DEFECTS IN CRYSTALLINE MATERIALS

Many things can go wrong when crystals are formed out of a solution or a smelt. As a result the crystals will not have a perfect shape, but will have defects. These can affect the material properties to a large extent. It is often easier to regulate the crystallization proc- 

In literature the defects in crystalline materials are called O-di-mensional or point defects, 1-dimensional, also called line defects or dislocations and 2-dimensional or packing defects. In this book we will confine ourselves to a brief description of some of the many kinds of defects. 

One of the causes of point defects is a temperature increase which results in an increased thermal movement of the atoms which can subsequently lead to the atoms escaping from their place in the lattice. Other causes are the effects of radiation and inbuilt, foreign atoms. In an atomic lattice a vacancy can occur due to the movement of an atom, an absence of an atom or molecule from a point which it would normally occupy in a crystal. In addition to this vacancy an atomic will form elsewhere. This combination of an atomic pair and a vacancy is called the Frenkel defect. In ionic crystals an anion and a cation have to leave the lattice simultaneously due to the charge balance. As a result a vacancy pair remains and this is called the Schottky defect. Both defects can be seen in figure 4.8. 

Solids are usually polycrystalline, which means that they are built up of many small, individually ordered crystals. Crystal defects are caused by disarranged grain borders. 

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Dislocation A linear defect in crystalline materials that represents atomic misalignment... 

TEM mode is used extensively in life sciences, which is very similar to the light microscope, and also in materials science. In materials science, the combination of imaging and diffraction provides a unique capability for understanding the properties of crystals and defects in crystalline materials. High-resolution TEM (HRTEM) imaging is possible to the atomic level and can now extend to sub-A level with aberration correctors [1]. 

Defects in these crystal structures are essential to determining the properties of the materials. The crystalline defects relevant to semiconductors will be discussed in detail in Chapter 7. Amorphous materials have no regular order so there are no well-defined defects in the material. Nonetheless, we will see in Chapter 8 that the continuum of distortions in the structures of amorphous semiconductors play a key role in determining their properties. Here we will list only the types of atomic-scale (point) defects in crystalline materials and leave more complex structures and detailed discussion to Chapters 7 and 8. Point defects in crystals include vacancies, interstitials, and antisites. Vacancies are missing atoms in the crystal structure. They are essential to the diffusion of atoms among lattice sites in many materials. Interstitials are atoms lying in spaces between atoms in the crystal structure. More open lattices such as the diamond structure accommodate interstitial atoms relatively... 

As with all other classes of materials, one of the primary keys (if not THE key) to engineering a semiconductor is control of defects in its structure. Defects can be divided into classes according to their dimensionality. Thus, zero (jwint), one (line), two (plane) and three (volume) dimensional defects occur in semiconductors and each is significant is considered in turn, although two and three-dimensional defects will be lumped together as they behave similarly. Furthermore, the behaviors of two and three-dimensional defects can be considered to be extensions of zero and one-dimensional behaviors. Therefore, we will spend more time on the latter two. In this chapter we wUl consider only defects in crystalline materials. Amorphous semiconductors, the ultimate in defective materials, are considered in the following chapter. 

Since positron annihilation spectroscopy is highly sensitive to atomic defects in solid materials, positron annihilation experiments have been carried out extensively on silicon (Si) and silicon dioxide (Si02), both of which are extremely important for the microelectronic device industry. While several reviews are available [1], those reviews are mainly focused on positron (not positronium) annihilation behavior because positronium (Ps) formation dose not occur in bulk crystalline Si. Recent positron annihilation experimental studies revealed that Ps formation occurs in some Si-based thin films, such as porous Si and hydrogenated amorphous Si furthermore, Ps formation is dominant in high-purity amorphous Si02 thin films. In this chapter, Ps annihilation characteristics in Si and Si02 thin films will be discussed from the experimental point of view. 

The hyperfine ESR data are valuable because they are the best measure of the electron wavefunetion at the defeet. The form of the hyperfine spectrum, which contains two broad lines, implies that the defect state is highly localized on a single silicon atom. Further analysis makes use of an approach that is successful in analyzing the hyperfine interaction in crystalline materials and describes the defect wave-function, 4, by a linear combination of atomic orbitals. The wavefunetion of a single silicon valence electron is written in terms of s and p orbitals as... 

In an amorphous semiconductor with a low defect density such as hydrogenated amorphous silicon (a-Si H), charge transport takes place in the electronic states in the vicinity of the conduction- and valence-band edges. However, no complete theory of the electronic structure near the band edges in a-Si H or any amorphous semiconductor has yet been devised. The problem appears to be extraordinarily complex. The disorder generates localized states near the band edges that are not present in crystalline material. 

