Tail states

In phenomenological terms, the presence of a distribution of traps is equivalent to an increase of the number of tail states of the DOS and, concomitantly, opens additional pathways to the relaxation of charge earners towards sleeper states. A zero order analytic description of the effect can be based upon the Hocsterey and Letson [75] formalism. The latter is premised on the argument that the carrier mo-... 

In a-C H, the tail states are dominated by n electrons, which results, as pointed out by Robertson [99, 100], in an enhanced localization as compared to a-Si H, giving rise to higher band tail density of states and also to higher defect density in the midgap. 

In amorphous semiconductors, information about the width of the band tail states (or disorder) may also be extracted from the optical absorption spectra. For photon energies near bandgap energy, the optical absorption coefficient of amorphous semiconductors exhibit an exponential dependence on the photon energy, following the so-called Urbach relationship ... 

This disorder decrease on alloying is in principle unexpected, because alloying introduces chemical disorder in addition to bond disorder. However, Silva et al. [14] reported results on the joint density of states as determined by EELS which showed a clear decrease in the density of tail states at 7-at.% N and showed a further increase for higher nitrogen content. 

While most of the research in metastable defect formation has focussed on light-induced defects, there has recently been growing interest in thermally generated defects. Smith and Wagner (1985 Smith et al., 1986) extended the proposed Staebler-Wronski mechanism of electron-hole recombination via band tail states, resulting in the formation of dangling... 

T. Tiedje, Information about Band-Tail States from Time-of-Flight Experiments Arnold R. Moore, Diffusion Length in Undoped a-Si H W. Beyer and J. Overhof, Doping Effects in a-Si H H. Fritzche, Electronic Properties of Surfaces in a-Si H CR. Wronski, The Staebler-Wronski Effect... 

As shown in Fig. 2- 3, localized electron levels arise (A and C in the figure) near the band edges at relatively high state densities tailing into the band gap these are called diffuse band tail states. Further, localized electron levels may occur due to dangling bonds and impurities (B in the figure) in the band gap, which are called gap states. 

Fig. 2-33. Electron energy and state density in amorphous semiconductors A and C = diffuse band tail states B = gap states, cmc = mobility edge level for electrons MV = mobility edge level for holes ...

Electrons in the diffuse band tail states migrate in accord with the hopping mechanism rather than the band mechanism. The energy gap between the mobility edge Cue for electrons and the mobility edge mv for holes is called the mobility gap, Emt, instead of the band gap, C(. 

T. Tiedje, Information about band-Tail States from Time-of-Flight Experiments... 

The tail states associated with F"-ion substitutions also introduce the possibility of variable-range hopping and hence a In a versus dependence. Indeed, Graener et... 

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