Effect of Impurity Atoms

The composition variation described in the previous chapter has a considerable impact upon the electronic properties of the solid. However, it is often difficult to alter the composition of a phase to order, and stoichiometry ranges arc frequently too narrow to allow desired electronic properties to be achieved. Traditionally, the problem has been circumvented by using selective doping by aliovalent impurities, that is, impurities with a different nominal valence to those present in the parent material. However, it is important to remember that all the effects described in the previous chapter still apply to the materials below. The division into two chapters is a matter of convenience only. 

Defects in Solids, by Richard J. D. Tilley Copyright 2008 John Wiley Sons, Inc. 

As in the previous chapter, most work has been carried out on oxides, and these figure prominently here. As the literature on oxides alone is not only vast but is also rapidly increasing, this chapter focuses upon a number of representative structure types to explain the broad principles upon which the defect chemistry depends. However, despite considerable research, the defect chemistry and physics of doped crystals is still open to considerable uncertainty, and even well-investigated simple oxides such as lithium-doped nickel oxide, Li Nij- O, appear to have more complex defect structures than thought some years ago. 

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The situation with II-VI semiconductors such as ZnO is similar to the situation with the elemental and the III-V semiconductors in respect of the location of the impurity atoms and their influences on the electric property. It is reported in ZnO that P, As, or S atom replaces either Zn or O site, and a part of them are also located at an interstitial site, as well as at a substitutional site [2,5-7], The effect of a few kind of impurities such as group-IIIA and -VA elements on the electric property of ZnO was extensively studied, especially when the impurity atoms were located at a substitutional site. The effects of the greater part of elements in the periodic table on the electric property of ZnO are, however, not well understood yet. The purpose of the present study is to calculate energy levels of the impurity atoms from Li to Bi in the periodic table, to clarify the effect of impurity atoms on the electric property of ZnO. In the present paper, we consider double possible configuration of the impurity atoms in ZnO an atom substitutes the cation lattice site, while another atom also substitutes the anion sublattice site. The calculations of the electronic structure are performed by the discrete-variational (DV)-Xa method using the program code SCAT [8,9],... 

Ion implantation (qv) has a large (10 K/s) effective quench rate (64). This surface treatment technique allows a wide variety of atomic species to be introduced into the surface. Sputtering and evaporation methods are other very slow approaches to making amorphous films, atom by atom. The processes involve deposition of a vapor onto a cold substrate. The buildup rate (20 p.m/h) is also sensitive to deposition conditions, including the presence of impurity atoms which can faciUtate the formation of an amorphous stmcture. An approach used for metal—metalloid amorphous alloys is chemical deposition and electro deposition. 

These problems have of course different weights for the different metals. The high reactivity of the elements on the left-side of the Periodic Table is well-known. On this subject, relevant examples based on rare earth metals and their alloys and compounds are given in a paper by Gschneidner (1993) Metals, alloys and compounds high purities do make a difference The influence of impurity atoms, especially the interstitial elements, on some of the properties of pure rare earth metals and the stabilization of non-equilibrium structures of the metals are there discussed. The effects of impurities on intermetallic and non-metallic R compounds are also considered, including the composition and structure of line compounds, the nominal vs. true composition of a sample and/or of an intermediate phase, the stabilization of non-existent binary phases which correspond to real new ternary phases, etc. A few examples taken from the above-mentioned paper and reported here are especially relevant. They may be useful to highlight typical problems met in preparative intermetallic chemistry. 

The effect of solute atoms on grain boundary migration cannot adequately be described by standard impurity drag theories. A more satisfactory agreement is obtained by taking an interaction ofthe impurities in the boundary into account. 

If adatom-impurity atom interaction is attractive, then the impurity atom can act as a trapping center. A diffusing adatom may be trapped. In heterogeneous catalysis, the reaction rate may be changed by the trapping effect of impurities as also by lattice defects and lattice steps and so on. 

The effect of impurities on the resistivity of metallic V203 is very large. Thus McWhan et al (1973b) found 140 p 2 cm per atomic per cent of Cr for the residual... 

The addition of small quantities of impurity atoms to a semiconductor has a dramatic effect on conductivity. It is, of course, such extrinsic effects that are the basis on which silicon semiconductor technology has developed. Their origins can be understood by considering a silicon crystal in which a small fraction of the... 

The new phases are characterized by considerable volume changes in relation to the basic material. The stresses occur on the interphase boundaries, and microcracks are formed. The example of such damage is metal embrittlement when forming hydride phases. The internal stresses also have an effect on kinetics of a new phase growth. Let us consider the residual stresses in a hollow cylinder. Maximal concentration of the impurity atoms occurs on the area boundary, where the new phase formation takes place. Its further growth is realized at the expense of impurity atoms diffusion. The task of defining kinetics of the new phase growth in the hollow cylinder is mathematically formulated as follows... 

The inclusion of impurity atoms in MgO is much more interesting from a chemical point of view when alkali metals are used to replace Mg ions. In fact, this results in trapped-hole centers. The MVO pairs have been extensively studied in the bulk of alkaline-earth oxides by optical studies, EPR and ENDOR measurements [185,186] as well as by embedded cluster calculations [187]. The LiVO ions create an effective dipole which polarizes the surrounding lattice, with the two ions moving toward each other. The presence of an O radical, however, is most interesting when one is dealing with surface properties. This center in fact is very reactive and is the subject of the next paragraph. 

From the theoretical side, there has been little achieved in quantitative predictions of the effect of impurity molecules on crystal nucleation. One exception is the work of Cole and Sluckin who considered the nucleation of freezing in an atomic liquid by a charged particle through the mechanism of electrostriction, in which the pressure is increased in the vicinity of an ion due to induced dipole forces, leading to a reduction of the barrier to nucleation. It is clear that much remains to be done in this field. 

In the third paper, Nonexponential motional damping of impurity atoms in Bose-Einstein condensate, the propagation of impurity atoms is considered in BEC. For atom velocities superior to that of condensate phonons, the thermal-isation rate of the condesate is not high enough to allow Markovian relaxation and the motional damping of the atoms is predicted to deviate from exponential. This is also a manifestation of non-mean-field effects in the condensate since the mean-field theories yield exponential decay for the motion of impurity atoms. Measurement of the motional decay rate should, therefore, provide a way for observing non-mean-field effects in the condensate. 

This value is independent of pH above pH 12 and only applies in solutions where the OH and H atom yield are completely converted into hydrated electrons. pH 12 is optimum as lower pH s do not convert all the H and OH yields (2) and higher pH s increase the effects of impurities in the alkali. A hydrogen-saturated solution at pH 12 can be sealed permanently in an optical cell. Such a cell has been used over a period of two years without showing any change in characteristics with an accumulated dose of several megarads. Effects of exposure to high dose rate irradiation (which produces hydrogen peroxide) are removed by exposure to low... 

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