Impurity centres

In conclusion to this part it seems noteworthy that in contrast to the effect of adsorption of molecular particles on electrophysical properties of oxide semiconductors, the major peculiarity of this effect for such chemically active particles as the simplest free radicals or atoms of simple gases (H2, O2, N2, CI2, etc.) is that they are considerably more chemically active concerning the impurity centres [47]. The latter are responsible for dope conductivity of oxide semiconductors. As for the influence of electric fields on their adsorption due to adsorption-induced surface charge distribution, they are of minor importance which is proved by results of the experiments on assessing field effect on adsorp-... 

CH3 -Zn with superstoichiometric (defect) zinc atoms (Zn -impurity centres of conductivity). The larger is the electric positivity of the metal in these complexes, the larger is the ionicity of the carbon-metal bond, carbon being at the negative end of the dipole. Thus, in the case of C - K bond, ionicity amounts to 51%, whereas for C - Mg and C - Zn bonds ionicity amounts to 35% and 18%, respectively [55]. Consequently, metalloorganic compounds are characterized by only partially covalent metal-carbon bonds (except for mercury compounds). 

In the discussion of defect equilibrium we discussed neutral defects such as V corresponds to an anion vacancy which has captured an electron. Such vacancies give rise to interesting optical phenomena. Defects associated with electrons or holes lead to colouration of the crystals and are known as colour centres. The term colour centre also includes impurity centres such as Tl, which are responsible for absorption and... 

We first note that an isolated atom with an odd number of electrons will necessarily have a magnetic moment. In this book we discuss mainly moments on impurity centres (donors) in semiconductors, which carry one electron, and also the d-shells of transitional-metal ions in compounds, which often carry several In the latter case coupling by Hund s rule will line up all the spins parallel to one another, unless prevented from doing so by crystal-field splitting. Hund s-rule coupling arises because, if a pair of electrons in different orbital states have an antisymmetrical orbital wave function, this wave function vanishes where r12=0 and so the positive contribution to the energy from the term e2/r12 is less than for the symmetrical state. The antisymmetrical orbital state implies a symmetrical spin state, and thus parallel spins and a spin triplet. The two-electron orbital functions of electrons in states with one-electron wave functions a(x) and b(x) are, to first order,... 

Cr ions. The impurity centres of Cr2+ in a CdGa2S4 crystal have been studied39 by HFEPR at frequencies up to 240 GHz. From the g-values, g = 1.93 and gL = 1.99, and the ZFS parameters it was shown that the chromium ions occupy one of the tetrahedrally coordinated cation positions. 

The presence of the region of weak dependence of the conductivity of alloyed semiconductors on temperature can be explained by tunneling of electrons from one impurity centre to another, unoccupied centre. The necessary condition of the impurity conductivity is the partial filling of the impurity levels. At low temperatures this conduction can be maintained only by semiconductor compensation, i.e. by the simultaneous presence of donor and acceptor impurities. In the case, for instance, of the n-type semiconduc-... 

Fig. 22. Energy levels for the compensated semiconductor of the n-type. Charge transfer is carried out by means of tunneling from an occupied donor level to a vacant donor level. The presence or the absence of the point designated "e indicates the presence or absence of the electron on the impurity centre.

Using the harmonic and the Franck-Condon approximations, it is possible to advance in calculating the sums of the (18) type using the method of generating functions developed by Kubo and Toyozawa [8, 9] for the calculation of the probabilities of optical and non-radiative transitions in the impurity centres in crystals. According to this method we can rewrite eqn. (18) as... 

These results show that the two gaps behaviour is present over all the range of the reduced Lifshitz parameter -0.8< z <+0.8. These results support the predictions that the doped materials remain in the clean limit for interband pairing although the large number density of impurity centres. This results falsifies the predictions of the multigap suppression because of impurity scattering due to substitutions. 

Inorganic phosphors can have their luminescent efficiency improved by the addition of an activator, e.g., 0.1% T1 in Nal (then referred to as Nal(Tl) excitation energy of Nal molecules is transferred to impurity centres in the crystal lattice, which de-excite with photoemission characteristic of the impurity. The higher atomic number of these phosphors improves the probability of photoelectric effect events. Nal is deliquescent and has to be sealed in an A1 pot with glass window. The useable light output is improved by painting white to improve reflection from the surface. 

