Defects antisites

Misplaced atoms If an atom is present on a crystal site that should be occupied by a different atom, that atom is a misplaced atom and may be called an antisite defect. Antisite defects usually form in covalent ceramics such as AIN and SiC, but can also occur in complex oxides that have several different types of cation, for example, spinels and garnets. (We do not expect to see cations on anion sites and vice versa.)... 

Substitutional defects are either impurities or antisites. Impurities , as one would expect, refers to atoms which are not constituents of the semiconductor host. They are not intrinsic to the nature of a solid, but result from its incomplete purification or intentional contamination. Thus, they are referred to as extrinsic defects. Antisites, as with vacancies, are intrinsic to compounds. Antisite defects are found only in crystals with more than one sublattice and having different atoms on each. Si has two sublattices, both fee Bravais lattices, translated by 1/4, 1/4, 1/4 with respect to each other. However, because the atoms on both are the same, moving a Si atom from one sublattice to the other has no effect. By contrast, GaAs has two sublattices, one on which only Ga atoms reside, the other contains only As. Thus, there are two antisite defects, a Ga on an As site (GaAs) or an As on a Ga site (Asoa)- Antisite defects may, and often do, have multiple charge states in the energy gap. 

For other intrinsic defects (antisites, interstitials,...), their concentration depends upon their formation energy and entropy exactly as for vacancies and the formation energy will change with the Fermi energy or in the presence of other defects as well. 

We consider an AB alloy which consists of an equal number of A and B sites. For the subsequent analysis, every site is uniquely associated with either an A or a B sublattive. The following is trivially generalised to A iBn alloys. The alloy is not quite stoichiometric, and has the composition A Bj.x, where for the validity of the independent defect approximation we must suppose x to be within a few percent of 0.5. Each site of each sublattice can be occupied by its own atom, an atom of the other kind (an antisite defect) or a vacancy. There are therefore six species for which we define the concentrations on each sublattice ... 

II. Triple defect on A two atoms vacate B sites and occupy new A and B sites, thus creating two B vacancies and an A antisite defect Denote the energy requred by Cia. 

At T=OK, the criterion of minimum energy dictates that only one defect, the constitutional defect, is present. In NiAl for example this is the Ni antisite (Cab) in Ni-rich alloys, and the Ni vacancy (Cva) in Al-rich alloys [6]. The constitutional defects will be the dominant ones at high temperatures. The criterion that the A antisite should be the constitutional defect in A-rich alloys is ... 

In principle LVM spectroscopy is not the only way to evidence the presence of hydrogen in bulk materials not intentionally doped with hydrogen. Shinar et al. (1986) attributed optically detected electron nuclear double resonance (ODENDOR) lines to hydrogen associated with a PGa antisite-related defect in GaP. However the identification of these ODENDOR lines is not unambiguous as it has been recently proposed that these lines could be P-related (Watkins, 1989). 

What is the "effective charge on a defect What is an antisite defect ... 

The position of a defect that has been substituted for another atom in the structure is represented by a subscript that is the chemical symbol of the atom normally found at the site occupied by the defect impurity atom. The impurity is given its normal chemical symbol, and the site occupied is written as a subscript, using the chemical symbol for the atom that normally occupies the site. Thus, an Mg atom on a Ni site in NiO would be written as MgNi. The same nomenclature is used if an atom in a crystal occupies the wrong site. For example, antisite defects in GaN would be written as GaN and NGa. 

An antisite defect is an atom on a site normally occupied by a different chemical species that exists in the compound. Antisite defects are a feature of a number of important materials, especially weakly ionic or covalently bonded ones. In a compound of formula AB the antisite defects that can occur are an A atom on a site normally occupied by a B atom (Fig. 1.16a), or a B atom on a site normally occupied by an A atom (Fig. 1.16b). 

Figure 1.16 Antisite point defects in an ionic crystal of formula MX (schematic) (a) A on B sites, AB (b) B on A sites, BA (c) Na+ on a Cl- site in sodium chloride, Na and (d) Cl- on an Na+ site in sodium chloride, Cl a.

In metallic and many semiconducting crystals, the valence electrons are delocalized throughout the solid, so that antisite defects are not accompanied by prohibitive energy costs and are rather common. For example, an important defect in the semiconducting material GaAs, which has the zinc blend structure (Supplementary Material SI), is the antisite defect formed when an As atom occupies a Ga site. 

In cases where the antisite defects are balanced, such as a Ga atom on an As site balanced by an As atom on a Ga site, the composition of the compound is unaltered. In cases where this is not so, the composition of the material will drift away from the stoichiometric formula unless a population of compensating defects is also present. For example, the alloy FeAl contains antisite defects consisting of iron atoms on aluminum sites without a balancing population of aluminum atoms on iron sites. The composition will be iron rich unless compensating defects such as A1 interstitials or Fe vacancies are also present in numbers sufficient to restore the stoichiometry. Experiments show that iron vacancies (VFe) are the compensating defects when the composition is maintained at FeAl. 

The creation of antisite defects can occur during crystal growth, when atoms are misplaced on the surface of the growing crystal. Alternatively, they can be created by internal mechanisms once the crystal is formed, provided that sufficient energy is applied to allow for atom movement. 

To illustrate exactly how these mles work, a number of examples follow. In the first, the formation of antisite defects, a simple example that does not involve changes in atom numbers or charges on defects, is described. Secondly, two reactions involving oxides, nickel oxide and cadmium oxide, both of which are nonstoichio-metric, but for opposite reasons, indicate how to deal with a solid-gas interaction... 

The creation of a complementary pair of antisite defects consisting of an A atom on a B atom site, AB, and a B atom on an A atom site, Ba, can be written in terms of a chemical equation ... 

Antisite defects can be created via the intermediate formation of a Frenkel defect by the following mles ... 

A point defect is a localized defect that consists of a mistake at a single atom site in a solid. The simplest point defects that can occur in pure crystals are missing atoms, called vacancies, or atoms displaced from the correct site into positions not normally occupied in the crystal, called self-interstitials. Additionally atoms of an impurity can occupy a normal atom site to form substitutional defects or can occupy a normally vacant position in the crystal structure to form an interstitial. Other point defects can be characterized in pure compounds that contain more than one atom. The best known of these are Frenkel defects, Schottky defects, and antisite defects. 

At all temperatures above 0°K Schottky, Frenkel, and antisite point defects are present in thermodynamic equilibrium, and it will not be possible to remove them by annealing or other thermal treatments. Unfortunately, it is not possible to predict, from knowledge of crystal structure alone, which defect type will be present in any crystal. However, it is possible to say that rather close-packed compounds, such as those with the NaCl structure, tend to contain Schottky defects. The important exceptions are the silver halides. More open structures, on the other hand, will be more receptive to the presence of Frenkel defects. Semiconductor crystals are more amenable to antisite defects. 

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