Defect point/planar

The topic of defects in semiconductors encompasses point, line, planar and volume defects. Point defects include those defects occupying, or sharing, a single lattice site these would include substitutional impurities... 

Thermodynamic considerations imply that all crystals must contain a certain number of defects at nonzero temperatures (0 K). Defects are important because they are much more abundant at surfaces than in bulk, and in oxides they are usually responsible for many of the catalytic and chemical properties.15 Bulk defects may be classified either as point defects or as extended defects such as line defects and planar defects. Examples of point defects in crystals are Frenkel (vacancy plus interstitial of the same type) and Schottky (balancing pairs of vacancies) types of defects. On oxide surfaces, the point defects can be cation or anion vacancies or adatoms. Measurements of the electronic structure of a variety of oxide surfaces have shown that the predominant type of defect formed when samples are heated are oxygen vacancies.16 Hence, most of the surface models of... 

Point Defects versus Planar Defects Structural Considerations... 

Ion implantation and subsequent thermal processing will form defects. Defects may be categorized as (1) point defects, (2) line defects, (3) planar defects, and (4) volume defects. Table 9.1 lists examples of these four types of defects, and Fig. 9.7 shows schematically some of the point defects in a two-dimensional simple cubic lattice. 

Defects in crystalline solids occur for structural reasons, because the atoms (or ions) are not arranged ideally in the crystal when all the lattice sites are occupied, and for chemical reasons, because inorganic compounds may deviate from the fixed composition determined by the valence of the atoms. There are different types of structural defects in a crystalline solid which are normally classified into three groups (1) point defects, (2) line defects, and (3) planar defects. Point defects are associated with one lattice point and its immediate vicinity. They include missing atoms or vacancies, interstitial atoms occupying the interstices between atoms, and substitutional atoms sitting on sites that would normally be occupied by another type of atom. These point defects are illustrated in Fig. 7.2 for an elemental solid (e.g., a pure metal). The point defects that are formed in pure crystals (i.e., vacancies and interstitials) are sometimes referred to as intrinsic or native defects. 

Extended defects are planar defects and may be considered to be formed by ordering and ehmination of point defects, e.g of oxygen vacancies, along specific... 

Numerous chemical reactions or micro-structural changes in solids take place through solid state diffusion, i.e. the movement and transport of atoms in solid phases. In crystalline solids, the diffusion takes place because of the presence of defects. Point defects, e.g. vacancies and interstitial ions, are responsible for lattice diffusion. Diffusion also takes place along line and surface defects which include grain boundaries, dislocations, inner and outer surfaces, etc. As diffusion along linear, planar and surface defects is generally faster than in the lattice, they are also termed high diffiisivity or easy diffusion paths. Another frequently used term is short circuit diffusion. 

The diffraction pattern obtained in the detector plane when the beam scan in a STEM instrument is stopped at a chosen point of the image comes from the illuminated area of the specimen which may be as small as 3X in diameter. In order to form a probe of this diameter it is necessary to illuminate the specimen with a convergent beam. The pattern obtained is then a convergent beam electron diffraction (CBED) pattern in which the central spot and all diffraction spots from a thin crystal are large discs rather than sharp maxima. Such patterns can normally be interpreted only by comparison with patterns calculated for particular postulated distributions of atoms. This has been attempted, as yet, for only a few cases such as on the diffraction study of the planar, nitrogen-rich defects in diamonds (21). 

Figure 1.1 Defects in crystalline solids (a) point defects (interstitials) (b) a linear defect (edge dislocation) (c) a planar defect (antiphase boundary) (d) a volume defect (precipitate) (e) unit cell (filled) of a structure containing point defects (vacancies) and (/) unit cell (filled) of a defect-free structure containing ordered vacancies. ...

In a similar fashion, the line and planar defects described above are all, strictly speaking, volume defects. For the sake of convenience it is often easiest to ignore this point of view, but it is of importance in real structures, and dislocation tangles, for instance, which certainly affect the mechanical properties of crystals, should be viewed in terms of volume defects. 

Point defects have zero dimension line defects, also known as dislocations, are onedimensional and planar defects such as surface defects and grain boundary defects have two dimensions. These defects may occur individually or in combination. 

The introduction to this chapter mentions that crystals often contain extended defects as well as point defects. The simplest linear defect is a dislocation where there is a fault in the arrangement of the atoms in a line through the crystal lattice. There are many different types of planar defects, most of which we are not able to discuss here either for reasons of space or of complexity, such as grain boundaries, which are of more relevance to materials scientists, and chemical twinning, which can contain unit cells mirrored about the twin plane through the crystal. However,... 

The lattice defects are classified as (i) point defects, such as vacancies, interstitial atoms, substitutional impurity atoms, and interstitial impurity atoms, (ii) line defects, such as edge, screw, and mixed dislocations, and (iii) planar defects, such as stacking faults, twin planes, and grain boundaries. 

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