Impurity and Aperiodicity Effects in Polymers.—The presence of various impurity centres (cations and water in DNA, halogens in polyacetylenes, etc.) contributes basically to the physics of polymeric materials. Many polymers (like proteins or DNA) are, however, by their very nature aperiodic. The inclusion of these effects considerably complicates the electronic structure investigations both from the conceptual and computational points of view. We briefly mentioned earlier the theoretical possibilities of accounting for such effects. Apart from the simplest ones, periodic cluster calculations, virtual crystal approximation, and Dean s method in its simplest form, the application of these theoretical methods [the coherent potential approximation (CPA),103 Dean s method in its SCF form,51 the Hartree-Fock Green s matrix (resolvent) method, etc.] is a tedious work, usually necessitating more computational effort than the periodic calculations... 

When the light is switched off, the photoconduction will decay as the carrier population gradually returns to equilibrium. By studying photoconduction kinetics it is often possible to determine the dominant mechanism of carrier loss neutralisation at electrodes, recombination of electrons with holes, or trapping at defects or impurity centres. 

On the viewpoint here presented poisons and promoters become impurity centres in the normal lattice of the catalyst, the former tending to raise the activation energy of adsorption, the latter to lower it. 

Dipolar instabilities of impurities in solids were discovered in 1965 by Lombardo and Pohl in Li-doped KCl [49]. Since then, a large amount of off-centre and on-centre instabilities of monoatomic impurities in insulator and semiconductor materials have been reported. In many cases the impurity centres are not well characterized and the observed instabilities could be due to close defects. In this article we only consider centres with spontaneous instabilities driven by PJT mechanisms. An exhaustive review of all these centres is beyond the scope of this report and we have selected representative examples based on the authors interest. Some early reviews can be found in [93,152,153]. 

Extrinsic semiconductors are materials containing foreign atoms (FAs) or atomic impurity centres that can release electrons in the CB or trap an electron from the VB with energies smaller than Eg (from neutrality conservation, trapping an electron from the VB is equivalent to the release of a positive hole in the otherwise filled band). These centres can be inadvertently present in the material or introduced deliberately by doping, and, as intrinsic, the term extrinsic refers to the electrical conductivity of such materials. The electron-releasing entities are called donors and the electron-accepting ones acceptors. When a majority of the impurities or dopants in a material is of... 

In a semiconductor, substitutional FAs from the same column of the periodic table as the one of the crystal atom they replace are usually electrically inactive and they are called isoelectronic with respect to the semiconductor. It can occur, however, that for some isoelectronic impurities or electrically-inactive complexes, the combination of the atomic potential at the impurity centre with the potential produced by the local lattice distortion produces an overall electron- or hole-attractive potential in a given semiconductor. This potential can bind an electron or a hole to the centre with energies much larger than those for shallow electrically-active acceptors or donors. The interaction of these isoelectronic impurities traps the free excitons producing isoelectronic bound excitons which display pseudo-donor or pseudo-acceptor properties. This is discussed later in this chapter in connection with the bound excitons, and examples of these centres are given in Chaps. 6 and 7. 

Fig. 2.7. Configuration coordinate diagram of the electronic energies of an impurity centre whose lattice equilibrium configurations in the ground and ionized states are represented by configuration coordinates Qgr and <5free with different values. The thermal ionization energy Eith of such a centre is smaller than the optical ionization energy Ei0 by the Franck-Condon energy Efc...

The absorption of impurity centres is observed in the transparency domains of semiconductors and insulators, which are limited by their intrinsic electronic and vibrational absorptions. Further, a brief account of the relevant physical processes and an overview of the intrinsic optical properties of these materials and of their dependence on temperature, pressure and magnetic field is given in this chapter. Some semiconductors have been or are now synthesized in quasi-monoisotopic (qmi) forms because of improvements in their physical properties like thermal conductivity. A comparison of their intrinsic optical properties with those of the crystals of natural isotopic composition is also given. The absorption related to free carriers, due mostly to doping is also discussed at the end of this chapter. A detailed account of the optical properties of semiconductors can be found in the books by Yu and Cardona [107] and by Balkanski and Wallis [4]. 

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