Defect trapping

Figure 8 (a) The cis-transoid form (left of the defect) is more stable than the trwis-cisoid form (right), (b) Spin defect trapped at one end of the chain. 

The foregoing discussion strictly refers to semiconductors in single-crystal form. Amorphous and polycrystalline counterparts present other complications caused by the presence of defects, trap states, grain boundaries and the like. For this reason, we orient the subsequent discussion mainly toward single crystals, although comparisons with the less ideal cases are made where appropriate. The distinction between metal and semiconductor electrodes also becomes important when we consider the electrostatics across the corresponding solid-liquid interfaces this is done next. 

The results of Table 18.2 lead to the conclusion that Paths 2, 5 and 7 are plausible pathways with sufficiently low activation energies compared to the experimental value (0.16 eV). Path 2 on the stoichiometric surface is advantageous for the available adsorption energies at the transition state, whereas Paths 5 and 7 on the defective surface are stable due to the electronegativity of the defect-trapped H atom [14]. 

Figure 9.4 Recombination pathways of photogenerated charge carriers in an n-type semiconductor-based photoelectrochemical cell. The electron-hole pairs can recombine through a current density in the bulk of the semiconductor, the depletion region, or through defects (trap states) at the semiconductor/liquid interface, iss- Charges can also tunnel through the electric potential barrier near the surface, 4 or can transfer across the interface, The bold arrows indicate the favourable current processes in the operation of a photoelectrochemical cell. The hollow arrows indicate the processes that oppose the excess of charge carriers generated by light absorption.

Hydrogen in metals is trapped by all maimer of lattice impurities, including other interstitials and lattice defects. Trapping reduces the mobility of hydrogen but its solubility is usually improved. This prevents precipitation, and the crystal degradation that it causes is avoided. 

Beside bands and excitonic energy levels, semiconductors contain traps for electrons and holes. These are interstitial atoms, impurities, dislocations, and defects. Traps are frequently located on the surface as a result of the disturbed stoichiometry. The trapped charge carriers can recombine, either... 

The fast component is attributed to direct electron-hole recombination and the slow component is associated with the relaxation of charge carriers through deep defect (trap) states. The differential absorption measurements do not show... 

Centers due to impurities. Another type of color centers are impurity ions, the bands of which are caused by electronic transitions to neighboring ions of the host lattice (U-centers) or of electron defects trapped by impurities (p2-cen-ters). In the case of high impurity concentrations colloidal segregations may form. 

However, Fierens [9] has shown, that the hydroxylation of the slag glass surface layer, as a result of water molecules chemisorption, which is enhanced by the surface electron defects (trapped electrons), can be considered as a topochemical process. The topochemical processes became important also at later hydration stage, when the pozzolanic reaction of slag glass network, impoverishes of the majority of alkaline elements, with calcium ions is occtrrring, and calcium ions are chemisorbed on the active sites of solid phase. 

Figure 5.2 Schematic of how the defect trapping technique used in the replacement fin approach traps dislocation defects against the sidewalls of trenches with high aspect ratios. Situation shown post-CMP and recess etch.

Fig. 4 Diffusion profiles in c-Si for the steady-state approximation for (1) molecule formation, and (2) defect-trapping. Case (3) shows the time-dependence of normal diffusion. Hhe depth is X and diffusion time t. 

The simple trapping model (STM) can be used to interpret the results from PL measurements [129], According to this model, for positrons implanted into a homogeneous solid with a bulk lifetime z, = 1/Ab and with N different kinds of homogeneously distributed microscopic defects with lifetimes Xi = jA and trapping rates ki, the annihilation spectrum will consist of A +1 exponentials. Through defect trapping, the bulk lifetime will be reduced to tq = l/(Ab -I- k), where... 

Apart from its function as a point defects trap, Ti diffuses towards structure dislocations to form Cottrell-type atmospheres that can block the restoration and the rise of the initial lattice. Weertman and Green [41] demonstrated that these dislocations, thus decorated by dense clusters of large solutes such as Ti, become neutral sinks that cause unbiased elimination of point defects and therefore an increase in the resistance to swelling. Furthermore, by screening dislocations from the arrival of point defects, these clusters are probably conducive to mutual recombinations close to the decorated dislocations which will also tend to increase the resistance to swelling ... 

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