Clean dislocations

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. 

Extended defects range from well characterized dislocations to grain boundaries, interfaces, stacking faults, etch pits, D-defects, misfit dislocations (common in epitaxial growth), blisters induced by H or He implantation etc. Microscopic studies of such defects are very difficult, and crystal growers use years of experience and trial-and-error teclmiques to avoid or control them. Some extended defects can change in unpredictable ways upon heat treatments. Others become gettering centres for transition metals, a phenomenon which can be desirable or not, but is always difficult to control. Extended defects are sometimes cleverly used. For example, the smart-cut process relies on the controlled implantation of H followed by heat treatments to create blisters. This allows a thin layer of clean material to be lifted from a bulk wafer [261. 

Figure 3, Area of clean gold (111) surface showing a surface Shockley partial dislocation (arrowed) - see 2. Atomic columns are black.

Fish. Solid biota samples, mainly fish, should be quickly killed by liquid N2 [28] or cervical dislocation [32] and kept at low temperatures (— 20°C). Some authors preferred desiccation of the sample at high temperature (70°C) [33,34] or lyophylisation [28]. The extraction and isolation steps would be combined when using lyophylisation and homogenisation, followed by a Soxhlet extraction, usually with MeOH, and a subsequent solid-phase extraction (SPE) clean-up, prior to the quantification. 

Clean single-crystal surface, 15 31 Clean surfaces, alcohol reactions, 29 37-38 Clear Air Act of 1970, 24 59, 62 Cleavage surfaces, dislocations on, 19 331-333 ClOj, ESR of, 22 309... 

Many kinds of disorder are known at surfaces. Clean, well-ordered surfaces present, among others, point defects (impurities, dislocations, etc.) and line defects (steps, crystallite boundaries, etc.). Surfaces with adsorbates or reconstruction-induced superlattices can have a variety of additional defects, e.g. 

The effects of an isolated defect on material quality (e.g. recombination activity of a clean, undecorated dislocation, capture cross sections of point defects) are well known for many defects present in crystalline silicon material, but the interactions of the impurities or structural defects form a major challenge in getting an improved understanding of the complex situation in the solidified silicon ribbon. Currently it is impossible to list a complete overview of the known interactions, but more information can be found in [49],... 

Both the above simulations considered identical tips and substrates. Failure moved away from the interface for geometric reasons, and the orientation of the interface relative to easy slip planes was important. In the more general case of two different materials, the interfacial interactions may be stronger than those within one of the materials. If the tip is the weaker material, it will be likely to yield internally regardless of the crystallographic orientation. This behavior has been observed in experiments between clean metal surfaces where a thin tip is scraped across a flat substrate [31]. When the thin tip is softer than the substrate, failure is localized in the tip, and it leaves material behind as it advances. However, the simulations considered in this section treated the artificial case of a commensurate interface. It is not obvious that the shear strength of an interface between two incommensurate surfaces should be sufficient to cause such yield, nor is it obvious how the dislocation model of Hurtado and Kim applies to such surfaces. 

There are three circumstances which make a geometrical reason for an altered catalytic activity probable. If the substrate is a metal with a clean surface, any change upon irradiation must be attributed to atomic point defects or dislocations since electronic defects are excluded by the conductivity of metals. Since dislocations are produced or destroyed by radiation only under special circumstances, the normal explanation for a metal is vacancies, subsurface interstitials, or multiple defects. If, with any nonmetallic type of solid, a catalytic activity is introduced only or especially by heavy-particle bombardment and if the induced activity is little changed by annealing at low temperature, then the arrangement of the atoms rather than the presence or absence of electrons must be important. Finally, if the induced catalytic effect depends... 

These results on dislocations illustrate the simplifying advantage of a metal in supporting no electronic defects, but show also the basic difficulty of a lack of convenient and specific markers for the defects produced. The possibility of interference by surface impurities was not eliminated, although metals can usually be studied under cleaner conditions than can any other class of substance. The extreme care necessary for clean experiments is perhaps not justified for radiation experiments with dislocations, because mechanical working is a simpler and more nearly universal way of introducing them (54). 

As was hinted at in the previous section, the ease with which dislocations may be produced at a crack tip can clearly alter the mechanical response of a solid containing such a crack. In particular, for the special case of the clean geometry and samples that can be found in a material such as Si, the consideration of the cleavage-dislocation emission dichotomy may encompass all of the relevant mechanisms to attempt model building. 

Most of the work in the previous sections of this chapter has dealt with mercury electrodes for the reasons discussed in Section 13.2.1. However, electrochemists are also interested in studying the interfacial structure of solids, because most electrochemical studies are carried out with solid electrodes (e.g., platinum or carbon). Such studies are difficult, because there are problems in reproducing a surface and in keeping it clean. Impurities in solution can diffuse to the electrode surface and adsorb, thereby significantly changing the interfacial properties. Moreover, the surfaces of solids, unlike those of mercury, are not atomically smooth, but have defects, such as dislocation lines, with a density of at least 10 to 10 cm. In comparison, a typical metal surface density has about 10 atoms cm. Especially important to the understanding of solid electrodes has been the use of so-called well-defined metal electrodes, that is, single crystal metals with very carefully prepared surfaces of known orientation (35). 

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