Defects electron traps

Shock-modified rutile is found to exhibit two characteristic resonances, which can be confidently identified as (1) an isotropic resonance characteristic of an electron trapped at a vacancy, and (2) an isotropic resonance characteristic of a Ti" interstitial. The data indicate a concentration of 2 X 10 cm , which is an order of magnitude greater than observed in hydrogen- or vacuum-induced defect studies. At higher pressures the concentration of interstitials is the same as at lower pressure, but more dispersion is observed in the wave shape, indicating higher microwave conductivity. 

X-Ray irradiation of quartz or silica particles induces an electron-trap lattice defect accompanied by a parallel increase in cytotoxicity (Davies, 1968). Aluminosilicate zeolites and clays (Laszlo, 1987) have been shown by electron spin resonance (e.s.r.) studies to involve free-radical intermediates in their catalytic activity. Generation of free radicals in solids may also occur by physical scission of chemical bonds and the consequent formation of dangling bonds , as exemplified by the freshly fractured theory of silicosis (Wright, 1950 Fubini et al., 1991). The entrapment of long-lived metastable free radicals has been shown to occur in the tar of cigarette smoke (Pryor, 1987). 

A detailed study of the C02- species on MgO has been carried out by Lunsford and Jayne 26). Electrons trapped at surface defects during UV irradiation of the sample are transferred to the CO2 molecule upon adsorption. By using 13C02 the hyperfine structure was obtained. The coupling constants are axx - 184, am = 184, and a = 230 G. An analysis of the data, similar to that carried out in Section II.B.2 for N02, indicates that the unpaired electron has 18% 2s character and 47% 2p character on the carbon atom. An OCO bond angle of 125° may be compared with an angle of 128° for CO2- in sodium formate. 

Even though a chemical gettering mechanism is proposed in some of these papers, it seems quite likely to us that the decrease of the concentration of these deep levels due to hydrogenation during MBE growth is due to a neutralization by hydrogen of the defects or impurities responsible for these electron traps. 

The origin of the color is as follows. The electron trapped at an anion vacancy in an alkali halide crystal is an analog of a hydrogen atom. The electron can occupy one of a number of orbitals, and transitions between some of these levels absorb light and hence endow the solid with a characteristic color. F centers and related defects are discussed further in Chapter 9. 

Subsequently, it was found that F-centres can also be produced by heating a crystal in the vapour of an alkali metal this gives a clue to the nature of these defects. The excess alkali metal atoms diffuse into the crystal and settle on cation sites at the same time, an equivalent number of anion site vacancies are created, and ionisation gives an alkali metal cation with an electron trapped at the anion vacancy (Figure 5.24). In fact, it does not even matter which alkali-metal is used if NaCl is heated with potassium, the colour of the F-centre does not change because... 

The excess free carriers (and excitons) do not represent stable excited states of the solids. A fraction of them recombine directly after thermahzation either radiatively or by multiphonon emission. In most materials, nonradiative transitions to defect states in the gap are the dominant mode of decay. The lifetime of free carriers T = 1/avS is determined by the density a of recombination centers, their thermal velocity v, and the capture cross section S, and may span 10-10 s. Electrons, captured by states above the demarcation level, and holes, captured by states below the hole demarcation level, may get trapped. The condition for trapping is given when the occupied electron trap has a very small cross section for recombining with a free hole. The trapping process has, until recently, not been well understood. 

The identification of an impurity, defect, or impurity-defect complex by some particular technique must nearly always be accomplished in conjunction with doping experiments. Thus, the well-known, sharp, zero-phonon photoluminescence lines at 0.84 eV in GaAs are almost certainly associated with Cr, as established by Cr-doping experiments (Koschei et al., 1976). However, some care must be taken here. For example, a dominant electron trap (EL2) in -doped GaAs is probably not associated with O, according to recent experiments (Huber et al., 1979). Thus, the doping must be accompanied by a positive identification of the relevant impurity concentration, say by SSMS, or SIMS. These general considerations apply to all the techniques discussed below. 

Thus, the electrical conductivity will be a measure of the number of free charge carriers of the catalysts. Adsorption processes which produce or destroy defects, or trap free electrons or holes, will alter the conductivity. Magnetic susceptibility, which will usually be changed by... 

Figures 3.5 and 3.6 present schematic classification of regimes observable for the A + B —> 0 reaction. We will concentrate in further Chapters of the book mainly on diffusion-controlled kinetics and will discuss very shortly an idea of trap-controlled kinetics [47-49]. Any solids contain preradiation defects which are called electron traps and recombination centres -Fig. 3.7. Under irradiation these traps and centres are filled by electrons and holes respectively. The probability of the electron thermal ionization from a trap obeys the usual Arrhenius law 7 = sexp(-E/(kQT)), where s is the so-called frequency factor and E thermal ionization energy. When the temperature is increased, electrons become delocalized, flight over the conduction band and recombine with holes on the recombination centres. Such...

It should be stressed that means the effective radius at which mobile defect is trapped by the Coulomb field of its partner but the electron tun-... 

The temporal evolution of spatial correlations of both similar and dissimilar particles for d = 1 is shown in Fig. 6.15 (a) and (b) for both the symmetric, Da = Dft, and asymmetric, Da = 0 cases. What is striking, first of all, is rapid growth of the non-Poisson density fluctuations of similar particles e.g., for Dt/r = 104 the probability density to find a pair of close (r ro) A (or B) particles, XA(ro,t), by a factor of 7 exceeds that for a random distribution. This property could be used as a good aggregation criterion in the study of reactions between actual defects in solids, e.g., in ionic crystals, where concentrations of monomer, dimer and tetramer F centres (1 to 3 electrons trapped by anion vacancies which are 1 to 3nn, respectively) could be easily measured by means of the optical absorption [22], Namely in this manner non-Poissonian clustering of F centres was observed in KC1 crystals X-irradiated for a very long time at 4 K [23],... 

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