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. 

Defects are often classified in terms of a dimensionality. This is known as the defect hierarchy. The classifications are given in Table 11.1. In spite of this table, remember that all these defects are three-dimensional. We will first summarize the different types of point defect that can occur in crystalline materials. To provide some idea of the importance of point defects, we will consider some specific examples. 

In amorphous condensed phases, the interpretation of the conduction is rather similar to that in crystalline materials if one views the amorphous phase as a crystalline solid that is highly disordered. In this view, the soKd contains a high concentration of defects with at least one kind being mobile. This explains why glasses exhibit, in many cases, a higher ionic conductivity than the corresponding crystalline solid with the same composition. 

Feng T, Pinal R, Carvajal MT. Process indnced disorder in crystalline materials Differentiating defective crystals from the amorphons form of griseofulvin. /. Pharm. Sci. 2008 8 3207-3221. 

On a two-dimensional level, there are also several types of important microstruc-tural features. Grain boundaries and interfaces between dissimilar phases control the properties of many systems. Defects, such as twins, stacking faults, and domain boundaries, often develop along specific habit planes in crystalline materials. 

In crystalline materials the lattice is defective because of, e.g. Schottky defects" and thus a net excess of anions (negative) or cations (positive) exists at the surface. The net charge is compensated by an equivalent ionic charge at the surface and, on contact with water, the crystal releases the compensating ions to form a double layer. This behaviour is typical of ion-exchange materials such as zeolites, clays etc. 

All of these two dimensional bubble raft demonstrations simulate the arrangement of atoms in crystalline materials. Dislocations, lattice defects, grain boundaries and recrystallization are all phenomena that occur in three dimensional crystalline materials. 

Disclination A topological defect in a material s liquid crystalline structure characterized by a discrete change in molecular orientation. 

Transmission electron microscopy has been widely used for structural characterization of the irradiated microstmctures and is one of the primary techniques used for characterization of nano-sized defects in structural materials. These microscopes have been used for decades to characterize dislocations (size, density, type), voids, bubbles, and precipitates in materials, as well as grain sizes on the nanoscale and misorientalion of grain boundaries. In addition to imaging, semiquantitalive chemical compositions can be obtained using characteristic X-rays due to electron beam excitation. Electron diffraction provides structural information such as phases and ordering of the structure (amorphous versus crystalline). Improvements over the decades in... 

The Raman peak position and linewidth for a nanocrystalline material are affected by phonon confinement. In bulk-defect-ffee crystalline materials, only q = 0 phonons are Raman active. Contrary to this, in the case of a nanocrystalline material, q vectors in the range of Aq = [L crystallite size) are Raman active due to the uncertainty principle, which results in frequency shifts and broadening. The phonon-confinement model studied by Richter et and extended by Campbell and Fauchet generates the first-... 

The im< e mode produces an image of the illuminated sample area, as in Figure 2. The imj e can contain contrast brought about by several mechanisms mass contrast, due to spatial separations between distinct atomic constituents thickness contrast, due to nonuniformity in sample thickness diffraction contrast, which in the case of crystalline materials results from scattering of the incident electron wave by structural defects and phase contrast (see discussion later in this article). Alternating between imj e and diffraction mode on a TEM involves nothing more than the flick of a switch. The reasons for this simplicity are buried in the intricate electron optics technology that makes the practice of TEM possible. 

X-ray Diffraction (XRD) is a powerful technique used to uniquely identify the crystalline phases present in materials and to measure the structural properties (strain state, grain size, epitaxy, phase composition, preferred orientation, and defect structure) of these phases. XRD is also used to determine the thickness of thin films and multilayers, and atomic arrangements in amorphous materials (including polymers) and at inter ces. 

Many inorganic solids lend themselves to study by PL, to probe their intrinsic properties and to look at impurities and defects. Such materials include alkali-halides, semiconductors, crystalline ceramics, and glasses. In opaque materials PL is particularly surface sensitive, being restricted by the optical penetration depth and carrier diffusion length to a region of 0.05 to several pm beneath the surface. 

The work on colour centres outlined in Section 3.2.3.1, much of it in the 1930s, and its consequences for understanding electrically charged defects in insulating and semiconducting crystalline materials, helped to stimulate ceramic researches in the electrical/electronic industry. The subject is enormous and here there is space only for a cursory outline of what has happened, most of it in the last 80 years. 

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