Recombination centres

If tlie level(s) associated witli tlie defect are deep, tliey become electron-hole recombination centres. The result is a (sometimes dramatic) reduction in carrier lifetimes. Such an effect is often associated witli tlie presence of transition metal impurities or certain extended defects in tlie material. For example, substitutional Au is used to make fast switches in Si. Many point defects have deep levels in tlie gap, such as vacancies or transition metals. In addition, complexes, precipitates and extended defects are often associated witli recombination centres. The presence of grain boundaries, dislocation tangles and metallic precipitates in poly-Si photovoltaic devices are major factors which reduce tlieir efficiency. 

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...

Fig. 3.7. Trap-controlled carrier recombination 1 - excitation of solid with creation of electron ( ) and hole (0) pair 2, 3 - their localization (trapping) by defects 4 - thermal ionization of electron from a trap 5 - its recombination with the recombination centre.

Deep levels interact with free carriers as either recombination centres or traps. Consequently, deep levels can significantly influence the photoelectric or electronic properties of a semiconductor. For example, in the active region of light emitting diodes deep levels can act as efficient non-radiative recombination centres and significantly limit the internal quantum efficiency. Other applications may utilise deep levels for the benefit of device performance. 

In Minero s Case 2, the concentration of the substrate is high or it is very hydrophobic, although it should be borne in mind that such a scenario may result in the oxidised substrate behaving as an extrinsic recombination centre. A direct consequence of this is that Minero s model is no longer applicable as his simplified kinetic scheme did not consider this reaction. Thus, the utility of the resultant expressions is limited to mainly very hydrophobic or poor water soluble compounds at low concentrations. Under these conditions, equation (9.59) simplifies to... 

The photogenerated Tij-O radicals may, in principle, act as (i) surface recombination centres for the conduction band electrons, (ii) electron acceptors towards reduced species present in the solution, or, for example, (iii) undergo conproportionation... 

The latter explains also why a number of ions and molecules undergo photo-oxidation much easier than water and OH anions, especially at potentials close to the flat-band potential of Ti02- The ability of such species to react directly with the precursors to the peroxo-titanates, i.e., with the Tis-0 radicals or positive holes, permits not only to avoid the slow step in the process of photo-oxidation, but also to restrict the activity of the recombination centres by reducing the surface concentration of the peroxides. 

Additionally, transition metals are present in all materials, although mostly in concentrations not limiting material quality Nevertheless, some are effective recombination centres as point defects or in the form of precipitates and affect the as-grown material quality. 

It is known that clean dislocations without decoration reveal almost no recombination activity [55], but increasing decoration with impurities leads to recombination centres deep in the band gap, which significantly reduce carrier lifetime [56]. It can, therefore, be concluded that one of the most detrimental defects in EFG and SR apart from recombination active large angle grain boundaries are decorated dislocations. 

Low solubility of solvent into the silicon During epitaxial process, atoms of the solvent are incorporated in the Si crystal. Their incorporation can modify the electronic properties of the layer. Actually, many metallic impurities act as recombination centre or as dopant and reduce the lifetime of minority carriers. Solvent purity is also an important parameter to avoid other impurities. 

Figure 11 Photographic latent image formation in undoped and formate-doped and gold-sulfide (S) sensitized AgBr crystals. Top undoped crystal with electron-hole recombination. Centre formate doped crystal and hole scavenging step by formate (HCOp). Bottom formate-doped crystal and delayed reduction step of additional silver ions by carboxyl radicals CO- [16],...

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