Planar and volume defects

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... 

Crystal imperfections, such as point defects, line defects, planar and volume defects are limiting factors for the applicability of semiconductors in microelectronics. The impact of these defects on band structure, carrier mobility, and optical properties is even found to create principal obstacles for the applicability of certain materials. One example is the II-VI material system for which a prominent obstacle is the creation of extended defects during device operation, connected with the diffusion of doping-related point defects [1], causing lifetime shortening and making the device unreliable. 

The concept of a defect has undergone considerable evolution over the course of the last century. The simplest notion of a defect is a mistake at normal atom site in a solid. These stmcturally simple defects are called point defects. Not long after the recognition of point defects, the concept of linear defects, dislocations, was invoked to explain the mechanical properties of metals. In later years, it became apparent that planar defects, including surfaces, and volume defects such as rods, tubes, or precipitates, also have important roles to play in influencing the physical and chemical properties of the host matrix. More recently, it has become apparent that interactions between point defects are of considerable importance, and the simple model of isolated point defects is often inadequate with... 

Each of these is confined to a site or point, thus, they are called point defects. There are more disl(x ations disturbing the periodicity of lattice sites grain boundaries and surfaces spatially confining the crystal which would have to be infinite if ideal voids and inclusions that are three-dimensional aggregates of point defects of a kind. Depending on their geometries, they are often called line defects, planar defects and volume defects, respectively. 

Defects play a major role in the performance of materials, some wanted and some unwanted. Therefore, it is necessary to understand what they do, how they form, and how to control them. Defect are categorized by their dimensionality point defects (zero dimensional), line defects (1-D), planar defects (2-D), and volume defects (three dimensional [3-D]). 

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. 

Ordered arra3rs are not completely uniform but have defects, as do crystals, as shown in Figure 11.16. These defects fall into four main categories (1) point defects or vacancies (i.e., places where a particle is missing) (2) line defects or dislocations, (3) planar defects (i.e., grain boimdaiies), and (4) volume defects like cracks. The point defects are... 

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. 

Frequently the most important planar defects in a crystal are the external surfaces. These may dominate chemical reactivity, and solids designed as catalysts or, for example, as filters must have large surface areas in order to function. Rates of reaction during corrosion are frequently determined by the amount of surface exposed to the corrosive agent. (A further discussion of surface physics and chemistry, although fascinating, is not possible within the scope of this volume.)... 

Embedded atom potentials have been extensively used for performing atomistic simulations of point, line and planar defects in metals and alloys (e.g. Vitek and Srolovitz 1989). The pair potential ( ), atomic charge density pBtom(r), and embedding function F(p) are usually fitted to reproduce the known equilibrium atomic volume, elastic moduli, and ground state structure of the perfect defect-free lattice. However, the prediction of ground state structure, especially the competition between the common metallic structure types fee, bcc, and hep, requires a more careful treatment of the pair potential contribution ( ) than that provided by the semiempirical embedded atom potential. This is considered in the next chapter. 

These different contrast mechanisms can all be used to reveal the scale of liquid crystalline polymer microstructures. In specimens that exhibit a mosaic texture, and in those that contain predominantly planar defects, domain size is easily defined in terms of areas that uniformly show extinction between crossed polars. However, if the defects are predominantly linear, as in specimens that exhibit schlieren textures, such simple characterization of microstructural scale is no longer possible. Here it is more convenient to look at the length of disclination line per unit volume, which is equivalent to the number of lines intersecting unit area, and analogous to the dislocation density as defined for crystalline solids. Good contrast is essential in order to obtain an accurate count. Technologically, microstructural scale is of growing interest because of its relevance to processability, mechanical properties and optical transparency. 

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