Complex defects

Common teniiinology used to characterize impurities and defects in semiconductors includes point and line defects, complexes, precipitates and extended defects. These teniis are somewhat loosely defined, and examples follow. 

If it is assumed that the ions and the interstitial ions are associated, i.e. not free to migrate in the lattice independently of one anotlrer, then we write for the equilibrium constant which involves the defect complex, — - U +... 

Note that we can use the same statistical mechanical approach to calculate SchottslQi" pairs, Frenkel pairs, divancies (which are associated vacancies), impurity-vacancy complexes, and line dislocation-point defect complexes. 

Much of the microscopic information that has been obtained about defect complexes that include hydrogen has come from IR absorption and Raman techniques. For example, simply assigning a vibrational feature for a hydrogen-shallow impurity complex shows directly that the passivation of the impurity is due to complex formation and not compensation alone, either by a level associated with a possibly isolated H atom or by lattice damage introduced by the hydrogenation process. The vibrational band provides a fingerprint for an H-related complex, which allows its chemical reactions or thermal stability to be studied. Further, the vibrational characteristics provide a benchmark for theory many groups now routinely calculate vibrational frequencies for the structures they have determined. 

The effect of isotopic or chemical substitutions on the vibrational frequency leads to the identification of the species that comprise a defect complex. For example, the large frequency shift that results upon the substitution of D for H leads to an unambiguous identification of the H motions. For a series of chemically similar complexes, such as the acceptor-H complexes, the frequency shifts that occur for group III substitutions show that the acceptor is indeed involved in the complex. 

One of the primary interests in defect complexes that contain hydrogen has concerned the possible motion of the light hydrogen atom. The stress-induced alignment techniques pioneered by Watkins and Corbett (1961) and Corbett et al. (1961) provide a means to detect and characterize such motions. 

As an introduction to the theory as it relates to these defect complexes, we point out that the most conspicuous experimental feature of a light impurity such as hydrogen is its high local-mode frequency (Cardona, 1983). Therefore, it is essential that the computational scheme produce total energies with respect to atomic coordinates and, in particular, vibrational frequencies, so that contact with experiment can be established. With total-energy capabilities, equilibrium geometries and migration and reorientation barriers can be predicted as well. 

IV. Hydrogen—Deep-Level-Defect Complexes in Silicon... 

VI. Hydrogen—Shallow-Level-Defect Complexes in Compound Semiconductors... 

This reveals that two alternative defect structures can be imagined, one with free holes and one with Ni3+ defects. A further possibility is that the hole may be lightly bound to an Ni2+ ion to give a defect complex that could be written (NiNi + h ). All of these descriptions are valid. The one adopted would be the one most consistent with the measured properties of the solid. 

These point defect models need to be regarded as a first approximation. Calculations for stoichiometric GaAs suggest that balanced populations of vacancies on both gallium and arsenic sites, VGa and VAs, exist, as well as defect complexes. Calculation for nonstoichiometric materials would undoubtedly throw further light on the most probable defect populations present. 

However, a detailed model for the defect structure is probably considerably more complex than that predicted by the ideal, dilute solution model. For higher-defect concentration (e.g., more than 1%) the defect structure would involve association of defects with formation of defect complexes and clusters and formation of shear structures or microdomains with ordered defect. The thermodynamics, defect structure, and charge transfer in doped LaCo03 have been reviewed recently [84],... 

Atoms in the free surface of solids (with no neighbors) have a higher free energy than those in the interior and surface energy can be estimated from the number of surface bonds (Cottrell 1971). We have discussed non-stoichiometric ceramic oxides like titania, FeO and UO2 earlier where matter is transported by the vacancy mechanism. Segregation of impurities at surfaces or interfaces is also important, with equilibrium and non-equilibrium conditions deciding the type of defect complexes that can occur. Simple oxides like MgO can have simple anion or cation vacancies when surface and Mg + are removed from the surface,... 

NADH coenzyme Q reductase defect (complex I) Succinate coenzyme Q reductase defect (complex II) Coenzyme Q cytochrome C reductase defect (complex III)... 

The identification of an impurity, defect, or impurity-defect complex by some particular technique must nearly always be accomplished in conjunction with doping experiments. Thus, the well-known, sharp, zero-phonon photoluminescence lines at 0.84 eV in GaAs are almost certainly associated with Cr, as established by Cr-doping experiments (Koschei et al., 1976). However, some care must be taken here. For example, a dominant electron trap (EL2) in -doped GaAs is probably not associated with O, according to recent experiments (Huber et al., 1979). Thus, the doping must be accompanied by a positive identification of the relevant impurity concentration, say by SSMS, or SIMS. These general considerations apply to all the techniques discussed below. 

The defect structure of Fei O with the NaCl-type structure had been estimated to be a random distribution of iron vacancies. In 1960, Roth confirmed, by powder X-ray diffraction, that the defect structure of wiistite quenched from high temperatures consists of iron vacancies (Vp ) and interstitial iron (Fcj) (there are about half as many FCj as Vpe). This was a remarkable discovery in the sense that it showed that different types of crystal defects with comparable concentrations are able to exist simultaneously in a substance, Roth also proposed a structure model, named a Roth cluster, shown in Fig. 1.84. Later this model (defect complex = vacancy -F interstitial) was verified by X-ray diffraction on a single crystal and also by in-situ neutron diffraction experiments. Moreover, it has been shown that the defect complex arranges regularly and results in a kind of super-structure, the model structure of which (called a Koch-Cohen model) is shown in Fig. 1.85 together with the basic structures (a) and (b). 

Fig. 1.84 Defect complexes of wustite proposed by Roth (Roth cluster). ...

Fig. 1.85 Defect complexes of wustite. (a) Basic structure (Roth cluster) (b) edge-sharing tetrahedra (6 2 complex) (c) cornersharing tetrahedra (Koch-Cohen complex).

Based on these experimental results, Catlow and Fender calculated the stability of various kinds of defect complexes by the use of Mott s method. 

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