Point defects typical examples

Materials that contain defects and impurities can exhibit some of the most scientifically interesting and economically important phenomena known. The nature of disorder in solids is a vast subject and so our discussion will necessarily be limited. The smallest degree of disorder that can be introduced into a perfect crystal is a point defect. Three common types of point defect are vacancies, interstitials and substitutionals. Vacancies form when an atom is missing from its expected lattice site. A common example is the Schottky defect, which is typically formed when one cation and one anion are removed from fhe bulk and placed on the surface. Schottky defects are common in the alkali halides. Interstitials are due to the presence of an atom in a location that is usually unoccupied. A... 

These symbols, which will be used hereafter in this book, are after Kroger and Vink (see Fig. 1.9). By combinations of these defects, a variety of defect structure types is produced. If both the combination and the concentration of point defects are adequate, the composition of a compound is stoichiometric, but this is a special case, and compounds with point defects are usually non-stoichiometric compounds. Typical examples of defect structure types are ... 

Rapid advances in semiconductor techrwlogy, including thin film formation by deposition, interface preparation or microstructuring, demand characterization techniques that provide understanding of the fundamental processes involved, including information on structural order—disorder and spatial inhomogeneity. Raman spectroscopy is used both in process control and quality assessment [34]. Typical examples of semiconductor applications are composition determination, analysis of crystal structure, surface and interface analysis, phase determination, doping, point defects, temperature influence and mechanical stress. 

Until now we have considered the ideal structures of crystals only when each atom or ion is on a regular site in the crystal. Real crystals contain a variety of imperfections or defects. In crystalline ceramics and glasses, the structure and chemistry of the material will be determined by the kinetics of defect movement. For example, the kinetics of the glass-to-crystal transformation are slow if the temperature is low (typically less that 1000°C) because the transformation occurs by atoms moving—in ceranucs, this usually occurs by point defects moving. If point defects move too slowly, the structure with the lowest energy (the equilibrium structure) may never actually be achieved. How fast they move is determined by their structure. 

Intrinsic vacancies are much more numerous in metals. For example, in a 1-cm crystal of aluminum at room temperature there are about 9 billion vacancies. In a crystal of silicon in equilibrium at room temperature there are only about 1 x 10 intrinsic vacancies per cubic centimeter. This is considerably less than typical concentrations of extrinsic point defects (dopants) in silicon—about 0.0001% another fortunate fact. 

Defects in solids are ubiquitous and can be found both in the bulk and at the surface of materials.Two classes can be distinguished point defects and extended defects. The former, also called local defects, produce a modification of the site environment of an otherwise perfect lattice for instance, the absence of an atom in a lattice position (vacancy), the presence of an atom in an interstitial position (interstitial defect), or the substitution of an atom for another atom of a different chemical species at a regular lattice site (substitutional defect). Figure 45 shows typical examples of local defects in an ionic solid. 

Figure 1. Typical point defects in a binary ordered alloy (a) vacancy ( ) (b) split interstitial (c) bound pair of antisite defects (d) bound triple defect consisting of two vacancies and one antisite atom (e) vacancy-impurity ( ) bound pair (f) unbound wrong pair. Many other types of defects are possible, as will be discussed for example for Bll (CuTi, Section 5.3) and A15 (NbjSn, Section 9.1) compounds...

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