Mobile holes

The impurity atoms used to form the p—n junction form well-defined energy levels within the band gap. These levels are shallow in the sense that the donor levels He close to the conduction band (Fig. lb) and the acceptor levels are close to the valence band (Fig. Ic). The thermal energy at room temperature is large enough for most of the dopant atoms contributing to the impurity levels to become ionized. Thus, in the -type region, some electrons in the valence band have sufficient thermal energy to be excited into the acceptor level and leave mobile holes in the valence band. Similar excitation occurs for electrons from the donor to conduction bands of the n-ty e material. The electrons in the conduction band of the n-ty e semiconductor and the holes in the valence band of the -type semiconductor are called majority carriers. Likewise, holes in the -type, and electrons in the -type semiconductor are called minority carriers. 

In perfect semiconductors, there are no mobile charges at low temperatures. Temperatures or photon energies high enough to excite electrons across the band gap, leaving mobile holes in the Fermi distribution, produce plasmas in semiconductors. Thermal or photoexcitation produces equal... 

Another interesting applications area for fullerenes is based on materials that can be fabricated using fullerene-doped polymers. Polyvinylcarbazole (PVK) and other selected polymers, such as poly(paraphcnylene-vinylene) (PPV) and phenylmethylpolysilane (PMPS), doped with a mixture of Cgo and C70 have been reported to exhibit exceptionally good photoconductive properties [206, 207, 208] which may lead to the development of future polymeric photoconductive materials. Small concentrations of fullerenes (e.g., by weight) lead to charge transfer of the photo-excited electrons in the polymer to the fullerenes, thereby promoting the conduction of mobile holes in the polymer [209]. Fullerene-doped polymers also have significant potential for use in applications, such as photo-diodes, photo-voltaic devices and as photo-refractive materials. 

The number of mobile holes is equal to the number of impurity Ni2+ ions, and so the fraction c in the Heikes equation is equal to x in LaNi,Coi -,(+. In accord with the theory, the Seebeck coefficient, a, is positive and greatest at low values of x and decreases as x increase (Fig. 1.12). Substituting a value of c = 0.02 into the equation yields a value of a = +335 pV K-1, in good agreement with the experimental value of 360 pV K-1 (Robert el al., 2006). Note that the above example also shows that an experimentally determined value of the Seebeck coefficient can be used to estimate the concentration of impurity defects in a doped oxide. 

Aliovalent additives are often called donor dopants, when they tend to provide electrons and enhance intrinsic n-type semiconducting behavior, or acceptor dopants, when they tend to give a population of mobile holes and enhance /j-typc semiconducting behavior. The process of creating electronic defects in a crystal in this way is called valence induction. 

The most fundamental transition that can take place is the transfer of an electron from the valence band to the conduction band. This creates a mobile electron and a mobile hole, both of which can often be treated as defects. Transitions of this type, and the reverse, when an electron in the conduction band drops to the valence band, eliminating a hole in the process and liberating energy, are called interband transitions. Apart from the electrons and holes themselves, interband transitions do not involve defects. All other transitions do. 

We forewarn the reader that the formation of high-mobility holes is not peculiar to these four cycloalkanes For instance, cyclooctane [61], squalane [62,63,64], and CCI4 [65] also yield such holes. However, in these other liquids, the holes are unstable and, consequently, more difficult to study (the lifetimes are 5-20 nsec vs. 1-3 psec). This explains why convincing demonstrations for the occurrence of high-mobility holes are slow to come. For example, squalane (by virtue of its high viscosity) has been frequently... 

If the O2 becomes strongly adsorbed as the result of the capture, it could form an effective recombination center for a mobile hole... 

The evidence seems conclusive that reduction centers react with holes and halogen, and the increase in photographic sensitivity can be accounted for on this basis. The reaction decreases recombination and could also increase sensitivity in another way. A mobile hole, or halogen atom, reacts with a silver atom pair according to the equation... 

In an unsensitized grain, shallow trap states provided by crystal imperfections are important in the trapping of both electrons and holes. Hamilton assumes that the fraction of holes trapped is approximately 1, that is, the concentration of mobile holes is near 0. Nucleation to form silver is inefficient, and a high level of free-electron/trapped-hole recombination occurs. There is a certain probability, however, that a trapped electron will unite with a silver ion to form an atom which may either dissociate back into electron and silver ion or trap another electron and, with a second Ag, form a silver atom pair. This pair is relatively stable and can grow by... 

Some excited dye molecules can inject holes into the valence band of the silver halide. Photohole injection has been demonstrated experimentally by the photobleaching of chemically produced fog silver (248) to produce direct positive photographs. This action depends on the favorable location of the SQ ground state of the dye relative to the valence band of the silver halide. The injection is accomplished by the transfer of an electron from the valence band to the vacant SQ level of the excited dye molecule, leaving a mobile hole in the valence band which can oxidize a silver atom. Conversely, if the SQ level of a dye molecule is located sufficiently above the top of the valence band, a mobile hole in the silver halide can become trapped by the nonexcited dye. Hole-trapping by dyes has been detected by ESR signals (249,250). 

The pronounced dependence of the crossover potential on chemical sensitization, and particularly on the gaseous environment, shows that the crossover does not represent a division between dyes that can cause the appearance of conduction electrons in the silver halide and dyes that cannot. Instead, it represents an energy level determined by a kinetic balance between the formation and loss of photoelectrons and/or silver in the silver halide, a kinetic balance between sensitization and desensitization (259,265). One cancels the other and the net formation of latent image is zero. The actions of oxy-gen/moisture and of mobile holes are important sources of desensitization. 

The proposed mechanism of dye-promoted desensitization by oxygen is reaction of oxygen molecules with photoelectrons trapped by the dye (83), followed by reaction of the superoxide radical with a hydrogen donor or a mobile hole (eqs. 6 and 5). Desensitization increases with increasing depth of the electron traps provided by the dye. The deeper the trap, the longer is the time that an electron can remain in the trap and the greater is the probability that an oxygen molecule will have time to diffuse to the trap and react with the electron before the... 

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