Electron and Hole Centers

Since the original studies of F centers many other color centers have been characterized that may be associated with either trapped electrons or trapped holes. These are called electron excess centers when electrons are trapped and hole excess centers when holes are trapped. 

The F center is an electron excess center and arises because the crystal contains a small excess of metal. Similar metal excess F centers exist in compounds other than 

The color center is the [A104]4 group, which can be thought of as [A104]5 together with a trapped hole. The color arises when the trapped hole absorbs radiation. 

Two complex centers form (Dy-B04 — Vsr h ), which are hole centers formed thermally from (Dy-B04 — V r), and (BO3-V0 e ), which are electron centers 

4The name Blue John is a corruption of the French term bleu-jeune, which was used to describe the blue 

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As seen from Fig. 5, upon absoption of photons with the energy hv > Eg, an electron and hole centres are formed. They migrate to different sites on the PC surface, thus becoming spatially separated. Note, that what solid state physisists call surface electron and hole centers, in fact are some definite chemical species with strong reducing and oxidizing... 

For spin-1/2 excitations such as electrons and holes in normal semiconductors or polarons in conjugated polymers, a single resonance is found centered at the Field... 

For all TSR processes (with the corresponding redistribution of electrons and holes over the states in the gap) to be discussed, it seems useful to give a criterion permitting the classification of states under consideration as traps and recombination centers, respectively. 

The recombination of electrons and holes is a rather complicated process. We have to distinguish between (a) the direct recombination of electrons and holes, occurring in particular at high concentrations of charge carriers (b) recombination via defect states which depends, among other factors, on the densities and capture cross-sections of the defects (recombination centers) located in the bulk of the solid or on its surface. 

Therefore, we are able to discuss the existence of defect states (recombination centers and traps) in the forbidden gap of solid organic dyes characterized by different trapping probabilities (ranging from 10-12 cm2 to 10-20 cm2) for electrons and holes. Hence, asymmetric trapping of electrons and holes leading to n- and -photoconductivity is very probable. 

Quantitatively, it should be possible to describe the p- and -type conductivity of dye films with known densities of empty and filled centers pr, nT) by capture cross-sections of these centers for free electrons and free holes (s , Sj,), because the free-carrier lifetime (rn, rv) of electrons and holes is given by... 

A description of the emission and capture processes at a trap will be useful before discussing the various experimental methods. Figure 1 depicts the capture and emission processes that can occur at a center with electron energy ET. The subscripts n and p denote electron and hole transitions, and the superscripts t and differentiate between thermally and optically stimulated processes. It is assumed here that only thermal capture processes are occurring. 

The capture and emission processes at a deep center are most often analyzed by fabricating a device such that the centers under study are in the depletion region of either a p-n or metal-semiconductor junction. This is because the absence of mobile electrons and holes in the space-charge region causes capture to become negligible compared to emission, permitting these two effects to be separated. 

These results clearly suggest that metal ions physically implanted do not work as electron and hole recombination centers but only work to modify the electronic property of the catalyst [11,12,14,15],... 

It is important to emphasize that the photocatalytic reactivity of the metal ion-implanted titanium oxides under UV light (A < 280 nm) retained the same photocatalytic efficiency as the unimplanted original pure titanium oxides under the same UV light irradiation conditions. When metal ions were chemically dopec into the titanium oxide photocatalyst, the photocatalytic efficiency decreased dramatically under UV irradiation due to the effective recombination of the photo-formec electrons and holes through the impurity energy levels formed by the doped metal ions within the bandgap of the photocatalyst (in the case of Fig. 10.3).14) These results clearly suggest that metal ions physically implanted do not work as electron and hole recombination centers but only work tc modify the... 

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