Bulk defects

An example of bulk defects are voids where there are simply no structural atoms. Another example consists of impurities that can cluster together to form small regions of a different phase such as precipitates. Impurities and precipitates also play an important role in the corrosion resistance of metals. 

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Obviously, only parts of the defects created on the surface are paramagnetic, and thus EPR active. Nonetheless subsequent XiCl4 deposition on samples covered with a non-epitaxial MgCl2 film quenches the EPR signal by 40% [21], This can be taken as a clear indication that some of the defects are localized on the surface of the MgCl2 film, while most of the defects are bulk defects not influenced by adsorbed TiCl4. 

Robertson has summarized the three recent classes of models of a-Si H deposition [439]. In the first one, proposed by Ganguly and Matsuda [399, 440], the adsorbed SiHa radical reacts with the hydrogen-terminated silicon surface by abstraction or addition, which creates and removes dangling bonds. They further argue that these reactions determine the bulk dangling bond density, as the surface dangling bonds are buried by deposition of subsequent layers to become bulk defects. 

Extensive study has been devoted to paramagnetic defects that are formed on high-surface area alkaline earth oxides, particularly magnesium oxide. The work carried out by Wertz el al. 187, 188) and Henderson and Wertz 139) on bulk defects formed in MgO single crystals has been quite valuable in the identification of the surface defects. Both the bulk and surface defects may be divided into two classes those in which an electron... 

The REM and SREM techniques have recently been shown to be very powerful for the study of flat surfaces of large crystals or bulk specimens (19,20). Single-atom surface steps may be seen clearly with a lateral resolution of 1 nm or better and the interactions of surface steps with bulk defects can be investigated. 

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... 

The lattice imperfections play an important role in reactions in solid state. The reactivity of solids are due to the defect or fault in the lattice. The more perfect a crystal is, the smaller is its reactivity. The defect may be point defect, dislocations, stacking faults, bulk defects etc. 

The nature of bulk defects is straightforward. Pinholes, cracks, or other surface imperfections result in rapid diffusion of saline into the implant resulting in corrosion of electrical conductors. This corrosion lowers the resistance of the original leakage path until normal operation of the electronic circuit is impared and the Implant falls. 

Some of the defects excluded from the TLK model are cavities of missing atoms, surface dislocations and bulk defects extending to the surface. The success of the TLK-model demonstrates that although such defects do exist and are interesting, they are in many contexts not terribly important. 

From our previous discussion of the net transport of defects of a given species from high concentration regions to low concentration regions by random motion, it is evident that the difference in bulk defect concentrations between the source and the sink interfaces constitutes a type of potential for the movement of defects. This potential acts to create a driving force proportional to the potential difference per unit distance for the motion of defects across the oxide layer. 

In terms of the bulk defect concentration, the hopping current for equal barrier heights in the forward and reverse directions [see eqn. (79)] takes the form... 

In the latter case, the vacancy is believed to have a stabilizing effect on the oxygen hole center at the surface (40). The EPR parameters of these bridge and terminal oxygen hole centers were found to be very close to those of the same bulk defects and O" adsorbed species, respectively (Table II). The reactivity of the hole centers at the surface was similar to that of O" adsorbed species (39, 40). 

Figure 10 groups the parameters according to geometry, bulk defects, surface phenomena, and extrinsic modifications. The geometry of a catalyst particle is given by its size, its habitus (meaning the anisotropy or deviation from a spherical shape), and by its pore system. Only for micro-and mesoporous samples is XRD a sensitive tool to determine the pore architecture (Chen et al., 2005 Davidson, 2002 Li and Kim, 2005 Liu et al., 2002 Ohare et al., 1998). In many solids that are more compact than most catalysts, only secondary effects are related to the pores. 

The advantage of the kinetic treatment lies in the fact that (i) also solutions far from equilibrium can be handled and (ii) the range of validity of Eq. (169) can be given (similarly as in the diffusion case, cf. Section VI.2./). Since in the above derivations bulk defect chemistry was assumed to be established at x = 0, the index bulk was used in Eq. (169) to allow for more general situations. Note that these explicit formulae predict defined dependencies on the control parameters which can be checked provided defect chemistry is known. For simple situations (see Refs.252,253) a power law relationship results ( is a constant)... 

Interest in the electronic properties of interfaces centers around a-Si H/Si3N4, because this combination is used in multilayers (Section 9.4) and field effect transistors (Section 10.1.2). The electronic structure of the interface is illustrated in Fig. 9.18. Apart from the band offset which confines carriers to the a-Si H layer, the distribution of localized interface states and the band bending are the main factors which govern the electronic properties of the interface. The large bulk defect density of the SijN also has an effect on the electronic properties near the interface. Band bending near the interface may result from the different work functions of the two materials or from an extrinsic source of interface charge - for example, interface states. 

The opposite effect happens in multilayer films of materials in which the luminescence is strongly quenched by bulk defects, for example, a-Ge H. The luminescence intensity increases in a-Ge H/a-Si H... 

Of these four types, (a)-(c) are observed at the atomic level, whereas bulk defects are easily observed by the naked eye, or using a light microscope. These bulk defects are produced through the propagation of the microscopic flaws in the lattice. For crystals with a planar defect such as polycrystalline solids, the grain boundary marks the interface between two misaligned portions of the bulk crystal (Figure 2.23). 

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