Impurity complexes

Sulphur, selenium and tellurium can be incorporated into Si in a variety of forms (Grimmeiss et al., 1981 Wagner et al., 1984). As isolated ions, they are all double donors, with levels around 260 and 550 meV from the conduction band. These impurities may also be introduced as pairs, which also act as a double donors (Pensl et al., 1986). Depending on the thermal history of the Si during diffusion of S, Se and Te, they may also be incorporated as higher-order impurity complexes (Grimmeiss et al., 1981 Wagner et al., 1984). 

A comprehensive review of the application of vibrational spectroscopy to study hydrogen-impurity complexes was made by Stavola and Pearton in Chapter 8. 

Much of the microscopic information that has been obtained about defect complexes that include hydrogen has come from IR absorption and Raman techniques. For example, simply assigning a vibrational feature for a hydrogen-shallow impurity complex shows directly that the passivation of the impurity is due to complex formation and not compensation alone, either by a level associated with a possibly isolated H atom or by lattice damage introduced by the hydrogenation process. The vibrational band provides a fingerprint for an H-related complex, which allows its chemical reactions or thermal stability to be studied. Further, the vibrational characteristics provide a benchmark for theory many groups now routinely calculate vibrational frequencies for the structures they have determined. 

Several types of ion-channeling experiments (see Chapter 9) also give useful information on atomic positions at impurities or impurity complexes. These include both scattering of channeled ions by atoms that disrupt the uniformity of a channel path and the production of nuclear reactions by collision of a channeled ion with an impurity nucleus (e.g., incident 3He colliding with dissolved 2H to give 4He plus a proton, which can be detected). Here again, one can study lattice positions of solute atoms and changes in populations of different sites. 

We proceed now to consider the formation of hydrogen impurity complexes, which are frequently related to the Si—H systems just described, but perturbed by the presence of another impurity atom. 

The electrical activity of a defect is characterized in part by its electrical level position, which can be determined by capacitance transient methods. When the capacitance transient spectra are monitored before and after exposure to atomic hydrogen, it is found in many systems that these levels disappear. This phenomenon has been associated with the formation of electrically inactive hydrogen-impurity complexes as summarized by Pear-ton et al. (1987) and in Chapter 5 of this volume. 

Cu(II) impurity complexes in amino acid single crystals have been the subject of several EPR studies181-183. Since nitrogen and proton hf structures are only partially resolved in the EPR spectra, no detailed information about the electronic properties of the complex in the neighborhood of the metal ion can be evaluated. ENDOR spectroscopy has therefore been applied58,63 to draw detailed pictures of the positions and the molecular environment of Cu(II) impurities in amino acid crystals. 

Component Raw NaNT solution /wt-% Purified NaNT solution /wt-% Copper impurity complex /wt-%... 

The level and structural complexity of the impurities. Complex impurities require higher analyte concentrations to acquire two-dimensional experiments in reasonable time frames. Low-level impurities may not give the desired spectral quality. 

Water is the major offender for column contamination problems. I have diagnosed many problems, which customers have initially blamed on detector, pumps, and injectors, that turned out to be due to water impurities. Complex gradient separations are especially susceptible to water contamination effects. 

On the other hand, this purification technique is mainly limited to quite clean reaction mixtures or to chemically known impurities complex mixtures with a lot of by-products need accurate and time-consuming standard purification methodologies. In addition, the SPE strategy is little used in multigram scale, mainly because of the need for careful scaling up and the shortage of suitable disposable kits. 

This strategy is attractive because the impurities will reflect exactly those produced by the host cell during product expression and which copurify with the product through the process. The immunoabsorption procedures required for this approach, however, are difficult to perform and validate at the part-per-million level (i.e., ng of impurity per mg of protein product). All of the product, product fragments and product-impurity complexes must be removed without the loss of any of the impurities. Any residual product will produce antibodies during immunization and render the assay nonspecific. Conversely, the immunoabsorbtion step must not remove any of the impurities. An exact demonstration of these criteria at the ppm level is not possible with currently available non-antibody based analytical methods. 

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