Electrons, shallow irradiation

This type of transition has been extensively investigated in ZnS doped with a monovalent d" cation (Cu+, Ag+, Au+) (usually called the activator) and a trivalent ion such as AP+ (coactivator) substituted for divalent zinc. The coactivator can also be Cl substituted for. The monovalent cations create deep acceptor states, Al + or Cl form shallow donor levels (0.1 eV for Al) (Figure 13). When electrons are transferred from the valence to the conduction band, for instance, under electron beam irradiation, they are trapped by the coactivator while holes formed in the valence band are captured by Cu+ or Ag+, which are oxidized to the divalent state. The energy of the photons emitted depends on the energy difference between the donor and acceptor levels and on the acceptor-donor distance ... 

The proposal that holes are detrapped at lower temperatures than the excess electrons is based on the observations discussed above in Very Shallow Traps and Shallow Traps (Sec. 4.3.2). One expects that the activation energy needed to detrap the hole from Gua in duplex DNA is relatively small, an order of magnitude less than that needed to detrap the electron. This fits well with the observation that upon warming 4 K irradiated crystalline DNA to 77 K, 10-30% of the radicals anneal out, i.e., at least one of the trapping sites [fide infra Gua(N3-H) ] is very shallow. 

The independent electron picture of photoionisation breaks down completely at photon energies around the threshold for excitation of a core level [1]. Consider for example a first row transition metal compound with a 3d" configuration under irradiation with photons whose energies match that required to excite electrons from the shallow 3p core level. There is interference between the direct photoemission channel ... 

Electron spin resonance reveals the unpaired electrons associated with impurities or structural defects and can be used to identify the lattice site positions of these features. Nitrogen is shown to substitute for carbon and acts as a shallow donor. The various ESR triplets due to nitrogen in several SiC polytypes give information on the lattice sites occupied. For the acceptor boron, ESR shows it to occupy Si sites only, in disagreement with DAP photoluminescence measurements which show only boron on carbon sites. It may be that boron substitutes on both sites and the two techniques have sensitivity for only one particular lattice site. The aluminium acceptor is not observed in ESR but gallium has been noted in one report. Transition metals, Ti and V, have been identified by ESR both isolated on Si sites and in Ti-N complexes. Several charged vacancy defects have been assigned from ESR spectra in irradiated samples. 

There have been a few reports on radiation damage in SiC [7]. In this area, the effects of ions and electrons have been considered. If irradiation is performed, six deep states are produced in 6H-SiC. These states have been denoted E1-E4, Z, and Z2. After thermal annealing, only the two Z states remain. It should be noted that these are the same Z states observed in as-grown bulk material. It should also be noted that the defects reported are rather shallow in energy and there are no reports of semi-insulating material produced by radiation damage. 

As can be seen in Figure 9.9b, the temperature dependence of (3 for PVA is similar to that of polyethylene (Matsuo et al. 2002). It can be seen that increases as a function of temperature up to -10°C. The increase in has been proved to be due to the positron irradiation effect on a polymer at low temperature. The secondary electrons that escape from the positron spur could be easily trapped in shallow potentials formed between the polymer chains when the motions of the molecular chains and groups are frozen at low temperature. Due to the positron irradiation time (experimental time), the probability of formation would become larger. As can be seen in this figure, becomes a maximum at around -10°C, and begins to decrease with increasing temperature. (3 attains a minimum at ca. 75°C and increases again beyond ca. 75°C. This is due to an apparent increase in the number of holes detected by positron annihilation, because of the thermal expansion of the holes at... 

For example, Vo has a very high formation energy in n-type ZnO (the Fermi level close to the conduction band), even under extreme Zn-rich conditions. Therefore, Vq concentration should be very low under equilibrium conditions in as-grown undoped ZnO. Moreover, Vo is a deep rather than a shallow donor and it carmot be responsible for n-type conductivity of undoped ZnO, contrary to the conventional wisdom dominated in ZnO communities for decades. In contrast, the anion vacancies can be formed abundantly in p-type material and may be the main cause of the selfcompensation. It should also be noted that Vq in n-type material can be formed after electron irradiation. Electron paramagnetic resonance studies indeed revealed the presence of Vo in electron irradiated ZnO as a signal with g= 1.99 [92-94]. 

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