levels, the so-called small polaron band. In that case, mobility is expected to increase when temperature decreases. The transition between the hopping and band regimes would occur at a critical temperature T, 0.40. We note, however, that the polaron bandwidth is predicted to be very narrow ( IO Viojo, or lO 4 eV for a typical phonon frequency of 1000 cm-1). It is therefore expected that this band transport mechanism would be easily disturbed by crystal defects. 

The cubic (ao = 8.441 A) end member Zn[Fe2]04 was seen to be n-type with mobile electrons moving at the bottom of a narrow 3d band. Transport properties... 

Transport in DNA samples with all bases the same could be either by free carriers, i.e., band transport, or by polarons. As will be further discussed in the next section, the polarons are expected to be large polarons, not small. In the conducting polymers there is overwhelming evidence that electrons (holes) from a metal contact are injected directly into polaron states in the polymer, because the polaron states have lower energies than the LUMO (HOMO) or conduction (valence) band edge. As has recently been shown theoretically [30], the injection takes place preferably into a polaron state made available when a polaron-like fluctuation occurs on the polymer chain close to the interface, rather than into a LUMO state, with subsequent deformation to form the polaron. It could also be expected for DNA that injection... 

Instead, transport is most likely a hopping process involving states associated directly with the silicon backbone. Trap-controlled band transport. 

Figure 1 Schematic presentation of the two different mechanisms governing charge transport in organic semiconductors. The so-called hopping transport assumes a thermally activated hopping between charge-traps and is always present, the resulting mobilities are very small. For certain materials a mechanism yielding much higher mobilities has been observed, which is commonly referred to as band-like , thus implying a similarity to band-transport observed in conventional semiconductors.

Evidence for band transport is often claimed to be brought when the temperature dependence in Equation (2.2.8) is observed. The most celebrated example for such a behavior is that by Karl and coworkers on highly pure crystals of acenes... 

The spin splitting of the energy bands of semiconductors without inversion symmetry has other interesting and presumably also technological consequences. Photoemitted electrons from GaAs are spin polarized when the light used for the irradiation is circularly polarized.[49,50] (such effects may also be observed for metals, see for example [51]). The so-called three step model for photomission considers the three processes, optical excition from a valence-band state to a state in the conduction band, transport of the excited electron to the surface, and — finally — emission through the... 

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