Defect native point

One of the major controversies in solid-state science is the nature of the dominant native point defect in silicon. Is the dominant native point defect in silicon the monovacancy or the silicon self-interstitial Well-developed arguments have been proposed for each type, but the current consensus is that both types are present and important. 

In view of the uncertainties regarding the native point defect in Si, it is necessary in discussions of self-diffusion and dopant diffusion to take account of both types of defects. 

A8.2 Native point defects in GaN and related compounds A8.3 O, C and other unintentional impurities in GaN and related compounds... 

FIGURE 1 Formation energy as a function of Fermi level for native point defects in GaN under Ga-rich conditions. EF = 0 corresponds to the top of the valence band. 

Iddir H, Ogiit S, Zapol P, Browing ND (2007) Diffusion mechanisms of native point defects in rutile TiO ab initio total-energy calculations. Phys Rev B 75 073203... 

Line defects in a crystalline material are known as dislocations. Dislocations are formed due to nonequilibrium conditions such as ion implantation and thermal processing. Under equilibrium conditions, there is no requirement for the presence of dislocations or any other defect (except native point defects) in the crystal. An edge dislocation may be viewed also as having an extra half-plane inserted into the crystal (see Fig. 9.9). 

In additimi to foreign cation and anion dopants, native point defects are also usually present in the material. Examples are vacancies, interstitials, or substituents. These defects can have a similar influence on the optical, electrical, and catalytic properties as dopants. They are formed by intrinsic defect-chemical reactions, or by a change in the lattice stoichiometry due to exchange of, e.g., oxygen with the gas phase. Since virtually every defect affects the performance of the material in some way, the ability to understand and predict the relatiruiship between dopants and defect concentrations is of paramoimt importance for designing efficient photoelectrodes. 

The point defects, in turn, are classified as native (intrinsic) and substitution defects. The intrinsic point defects appear as a vacancy (the absence of an atom in a crystal lattice position) or as an interstitial defect (the presence of the host crystal atom in an interstitial position). The host crystal atoms can be substituted for another atom of a different chemical species at a regular lattice site or at the interstitial position (impurity center or substitution defect). The point defects can also be classified as neutral and charged relative to the host crystal lattice. The perturbation of a solid by... 

The native point defects in an oxide include M and O vacancies and M and O interstitial atoms or ions. Vacancies are denoted by v with a subscript M or O referring to vacant metal or oxygen sites, respectively. Interstitial ions or sites are described with a subscript "i". Vacancies and interstitials in an oxide are thus written... 

For larger defect concentrations one expects that defects begin to interact. The interactions can be coulombic or mechanical and be attractive or repulsive. These interactions may change the activity coefficients and formation enthalpies of defects, and they may lead to associations between defects which attract each other. A simple example is the association of a native point defect and a dopant or impurity ion with opposite effective charges. Energy minimisation may for large defect concentrations lead to clusters of different defects or elimination of defects in lines or planes, so as to form a new stmcture (e.g. shear stmctures). 

When a metal-deficient oxide MO with singly charged metal vacancies as the prevalent native point defects is doped with singly valent foreign cations,, the electroneutrahty condition is given by... 

As described in Chapter 1 impurities/dopants and native point defects may associate to form complex defects. An important driving force may be the coulombic attraction between impurities and point defects having opposite effective charges, but also relaxation of the lattice around the associate may play a role. 

Such a transition from intrinsic to extrinsic diffusion may take place when the temperature is lowered from high temperatures, where the native point defects predominate, to low temperatures, where the point defect concentrations are determined by the impurity concentration or, in other cases, frozen in. The temperature dependencies and the corresponding change in activation energy of the random diffusion in such a case are illustrated in Fig. 5.13. 

Figure 5-13. The dijfusion coefficient for oxygen diffusion by the vacancy mechanism in an oxygen deficient oxide in which oxygen vacancies are the predominant native point defects. At high temperatures the oxide exhibits intrinsic behaviour and at reduced temperatures extrinsic behaviour (i.e. the oxygen vacancy concentration is determined by the concentration of lower valent cations).

Kohan AF, Ceder G, Morgan D, Van de Walle CG (2000) First-principles study of native point defects in ZnO. Phys Rev B 61(22) 15019-15027... 

The first two types of stmcture elements are normal elements of the solid, while the others are native point defects. In general, a given solid contains several types of defects, which will be as many components in the thermodynamic sense of the term, and will form a solution with the normal elements. In practice, the problem boils down to the superposition of the equilibria of a base of the vector space. Usually, the defects are very dilute in comparison to the normal elements, so that they can be considered to be solvents with constant activity and the activities of the defects can be considered equal to their site fractions. 

Gontrary to native point defects, dislocations are not in thermodynamic equihb-rium and, hence, in principle are preventable. However, at present only silicon... 

D. T. J. Hurle, 1999, A comprehensive thermodynamic analysis of native point defect and dopant solubilities in gallium arsenide , j. Appl. Phys. 85, 6957-7022. 

Besides with each other, native point defects and dopants interact with structural defects ( gettering ) like single dislocations and precipitates resulting in... 

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