Interstitial silicon

Just as in the case of (16), an equation of the form (20) applies to any other association-dissociation reaction in which one of the dissociated species is mobile, the other fixed. When the two species are distinct but both mobile, as for hydrogen combining with, say, an interstitial silicon, a similar line of reasoning, whose details we omit, leads to equations of the same form as (16) and (20) but with D+ replaced by the sum of the diffusion coefficients of the two species. When the two mobile species are the same, as for the reaction H° + H° 5H2, it turns out that nA and n+ should each be replaced by the monatomic density n, D+ by the monatomic diffusion coefficient, and 4ir by 8tt in (16) but not in (20). 

The silicon which separates is pushed out into the interstitial positions, but the amount is clearly insufficient to form a second phase. This reaction is at least slightly exothermic. The presence of interstitial Frenkel defects is also a reason for the hi ly unusual properties of this defect phase. If all the valence electrons participate in forming chemical bonds in the Mn4Si7 lattice, the phase transition characterized by Eq. ( ) should cause interstitial silicon to act as an acceptor for some of the valence electrons, which may lead to the formation of holes and to the occurrence of p-type conductivity. This assumption is supported by the fact that the hole density obtained experimentally is approximately equal to the number of Frenkel defects (4.6 10 and 6.5 10 cm", respectively). For this reason, the crystal chemical formula MnSii 73 corresponds to the phase which exists in the temperature range up to about 1125 C. 

For example, for the case of a thin plane-parallel crystal plate of a large diameter, when the conditions in the plane parallel to the surface of the crystal can be considered to be uniform and the diffusion can be treated only along the normal to the surface (the z coordinate axis), the mass balance of point defects in the crystal is described by the system of diffusion equations for intrinsic interstitial silicon atoms, oxygen atoms, carbon atoms, and vacancies ... 

In the system of equations (1), we took into account that the oxygen precipitates serves as sinks for oxygen atoms and vacancies and as sources of interstitial silicon atoms. At the same time, the carbon precipitates, in turn, also serve as sinks for carbon atoms and interstitial silicon atoms and as sources for vacancies. Kinetic model of decomposition of... 

If the parameter of crystal growth / G<, for stress relaxation precipitate generates interstitial silicon atoms. If the parameter of crystal growth / G> for stress relaxation precipitate adsorbs vacancies. In this case is suppressed the formation of dislocation loops. 

In some materials, semiconductors in particular, interstitial atoms play a crucial role in diffusion. Thus, Frank and Turnbull (1956) proposed that copper atoms dissolved in germanium are present both substitutionally (together with vacancies) and interstitially, and that the vacancies and interstitial copper atoms diffuse independently. Such diffusion can be very rapid, and this was exploited in preparing the famous micrograph of Figure 3.14 in the preceding chapter. Similarly, it is now recognised that transition metal atoms dissolved in silicon diffuse by a very fast, predominantly interstitial, mechanism (Weber 1988). 

Typically we fit up to the / = 3 components of the one center expansion. This gives a maximum of 16 components (some may be zero from the crystal symmetry). For the lowest symmetry structures we thus have 48 basis functions per atom. For silicon this number reduces to 6 per atom. The number of random points required depends upon the volume of the interstitial region. On average we require a few tens of points for each missing empty sphere. In order to get well localised SSW s we use a negative energy. 

Silicon can be doped with small amounts of phosphorus to create a semiconductor used in transistors, (a) Is the alloy interstitial or substitutional Justify your answer, (b) How do you expect the properties of the doped material to differ from those of pure silicon ... 

Carbon forms ionic carbides with the metals of Groups 1 and 2, covalent carbides with nonmetals, and interstitial carbides with d-block metals. Silicon compounds are more reactive than carbon compounds. They can act as Lewis acids. 

The nitrides reviewed here are those which are commonly produced by CVD. They are similar in many respects to the carbides reviewed in Ch. 9. They are hard and wear-resistant and have high melting points and good chemical resistance. They include several of the refractory-metal (interstitial) nitrides and three covalent nitrides those of aluminum, boron, and silicon. Most are important industrial materials and have a number of major applications in cutting and grinding tools, wear surfaces, semiconductors, and others. Their development is proceeding at a rapid pace and CVD is a major factor in their growth. 

The atomic and crystalline structure of the three covalent nitrides, aluminum, boron, and silicon nitrides, is less complex than that of the interstitial nitrides. Their bonding is essentially covalent. 

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