Impurity: also defect

However, most impurities and defects are Jalm-Teller unstable at high-symmetry sites or/and react covalently with the host crystal much more strongly than interstitial copper. The latter is obviously the case for substitutional impurities, but also for interstitials such as O (which sits at a relaxed, puckered bond-centred site in Si), H (which bridges a host atom-host atom bond in many semiconductors) or the self-interstitial (which often fonns more exotic stmctures such as the split-(l lO) configuration). Such point defects migrate by breaking and re-fonning bonds with their host, and phonons play an important role in such processes. 

Given the strontium chloride crystal, write the defect reaction(s) expected if lithium chloride is present as an impurity. Do likewise for the antimony chloride impurity. Also, write the defeet reactions expected if both impurities are present in equal quantities. 

In some ionic crystals (primarily in halides of the alkali metals), there are vacancies in both the cationic and anionic positions (called Schottky defects—see Fig. 2.16). During transport, the ions (mostly of one sort) are shifted from a stable position to a neighbouring hole. The Schottky mechanism characterizes transport in important solid electrolytes such as Nernst mass (Zr02 doped with Y203 or with CaO). Thus, in the presence of 10 mol.% CaO, 5 per cent of the oxygen atoms in the lattice are replaced by vacancies. The presence of impurities also leads to the formation of Schottky defects. Most substances contain Frenkel and Schottky defects simultaneously, both influencing ion transport. 

The low-temperature thermal conductivity of different materials may differ by many orders of magnitude (see Fig. 3.16). Moreover, the thermal conductivity of a single material, as we have seen, may heavily change because of impurities or defects (see Section 11.4). In cryogenic applications, the choice of a material obviously depends not only on its thermal conductivity but also on other characteristics of the material, such as the specific heat, the thermal contraction and the electrical and mechanical properties [1], For a good thermal conductivity, Cu, Ag and A1 (above IK) are the best metals. Anyway, they all are quite soft especially if annealed. In case of high-purity aluminium [2] and copper (see Section 11.4.3), the thermal conductivities are k 10 T [W/cm K] and k T [W/cm K], respectively. 

Previous sections of this chapter have shown that it is possible to introduce defects into a perfect crystal by adding an impurity. Such an addition causes point defects of one sort or another to form, but they no longer occur in complementary pairs. Impurity-induced defects are said to be extrinsic. We have also noted that when assessing what defects have been created in a crystal, it is important to remember that the overall charge on the crystal must always be zero. 

We depart briefly from our discussion of SI GaAs to consider an example that better illustrates some of the features of temperature-dependent Hall measurements. This example (Look et al., 1982a) involves bulk GaAs samples that have sc — F — 0-15 eV. We suppose, initially, that the impurity or defect controlling the Fermi level is a donor. Then any acceptors or donors above this energy (by a few kT more) are unoccupied and any below are occupied. Also, p n for kT eG. From Eq. (B34), Appendix B, we get... 

Each of the methods considered in this report has a certain impurity fingerprinting value. By this we mean that one or more distinctive parameters determined by the method will have the same value every time a particular impurity or defect is in the sample. For TDH measurements the distinctive parameter is essentially Ei0, the activation energy at T = 0. For PC and absorption measurement it is , the value at the temperature of measurement, and for the TSC, PITS, and OTCS methods it is basically the temperature of maximum peak height (for a given rate window). The latter three methods also yield the preexponential factor in the emission expression [cf. Eq. (30)], and this factor is often useful in distinguishing between traps of roughly equal activation energies. 

Defects which have extent of only about an atomic diameter also exist in crystals—the point defects. Vacant lattice sites may occur—vacancies. Extra atoms—interstitials—may be inserted between regular crystal atoms. Atoms of the wrong chemical species—impurities—also may be present. 

The investigation of impurities and defects, their energetic and interactions is seemed to be very important because appearance even one single defect can change not only value but also the type of nanotubes conductivity. 

It has been shown that the spin-Hall effect may arise from various spin-orbit couphngs, such as a spin-orbit (SO) interaction induced by the electron-impurity scattering potential,a Rashba SO conphng in two-dimensional systems, etc. Murakami et al. also predicted a nonvanishing spin-Hall cnrrent (AHC) in a perfect Luttinger bnlk p -type semiconductors (no impurities or defects)." Experimental observations of the spin-Hall effect have been reported recently in a n -type bnlk semiconductor and in a two-dimensional heavy-hole system. ... 

Several theories may be proposed to explain the difference in observed melting onsets, one of which is impurities present in the material under investigation. Often, impurities or defects in samples are perceived as specks or spots. Although these indeed are considered impurities, homogeneously or inhomogeneously dispersed impurities should also be considered. Based on the range of particle sizes observed, the concentration of these impurities in the various-sized particles will undoubtedly... 

In crystals, impurities can take simple configurations. But depending on their concentration, diffusion coefficient, or chemical properties and also on the presence of different kind of impurities or of lattice defects, more complex situations can be found. Apart from indirect information like electrical measurements or X-ray diffraction, methods such as optical spectroscopy under uniaxial stress, electron spin resonance, channelling, positron annihilation or Extended X-ray Absorption Fine Structure (EXAFS) can provide more detailed results on the location and atomic structure of impurities and defects in crystals. Here, we describe the simplest atomic structures more complicated structures are discussed in other chapters. To explain the locations of the impurities and defects whose optical properties are discussed in this book, an account of the most common crystal structures mentioned is given in Appendix B. 

In a crystal, perturbations can be classified as internal and external. The internal perturbations are disturbances from an equilibrium condition, taken as an ideal uniform distribution of impurities or defects which do not modify the crystal lattice and the average electronic density. Mechanical perturbations can be microscopic, like those introduced by impurities or defects producing large local volume changes, which reflect on crystal lattice spacings when their concentration is large, or macroscopic due to residual or accidental stresses. Permanent perturbations can also be produced by unrelaxed stresses... 

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