Field assisted tunneling

Superlattice structures yield efficient charge transport normal to the layers, because the charge carriers can move through the minibands the narrower the barrier, the wider the miniband and the higher the carrier mobility. Transport in MQWs with thick barriers requires thermionic emission of carriers over the barriers, or if electric fields are applied, field-assisted tunnelling through the barriers (Parsons et al, 1990). 

Regime two The fillers are well separated, but their mean distance is below a certain threshold so that electrical field-assisted tunneling can occur between neighboring reinforcement particles. For composite systems near the percolation limit, the conductivity is influenced by the properties of the polymer, the filler material, the interface, and by the dispersion of the filler in the matrix. 

Since in this case, electrons could only be excited in a single well the photocurrent was small. On the other hand, the quantum yield, that is, the number of transferred electrons per absorbed photons, reached values of up to = 0.63 [80]. This might appear surprisingly high for a relatively thick outer barrier layer. However, calculations and measurements of the temperature dependence of the photocurrent showed that at room temperature the mechanism of electron transfer out of the well was thermionic emission over the barrier [80]. The rate of thermionic emission at lattice temperatures in the range of 200—300 K was sufficient to keep up with the measured rate of interfacial electron transfer. Studies with very thin outer barriers (20 A) have shown that the mechanism of charge transfer was field-assisted tunneling, and the photocurrent was then independent of temperature. 

Figure 6 shows the effect of these avoided level crossings can be seen in hysteresis loop measurements. When the applied field is near an avoided level crossing, the magnetization relaxes faster, yielding steps separated by plateaus. As the temperature is lowered, there is a decrease in the transition rate due to reduced thermal-assisted tunneling. 

Nanocrystalline systems display a number of unusual features that are not fully understood at present. In particular, further work is needed to clarify the relationship between carrier transport, trapping, inter-particle tunnelling and electron-electrolyte interactions in three dimensional nan-oporous systems. The photocurrent response of nanocrystalline electrodes is nonlinear, and the measured properties such as electron lifetime and diffusion coefficient are intensity dependent quantities. Intensity dependent trap occupation may provide an explanation for this behaviour, and methods for distinguishing between trapped and mobile electrons, for example optically, are needed. Most models of electron transport make a priori assumptions that diffusion dominates because the internal electric fields are small. However, field assisted electron transport may also contribute to the measured photocurrent response, and this question needs to be addressed in future work. 

Figure 74 Comparison of field-assisted thermionic injection mechanisms in wide- (a) and narrow-band (b) materials. An x0 close to the geometrical contact region is distinguished where Eq. (198) is not applicable. The inapplicability can be associated with field or coordinate dependence of /1, D and s or the coexistence of some other processes such as bimolecular or tunneling recombination which are not included in Eq. (198). After Ref. 361. Copyright 1989 Jpn. JAP, with permission.

Figure 81 Comparison of the field-assisted thermal activation of an electron over the Coulombic barrier located at x=xm (cf. Fig. 74 ) (a) and tunneling through the barrier with (b), at a...

We assume that a source of charge carriers is the local electronic states (traps) in the electrode-nanowire interfaee layer, whose electrons emerge to the conduction band of the erystal due to an electrical field, induced phonon-assisted tunneling. 

In conclusion, the phonon-assisted tunnelling model describes well both the temperature-dependent I-V data and dependence of resistance on temperature in a wide range of temperatures. The electric field strength at which tunnelling occurs was estimated to be in the range of 1-5 MV/m. The densities of localized states in the interface layer were found to be about lO cm in the Bi-rich and 3x10 cm in the Sb-rich BiSbTe nanowires. 

Localization of energy exchanges in field-assisted double-barrier resonant tunneling... 

Localization of Energy Exchanges in Field-Assisted Double-Barrier Resonant Tunneling 181... 

Gordon-Volkov Wavefunctions for Field-Assisted Resonant Tunneling... 

We end with a word about possible inferences for an experimental study of field-assisted resonant tunneling. We have observed that essentially the same conclusions can be drawn if the double-barrier potential is affected by a bias field. The discussion is therefore applicable to situations similar to those mentioned in the Introduction, 7]. If the quanta are phonons or plasmons with different frequencies for the various interfaces in the sample, only the frequencies characteristic of the most external discontinuities should be involved. 

High-field transport properties of single wall carbon nanotubes (SWCNT) are analyzed on the basis of phonon-assisted tunneling (PhAT) model. This model enables to explain not only temperature-dependent current-voltage characteristics of SWCNT, but also the crossover from a semiconducting-like temperature dependence conductivity to a metallic-like one as temperature is increased. 

The central problem in the study of ionic conduction is to discover the details of the atomic transport processes involved in the growth of films. This will be discussed in the first part of this review. The electronic conductivity is of considerable theoretical interest and is of great practical importance for microelectronic devices. We discuss the system in which a thin metal counterelectrode replaces the electrolyte solution in which the oxide was made. Thermionic and field assisted emission, tunneling processes, impurity band conduction, and space-charge limited currents, have to be considered. We shall draw on results for oxide films made by other processes, such as evaporation and thermally promoted reaction with oxygen. 

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